VHF Broadcast Re-planning

G - Case study: London

This annex sets out the technical detail of the modelling on which the conclusions of this report are based.  It should be noted that the coverage calculations presented here, and in the next annex, refer to ‘normally resident populations’ within an interference-protected 60 dBV/m contour.

In practice, contours of 54 dBV/m and 60 dBV/m are used for local and national services respectively, and services are protected from interference to varying degrees.  RAJAR audience figures are given on the basis of adult (i.e.15+) populations.

The use of the single 60 dBV/m contour level, the need to apply a geographical limit in exploring the effects of frequency changes and the requirement that the study ignore continental interference all imply that the impact of the changes described will tend to be underestimated.

It should also be noted that the scenarios described below, relating to the introduction of new London services of various scale, are treated independently.

G.1 Introduction

VHF/FM services were first provided in the London area from the BBC high-power station at Wrotham, in Kent, which went into full service in 1955, providing the (then) three national services in the band 88.0–94.6 MHz.

Figure G.1  Transmitter locations in the London area

The Wrotham site was intended to provide coverage of the greater part of South-East England and while coverage in Central London is generally adequate, there have been criticisms over the years that impulsive interference (e.g. from car ignition systems), multipath and building blockage give rise to reception difficulties.  These problems were addressed in the mid-1990s by the addition of a ‘filler’ station at Crystal Palace, which made use of special techniques to allow the use of frequencies only 300 kHz below those of Wrotham.

BBC local radio was introduced in London in 1970, using a 2 kW transmitter at the Crystal Palace television transmitter site in the then recently released sub-band at 94.6–97.6 MHz. Independent radio followed with the opening of Capital and LBC (later News Direct) in 1973 in the same sub-band.

Independent services expanded rapidly in the early 90s: Melody (1990, Croydon), Kiss (1990, Croydon) and Jazz (1990, Croydon).  With the availability for broadcast use of frequencies above 105 MHz, further stations were licensed in the mid-1990s: ‘Heart 106.2’ (1995, Croydon), Xfm (1997, Crystal Palace) and ‘Virgin 105.8’ (1995, Croydon)–bringing the total to eight London-wide ILR stations.

Finally, six lower-power stations have been introduced, serving limited areas within London: London Greek Radio in North London (1989, Alexandra Palace), Choice FM in Brixton (1990, Croydon), Millennium Radio in Greenwich (1990, originally RTM, Shooters Hill), ‘Active 107.5 FM’ in Romford (1998, Lambourne House), ‘107.8 FM Thames Radio’ in Kingston (1997, Tolworth Tower) and ‘FLR 107.3’ in Lewisham (1999, Bredgar).

In addition to these local services, Classic FM is transmitted from both Wrotham and Crystal Palace, with the same parameters as the BBC services.

G.1.1 Frequencies potentially available

The plot below shows the variation of predicted pfs with frequency for a receiver in Central London (NGR:TQ300800 – Charing Cross), calculated for each 100 kHz channel in Band II.  Calculations are for 50%-time, and are based on the Radio Authority transmitter database, thus including some proposed and Geneva Plan stations in addition to existing services.  It should be recalled that a pfs of between 60–70 dBV/m would normally be the maximum that would allow the use of a channel to offer a new service.

Figure G.2  pfs in London

The national network allocations at Wrotham and Crystal palace are immediately apparent, separated by 300 kHz.  The Geneva Plan allocations for ‘London North’ (on 89.7 and 91.9 MHz) can be seen at a similar strength.

The high-power local services from Crystal Palace and Croydon can be seen giving a pfs of around 130 dBV/m (94.9, 95.8, 97.3, 100.0, 102.2, 104.9, 105.4, 105.8 and 106.2 MHz).

G.2 Scenario 0: Allocations possible with minimal disruption

The first scenario sought allocations for new, London-wide, services and assumed that only minimal changes to the existing network were permissible.  Any new services would have to be accommodated within the existing pattern of spectrum use.

On this basis, a large number of channels within the band can immediately be ruled out: it will not be practically possible to operate any station in the London area at a spacing of less than 400 kHz from an existing London service.

G.2.1 National network sub-bands

The implications of this can be seen in the following table, which indicates allocations serving, and close to, London in the BBC National Network (Radio 2) sub-band.  It should be recalled that the same pattern of use will apply for Radio 3 (90.2–92.5 MHz), Radio 4 (92.5–94.6 MHz), Radio 1 (97.7–99.8 MHz) and, to a lesser extent, Classic FM (99.9–102.0 MHz).  It follows that if an allocation suitable for use in London is found in one sub-band, it will also be available in the others.

Table G.1   BBC National Network sub-band

The frequencies between 88.6–89.4 MHz that have been marked ‘XX’ in Table G.1 are unusable due to the presence of Wrotham, with its relay at Crystal Palace.  Remaining channels might seem to offer potential frequencies for use by London stations, but if the interference power on each channel is examined it can be seen that it is not straightforward.

The Guildford transmitter, for instance, gives field strengths of ~45–50 dBV/m in Central London, giving a pfs of up to 83 dBV/m, even on the adjacent channel.  Rowridge (on the Isle of Wight) and Oxford both give rise to a pfs of ~77 dBV/m in the adjacent channels.  Smallfiller stations at Caterham and Kenley give adjacent channel pfs of some 75 dBV/m in parts of Central London.  The only channel that appears to be usable in most parts of London is 89.9 MHz, with a highest field strength of some 30 dBV/m from Wenvoe in South Wales, though other channels might be used for small-scale services in areas where local geography affords some shielding from incoming interference.

Some frequencies in the BBC National Network sub-bands, however, have a 10.7 MHz relationship with existing allocations in the upper part of the band: thus, the 89.9 MHz channel discussed above lies 10.7 MHz below the Crystal Palace allocation at 100.6 MHz, and would be very likely to suffer severe interference due to the mechanisms discussed in Annex D.

The use made of the INR sub-band (99–102.0 MHz) is somewhat different to that made of the BBC bands.  As there is no requirement for universal coverage, the extent of the transmitter network is driven by commercial considerations and the Radio Authority’s policies on frequency use.  As has been noted above, the law of diminishing returns applies to the coverage of a radio after a fairly small number of high-power sites have been commissioned.  Consequently, many of the transmitter sites potentially available for INR use have never been brought into service.  This has allowed the accommodation of some local radio services within the sub-band: for example, it is possible to accommodate a London-wide station at 100.0 MHz due to the absence of an INR relay site at Guildford, which would have operated on 99.9 MHz.

G.2.2 Lower local radio sub-band

Table G.2, below, illustrates some of the existing allocations in the sub-band between the two BBC National Network bands.  These frequencies are used for both commercial and BBC services.

Table G.2   Lower local sub-band

As above, the channels made unusable by the presence of an existing London service are indicated.  There are substantial limitations on the use of the remaining frequencies owing to their use by local stations in the surrounding area.  The field strengths in most of the London area from the transmitters at Guildford and South Benfleet (Southend) are such that it will not normally be possible to re-use these frequencies, or frequencies within 200 kHz of them.  This can be seen in Figure G.19(Guildford) and in Figure G.3below (South Benfleet).

Figure G.3   Field strength in London from South Benfleet (95.3 MHz)

It can be seen that this immediately renders all frequencies other than 96.6 and 96.7 MHz unusable; these also, however, suffer high levels of interference from transmitters serving the St. Albans and Medway areas (though mitigated somewhat by the use of directional aerials at these sites).

G.2.3 Upper local radio sub-band

Some of the existing allocations in this band are illustrated in Table G.3 below.  It should be noted that some frequencies in this band would not normally be considered available for use in London owing to their relationship with London-area allocations 10.7 MHz below.

Table G.3   Upper local sub-band

The use of the upper part of the band (above around 107.0 MHz) for low-power allocations is the result of a deliberate policy by the Radio Authority, aimed at avoiding interference into the adjacent aeronautical band (see Annex E).

Below this ‘low-power’ segment, the band is densely occupied to about 104.5 MHz by high-power allocations, whereas the remainder of this sub-band is more mixed.  The allocations to sites in the immediately-surrounding area (such as Hannington, Reigate and Bluebell Hill) give rise to pfs levels in London that are sufficient to preclude the use of these, or adjacent frequencies for new allocations.

G.2.4 Analysis and conclusions

This scenario has examined the possibility of introducing new high-power services to the London area.  The work undertaken indicates that there is very little possibility of accommodating this type of new service in the London area under the existing pattern of use.  Consequently, no value analysis has been undertaken for this scenario.  However, it should be noted that whilst the introduction of high-power services does seem to demand a change to existing use, lower-power services may yet be successfully introduced without such change.  This possibility is examined in more detail in Scenario C.

G.3 Scenario A: Allocations for large-area services

The next scenario investigated aimed to accommodate the largest possible number of new, London-wide, services, regardless of the implications for smaller services within London, or for services in other areas.  It must be emphasised that the re-planning described below is a hypothetical exercise for the purposes of exploring the likely availability of additional spectrum in London if a formal re-planning of the spectrum were undertaken.  The investigation of such a large number of allocations is only a tool, with which spectrum use and flexibility may be investigated.

While London lies in something of a bowl, this affords only limited shielding from the surrounding area, as evidenced by the fact that the Crystal Palace television transmitter serves an area including Crawley, Bracknell, Harlow and Southend, and that some doggedly enthusiastic listeners enjoy London local services along the south coast.  Consequently, the maximisation of services within the London area implies some penalty to be paid in the surrounding area; an attempt is made to quantify this penalty below.

G.3.1 BBC National Network sub-bands

The assumption made in this study is that existing allocations of greater than 20 kW are immutable.  All such stations are in the National Network sub-bands, and as indicated in Table G.1, the opportunity for accommodating new services is this band is severely limited.

With the high-power transmitters at Sutton Coldfield, Rowridge, Crystal Palace, Oxford, Tacolneston, Wenvoe, Swingate and Peterborough generating significant levels of interference in London, the only frequency group that seems to offer any potential is that used by the relay transmitter at Guildford (88.1 MHz group).  It is relevant to note that, as noted above, this relay site is not used by Classic FM.  This ‘unused allocation’ at 99.9 MHz allows the use of the adjacent channel (100.0 MHz) for a London service.

The use of the ‘Radio 2’ or ‘Radio 3’ frequencies for a new London service would create a 10.7 or 10.6 MHz relationship with the Radio 1 or Classic FM transmissions from Wrotham.  Neither of these relationships would normally be allowed under current planning assumptions.  The use of the Guildford ‘Radio 4’ frequency (92.5 MHz) avoids the creation of such taboo relationships.

The changes, and consequent impacts, required to allow the introduction of a new London-wide local service on 92.5 MHz are described below.  Equivalent services might also be introduced in the Radio 2 and Radio 3 sub-bands (on 88.1 and 90.3 MHz) if it was deemed possible to ignore the 10.7/10.6 MHz relationships.  The use of the frequency in the Radio 1 sub-band (97.7 MHz) would additionally require a change of frequency, or closure, by an ILR service at Dunstable.

Croydon 92.5MHz

Table G.4

The current pattern of coverage in the Guildford area is complex, with contributions from Wrotham, Crystal Palace, Oxford, Rowridge and Mickleham. If the Guildford transmitter is closed, a major deficiency (some 250,000) would result. It is therefore assumed that a low-power service will continue to be provided to Guildford town, on a new frequency of 93.7 MHz, reducing the composite loss of coverage to approximately 100,000.

The change in (gross) coverage from the Guildford site can be seen below, below.

Croydon 92.5 MHz

Figure G.4   Coverage of Guildford (92.5 and 93.7 MHz)

The use of this frequency will, in itself, degrade the existing coverage from Oxford and Wrotham.  In particular, coverage from Wrotham is lost in Aldershot.

The existing relay at Kenley, at a separation of 300 kHz, would cause local interference to the proposed new service, and is assumed to be closed.  The resulting net population loss (~1,000 people) is limited by the service remaining from Crystal Palace. Alternatively, the new London service might be offered on the understanding that a minor coverage deficiency would exist in the Kenley area.

The Manningtree transmitter, serving the Ipswich area is currently co-channel with Guildford: this relationship will not be tenable with the new London station, and it is therefore assumed that the Manningtree allocation is moved to 92.8 MHz, adjacent to Rowridge and Sutton Coldfield.  This change results in a relatively minor loss of coverage to the scattered rural population in the north of the Manningtree service area.  While the gross loss of population coverage is around 20,000, the net loss (taking the coverage from Tacolneston into account) will be of the order of 1,000-2,000.

Analysis

If high-power allocations are considered immutable, only one frequency in the BBC national Network bands appears to be potentially usable for a new London-wide service.

The use of this frequency to provide a new service to some 7 million potential listeners in London would result in a loss of service (BBC Radio 4) to some 100,000 people in the area to the South and West of Guildford.

The remaining Radio 4 service will be provided from the transmitters at Wrotham, Oxford, Rowridge and Crystal Palace, and will require frequent retuning by listeners travelling over a small area.

The changes will also cause minor losses in coverage in South London and Suffolk.

An additional consideration in the use of this frequency is that the coverage provided by the four BBC FM National Networks would no longer be comparable.

If a policy decision is made that the aim of universal coverage of BBC services is no longer to be sought, it would be necessary to undertake the re-planning of the network as a whole.  The piecemeal use of the more readily-identified frequencies would be likely to lead to an inefficient use of the spectrum.

G.3.2 Local radio sub-bands

It can be seen from Annex C that the protection ratio falls to zero at around 270 kHz offset, which might seem to imply that London-wide services could be radiated on a raster of frequencies at this interval (or the nearest integer channel increment of 300 kHz).  It would, of course, be necessary for the services to be radiated from the same site, so as to ensure that the ratio between them was constant throughout the service area.

In practice, however, an allowance must be made for multipath and standing wave effects, which could give rise to substantial differences in field strength between adjacent channels, resulting in interference.  In practice, this has been found to imply that the closest spacing for co-area coverage is 400 kHz.

The scenario considered below attempted to populate as much of the spectrum as possible with London-wide coverage stations at a 400 kHz spacing, regardless of the consequences for other services and other areas.  The assumption made in this study is that existing allocations of greater than 20 kW are immutable.  All such stations are in the national network sub-bands at 88.0–94.6 MHz (BBC Radios 2–4) and 97.7–101.9 MHz (BBC Radio 1 and Classic FM).  These bands were therefore excluded from consideration, except as providing potential frequencies for allocations that were required to be modified as a result of the scenario.

For ease of description the ‘scenario’ is considered in two phases – it should be borne in mind that the two phases are not wholly separate, as some of the allocations displaced in one phase are accommodated in the other.

As has been noted, the requirements for this study have not included the consideration of interference to, or from, continental (or more distant UK) services.  In cases where such potential problems have been identified, they are noted inparentheses in the tables below.

Where a percentage value for ‘loss of coverage’ is quoted, this refers to population coverage, based on 1991 census figures.

G.3.3 Phase 1 (94.9 to 97.3 MHz): General

The first phase addresses frequencies between 94.7 and 97.6 MHz.  As there are existing allocations at 94.9 (Crystal Palace) and 97.3 MHz (Croydon), the new services were fitted to a 400 kHz raster that included these frequencies, giving allocations at 95.3, 95.7, 96.1, 96.5 and 96.9 MHz.  It was assumed that all these services would be radiated from the Croydon site, though Crystal Palace could equivalently be assumed.

As seen above, the existing services at South Benfleet (Southend) and Guildford give rise to high levels of potential interference in the Greater London area.

The ‘proposed’ new allocation at 95.3 MHz is co-channel with an existing service at South Benfleet, while that at 96.5 MHz is adjacent to an allocation at Guildford.  In each case very severe mutual interference would occur.  In accordance with the ‘rules’ of this exercise, the services at South Benfleet and Guildford must be re-located.  It can be seen from Figure G.3 and Figure G.19 above that the field strengths from both these sites exceed 53 dBV/m over a large area of Greater London.  The maximum separation from a London station that can be achieved under the ‘400 kHz raster’ plan is 200 kHz, at which the required protection ratio is 7 dB (Stereo, continuous interference).  It is therefore clear that it will not be possible to accommodate these services in the sub-bands under consideration, and that if they are to be accommodated at all it will have to be in the National Network bands.  The other option would be for the aerial radiation pattern at these sites to be modified to reduce interference in London; given the relative geography of the sites and target service areas concerned, this may not be straightforward.

It will be seen in the second part of the scenario that frequencies may be available on which these services might be accommodated.  For the present, it may be assumed that the services have been deleted.

A lesser problem is caused by existing assignments such as that at Rowridge (96.1).  While this transmission would cause unacceptable interference to the ‘proposed’ co-channel London assignment, the field strengths are sufficiently low in London to permit the two assignments to co-exist at a 200 kHz separation.  In the scenario described below, the Rowridge assignment is therefore re-located to 95.9 MHz.

G.3.4 Phase 1 (94.9 to 97.3 MHz): Detailed examination

Not only do these new allocations necessitate the re-planning of many services on or near the relevant frequencies, but 10.7 MHz relationships are created with a few existing small-scale services in the upper part of the band.

It should be noted that, for this scheme, London-wide stations have been ‘forced’ onto these frequencies regardless of practicality – in many cases, there are serious knock-on effects over a large part of the network.

The new allocations suggested are at 95.3, 95.7, 96.1, 96.5 and 96.9 MHz, and it has been assumed that all are radiated from Croydon.

In particular, it has been assumed that some existing services radiated from the sites at South Benfleet, Guildford, Windsor and Wrotham cannot be accommodated within 200 kHz of a London-wide service, and these allocations will need to be closed or accommodated elsewhere in the band.

Where protected field strength (pfs) values are quoted in the text below, they refer to the pfs on the proposed ‘new’ allocation in Central London (predicted at NGR TQ300800), and are based on the parameters in the Radio Authority database (the ‘statlist’).  These parameters relate to the allocated characteristics, rather than those actually in use, and may therefore overestimate levels of interference.

G.3.4.1 Croydon 95.3 MHz

Table G.5

Population coverage from Croydon: 7,083,969

Co-channel: This allocation is co-channel with BBC Essex from South Benfleet, which gives rise to a pfs of some 98 dBV/m in Central London.  This service is, therefore, re-located (to 104.0, see phase II, below).  Though the composite loss of coverage, with Great Braxted, is relatively small, it is significant. It may be possible to achieve better-targeted coverage from a site in Kent, with a directional aerial.

The BBC local service in Brighton might suffer some additional interference limitation and it is assumed that this will be re-located 100 kHz lower on 95.2 MHz.

Lower adjacent (95.2 MHz): Oxford (BBC Thames Valley) gives a pfs in Central London of ~70 dBV/m.  The allocation at Oxford is therefore changed to 95.1 MHz where it would be co-channel with Stoke Holy Cross (Norwich), requiring the latter to be moved to 95.2 MHz.  The population coverage of Stoke Holy Cross is reduced by 10% by the ACI from Croydon, though it should be noted that the same relationship exists between the two sites at 106.1/106.2 MHz [8].  The coverage of Oxford is reduced by only 5%, but has an impact on Horsham, co-channel (see below).  The interference field strength from Oxford in London will still be somewhat high, and some modification to the aerial pattern may be required.

Upper adjacent (95.4 MHz): Windsor (BBC Thames Valley) gives a pfs of ~84 dBV/m and has been tentatively re-located to 103.7 MHz.  The ACI on this channel, however, reduces the population coverage by 74%, and, in practice, a better allocation would need to be sought.

Lower 2^nd adjacent (95.1 MHz): An unused GE84 assignment exists at Canterbury.  Horsham gives rise to a pfs of ~20 dBV/m and the newly-moved Oxford allocation to a pfs of ~44 dBV/m.  Co-channel interference from Oxford reduces the Horsham population coverage by some 44%.

Upper 2^nd adjacent (95.5 MHz): Sandy Heath gives a pfs in Central London of ~42 dBV/m. The Wye (Ashford) allocation at 96.1 MHz is moved to this frequency later in the scenario, and gives a pfs of ~12 dBV/m.

G.3.4.2 Croydon 95.7 MHz

Table G.6

Population coverage from Croydon: 7,377,184

This assignment represents a shift in frequency of the existing Croydon allocation at 95.8 MHz (population coverage: 7,375,301).

Co-Channel: Peterborough gives a pfs~70 dBV/m and is therefore moved to 95.8 MHz, maintaining the relative relationship with Croydon.  An unused GE84 plan assignment exists at Midhurst.

Lower adjacent (95.6 MHz): Sutton Coldfield gives a pfs of ~50 dBV/m.

Upper adjacent (95.8 MHz): Newly-moved Peterborough gives a pfs of ~59 dBV/m, but this is also the case under the existing plan.

Lower 2^nd adjacent (95.5 MHz): Sandy Heath give a pfs of ~42 dBV/m.  (The Wye (Ashford) allocation at 96.1 MHz is moved to this frequency later in the scenario, but gives a pfs of only ~12 dBV/m).

Upper 2^nd adjacent (95.9 MHz): Thanet gives a pfs of ~30 dBV/m. Rowridge is moved to this channel from 96.1 MHz (see below).  There is an unused GE84 assignment at Banbury.

G.3.4.3 Croydon 96.1 MHz

Table G.7

Population coverage from Croydon:7,081,528

Co-channel: The 10 kW assignment at Rowridge gives a pfs of ~74 dBV/m.  This assignment is therefore moved –200 kHz to 95.9 MHz.  The coverage of Rowridge remains unchanged by this shift.  It should be noted, however, that continental interference considerations are likely to make this change unrealistic.

The assignment at Colchester (Wivenhoe Park, 0.5 kW) gives a pfs of ~71 dBV/m and is moved +100 kHz to 96.2 MHz giving a pfs ~59 dBV/m.  The already interference-limited coverage of Colchester is reduced by 16% (see Figure G.5 below).

Figure G.5   Coverage of Colchester (96.1 and 96.2 MHz)

Wye (Ashford) generates limited interference in London due to the directionality of the transmitter aerial, but will suffer substantial incoming interference.  The allocation is therefore moved to 95.5 MHz (co-channel with Sandy Heath).

Lower adjacent (96.0 MHz): Madingley (Cambridge) gives pfs ~60 dBV/m. No frequency change has been proposed for this assignment, but the population coverage is reduced by 21% due to the ACI from Croydon.  The coverage losses are illustrated in Figure G.6 below.

Figure G.6   Coverage of Cambridge (96.0 MHz)

Upper adjacent (96.2 MHz): While the assignments at Aylesbury and Tunbridge Wells do not give rise to excessive pfs levels in London (~61 and ~56 dBV/m respectively) it is necessary to move both these allocations +100 kHz to avoid incoming interference.  The newly-moved allocation at Colchester gives a pfs of ~59 dBV/m.

Lower second adjacent (95.9 MHz): Thanet gives a pfs of ~30 dBV/m.  There is an unused GE84 assignment at Banbury.  Rowridge is moved to this channel from 96.1 MHz (see above).

Upper second adjacent (96.3 MHz): The allocation at South Benfleet gives rise to very high pfs in the Eastern part of London, and (as noted above) must be re-located at a separation of >200 kHz from a London service.  The newly-moved allocations at Aylesbury and Tunbridge Wells give rise to pfs values below 54 dBV/m.

G.3.4.4 Croydon 96.5 MHz

Table G.8

Population coverage from Croydon: 7,074,279

Co-channel: Marlborough gives a pfs of ~71 dBV/m and is therefore moved +100 kHz to 96.6 MHz (this would be co-channel with St.Albans, but this allocation is itself moved +100 kHz, see below).

Lower adjacent (96.4 MHz): The Guildford assignment (3 kW) gives rise to a pfs greater than 80 dBV/m, and must be re-located at a separation of >200 kHz from a London service.  A frequency of 103.9 MHz is tentatively suggested in phase II below: the dramatic loss of coverage would appear, however, to make this suggestion impractical, though a service is maintained in Guildford itself.

The directional aerial at Sutton Coldfield (10 kW) limits the pfs in London to  ~38 dBV/m. Interference to Birmingham from London may be a greater problem, though the same relationship applies between Croydon (100.6 MHz) and the regional service from Sutton Coldfield (100.7 MHz).

Great Barton (Bury St. Edmunds) gives a pfs in London of ~56 dBV/m.

Upper adjacent (96.6 MHz): St. Albans generates substantial levels of interference in London, and as a minimum will need to move +100 kHz to 96.7 MHz, co-channel with Gt.Massingham (Kings Lynn, 3 kW).  The service area of St. Albans is currently limited by CCI from Northampton, and this change actually increases the coverage slightly (by 13%).

Lower second adjacent (96.3 MHz): As noted above, the South Benfleet allocation must be re-located at >200 kHz from a London assignment.

Upper second adjacent (96.7 MHz): The local (Medway area) service from Wrotham gives a pfs of ~69 dBV/m in Central London, and will suffer very severe interference in its own service area.  This allocation must therefore be re-located at >200 kHz from a London assignment – 103.8 MHz is tentatively suggested, though this results in a severe loss (54%) of coverage (particularly in Gillingham and Sittingbourne).  It should be also be noted that continental interference considerations are likely to make this change unrealistic.

Crabwood Farm (Winchester) and Gt.Massingham (Kings Lynn) give a pfs of around 30 and 25 dBV/m respectively.

G.3.4.5 Croydon 96.9 MHz

Table G.9

Population coverage from Croydon: 6,950,898

A low-power service from Croydon is already using this channel, at ~50 W, with a service area that is (presumably) limited by co-channel interference from Sandy Heath, and possibly second-adjacent channel interference from Wrotham (96.7 MHz).  The allocation will be increased in power by some 13–16 dB (1to 2 kW) and the currently directional aerial made omnidirectional.

Co-channel: The allocation at Sandy Heath is assumed to be moved +100 kHz (to 97.0 MHz) to minimise mutual interference, though this leads to a substantial reduction in coverage in the main target area.  The pfs in London will remain rather high at ~64 dBV/m, and some power reduction or modification to the aerial pattern may be required at either site.

Lower adjacent (96.8 MHz): Wenvoe gives a pfs of ~57 dBV/m.

Upper adjacent (97.0 MHz): Reading gives a pfs of some 74 dBV/m. This allocation should therefore be moved +100 kHz.  As this would result in interference to Haslemere, this allocation is moved to 97.0 MHz, adjacent to both Croydon and Reading.  The coverage of Reading is almost unchanged, while that of Haslemere suffers some peripheral losses.

Lower second adjacent (96.7 MHz): It has been noted above that the Wrotham (Medway) allocation must be re-located at >200 kHz from a London service.  Crabwood Farm (Winchester) and Gt.Massingham (Kings Lynn) give a pfs of around 30 and 25 dBV/m respectively.

Upper second adjacent (97.1 MHz): Haslemere and Foxhall Heath (Ipswich) both give a pfs of ~34 dBV/m.  Newly-moved Reading gives a pfs of ~48 dBV/m.

G.3.4.6 97.3 MHz

Population coverage from Croydon: 7,398,324

This is an existing London-area coverage allocation at Croydon and will remain unchanged.

Lower second adjacent (97.1 MHz): Foxhall Heath (Ipswich) gives a pfs of ~34 dBV/m.  Newly-moved Reading gives a pfs of ~48 dBV/m.

G.3.4.7 Potential interference problems

10.7 MHz relationships

Because of the changes above, a number of 10.7 MHz relationships are created with existing services in the upper part of the band.  Any problems will generally be with the lower (i.e. new) service.

95.3 MHz: no problems

95.7 MHz: no problems

96.1 MHz: 10.7 with Shooters Hill

96.5 MHz: 10.8 with Bredgar

96.9 MHz: 10.6 with Lambourne House

The assumption is made that these small-scale services will be moved, or closed, to avoid these relationships.

G.3.5 Phase 2 (102.2 to 108.0 MHz): General

The second part of the scenario addresses frequencies between the existing Croydon allocation at 102.2 MHz and the upper edge of the band.  The lower part of this band is used for both BBC and commercial local services, while the portion above 105 MHz was only released in the mid-1990s and is used only for commercial services.  The effect of the coherent planning made possible by the responsibility of a single authority for the planning, from scratch, of a block of spectrum can be seen clearly in the pfs plot of Figure G.2: it has been possible to plan a ‘block’ of uniform, London-wide allocations at the minimum 400 kHz spacing between 105.4 and 106.2 MHz.

This ‘block’ of allocations falls on the same 400 kHz raster as the 102.2 MHz allocation, and this has therefore been used as the starting point of the second part of the scenario.  The attempt has been made, as before, to accommodate the maximum number of allocations without regard to the implications for other services or areas.

The possibilities in the upper part of the band are constrained by the need to avoid 10.7 MHz relationships with the existing, and newly-planned, transmitters in the lower part of the band.  These restrictions are illustrated below:

Table G.10   Allocations above 102.2 MHz

It can be seen that the newly-created 10.7 MHz relationships fall between the existing (and most of the proposed) high-power allocations.  If the assumptions regarding the use of allocations with such relationships are not relaxed, however, it seems that 103.8 and 104.2 MHz will not be usable for all-London services.

The re-arrangement of surrounding allocations is described in detail below, but a significant point is that all displaced allocations can be accommodated within the band.  This is partly owing to the fact that there is no allocation at South Benfleet in this part of the band, and only one to Guildford.  The site at Reigate, on the North Downs would create very serious interference were it not for the very directional aerial used at this site.  There is also more scope for accommodating ‘displaced’ allocations owing to the necessary avoidance of two frequencies within the 400 kHz raster.

These ‘unusable’ frequencies potentially allow the Wrotham (Kent) allocation to be accommodated on 103.8 MHz, Guildford (96.4) on 103.9 MHz, South Benfleet (95.3) on 104.0 MHz and Windsor (95.4) on 103.7 MHz.

G.3.6 Phase 2 (102.2 to 108.0 MHz): Detailed examination

G.3.6.1 Croydon 102.6 MHz

Table G.11

Population coverage from Croydon: 7,035,058

Co-channel: The allocation at Bakers Wood (Chelmsford) gives a pfs of ~96 dBV/m, and is therefore moved –200 kHz to 102.4 MHz, co-channel with Heathfield, as is the existing Oxford allocation.

These changes cause a 28% loss of population coverage from Bakers Wood, but a 27% gain at Oxford, as illustrated in Figure G.7 and Figure G.8 below:

Figure G.7 Change in coverage at Bakers Wood (102.6/102.4 MHz)

Figure G.8   Change in coverage at Oxford (102.6/102.4 MHz)

Lower adjacent (102.5 MHz): No significant interferers occupy this channel: Stoke Holy Cross is moved to this channel from 102.4 MHz (see below).

Upper adjacent (102.7 MHz): There is an allocation at Reigate on this frequency; this elevated site on the North Downs produces a fairly limited field strength (~26 dBV/m) in Central London due to the use of a directional transmitting aerial.  While this might allow the use of a 200 kHz spacing with respect to a London service, the reciprocal interference path will cause a severe reduction in the Reigate service area.  The allocation is therefore moved to 103.6 MHz, co-channel with Blunsdon (Swindon) and Great Braxted (Mid-Essex).  The population coverage is consequently reduced by some 26%, but this loss is mostly outside the target service area (see Figure G.9 below).

Figure G.9   Loss of coverage from Reigate (102.7/103.6 MHz)

Lower second adjacent (102.4 MHz): This channel is occupied by Heathfield, giving a pfs of ~42 dBV/m and by Stoke Holy Cross, which is moved +100 kHz to avoid CCI with Bakers Wood.  The newly-moved allocations at Bakers Wood (Chelmsford) and Oxford will give rather high pfs levels in London, and these sites may require some alterations to their antenna patterns.

Upper second adjacent (102.8 MHz): The current allocation at Dunkirk (Canterbury) gives rise to a pfs in the order of 50 dBV/m, and may be left unchanged.

G.3.6.2       Croydon 103.0 MHz

Table G.12

Population coverage from Croydon:7,000,067

Co-channel: The allocation is co-channel with Cambridge, which is moved +100 kHz to 103.1 MHz.

Lower adjacent (102.9 MHz): The Hannington site gives a pfs of some 83 dBV/m, and is therefore moved –100 kHz to 102.8 MHz, co-channel with Dunkirk.

Upper adjacent (103.1 MHz): Bluebell Hill gives a pfs of ~76 dBV/m, and is re-located +100 kHz to 103.2 MHz, co-channel with Chillerton Down (Southampton).

Lower second adjacent (102.8 MHz): The current allocation at Dunkirk (Canterbury) gives rise to pfs in the order of 50 dBV/m, and may be left unchanged.  Hannington is newly moved to this channel.

Upper second adjacent (103.2 MHz): Chillerton Down (Southampton) may be left unchanged, and is now co-channel with Bluebell Hill.

G.3.6.3 Croydon 103.4 MHz

Table G.13

Population coverage from Croydon: 6,967,753

Co-channel: The allocation is co-channel with the low-power ‘filler’ at Henley.  Though possibly sufficiently shielded, this site has been re-allocated to 103.3 MHz, co-channel with Bow Brickhill.  Though the loss of population coverage is dramatic, at 73%, it can be seen below that most of the loss is in the suburbs of Reading, which remains served by a separate transmitter carrying the same service.  The ‘filler’ at Henley will, therefore, probably remain viable on the new frequency.

Figure G.10   Change in coverage at Henley (103.4/103.3 MHz)

Lower adjacent (103.3 MHz): This frequency is currently used by a low-power allocation at Alexandra Palace.  This would have to be closed, possibly making use of the new assignment.

Upper adjacent (103.5 MHz): Currently assigned to Great Braxted (Chelmsford) giving a pfs of ~80 dBV/m. This assignment is moved to be co-channel with Blunsdon (Swindon) on 103.6 MHz, which increases the coverage of the site.  Newton Barrow (Salisbury) remains on the channel.

Lower second adjacent (103.2 MHz): Chillerton Down (Southampton) is now co-channel with Bluebell Hill, the latter giving a pfs of ~50 dBV/m.

Upper second adjacent (103.6 MHz): Currently assigned to Blunsdon(Swindon) giving a pfs of ~30 dBV/m. Great Braxted is also moved to this channel from 103.5 MHz, as is Reigate from 102.7 MHz (see above).

G.3.6.4 103.8 MHz (not used)

Co-channel: The potential allocation has a 10.6 MHz relationship with Crystal Palace (93.2 MHz) and it will therefore be assumed to be unusable for a London-wide service.  It had, however, been hoped that it would be a potential ‘home’ for the local (Medway) service from Wrotham, displaced earlier in the scenario from 96.7 MHz.  It can be seen from Figure G.11 below, however, that the coverage from Wrotham suffers what is probably a fatal loss of coverage (54%) because of the frequency change.  In more detailed planning, it would be necessary to change the frequency of Zouches Farm (Luton), which is currently allocated this channel.

Figure G.11   Loss of coverage from Wrotham (96.7/103.8 MHz)

Lower adjacent (103.7 MHz): Lark Stoke (Stratford) gives low co-channel pfs values.  It is therefore proposed that this channel be used to accommodate the displaced Windsor (95.4 MHz) allocation.

Upper adjacent (103.9 MHz): This frequency is +10.7 MHz with respect to Crystal Palace, and it will therefore be assumed to be unusable for a London-wide service.  The frequency is currently used at Manningtree (Ipswich).  It is assumed that this frequency is used by the Guildford service displaced from 96.4 MHz.  Though this channel has a 10.7 MHz relationship with Crystal Palace (BBC Radio 4), any listeners affected will be able to retune to the Guildford Radio 4 service.

Lower second adjacent (103.6 MHz): Great Braxted (Chelmsford) has been moved to this channel.

Upper second adjacent (104.0 MHz): Reigate gives rise to fairly low values of pfs to the north.  As this ‘set’ of frequencies may not be used within London, it had been hoped to use this channel to accommodate the displaced South Benfleet (95.3 MHz) allocation.  Predictions indicate, however, that the coverage of South Benfleet would be reduced by 64% as a result.  Nevertheless, for the purposes of this exercise, the station has been tentatively assumed to occupy this channel.

South Benfleet will then be adjacent-channel to Manningtree on 103.9 MHz. Bow Brickhill is also moved to this channel from 104.5 MHz (see below).

G.3.6.5 104.2 MHz (not used)

Co-channel: The allocation has a 10.7 MHz relationship with Wrotham (93.5) and is therefore (conventionally) unusable, not only within London, but throughout the Wrotham service area.  Swingate (Dover) is currently allocated this frequency (which presumably gives rise to some potential problems in the Wrotham/Dover overlap area).

Lower adjacent (104.1 MHz): Hannington: No change.

Upper adjacent (104.3 MHz): Naish Hill (Wiltshire) gives low pfs values.  This channel is used to accommodate the displaced Guildford (104.6 MHz) allocation, which will be adjacent to Reading (104.4).  Though a directional aerial is used at Guildford, both service areas will be interference-limited.  The impact on the Reading (Fountain House) service area can be seen in Figure G.12 below.  The population coverage is reduced by interference from Guildford by nearly 70%.

This Reading allocation is used in conjunction with a 4 kW allocation at Hannington on 104.1 MHz; the latter, however, provides poor coverage in the Reading area.  The closure of the Reading service would therefore be problematic.

The impact of the additional interference on the coverage of Guildford is less dramatic, with a loss of only some 31%, in peripheral areas.  It seems, however, that it will be necessary to re-work this part of the scenario, or to assume that these services are closed.

Figure G.12   Reduction in Reading (104.4 MHz) service area due to Guildford

Lower second adjacent (104.0 MHz): Currently used at Reigate.  Add newly-moved South Benfleet (from 95.3 MHz) and Bow Brickhill (104.5 MHz)

Upper second adjacent (104.4 MHz): Reading gives pfs values of ~49 dBV/m in Central London.

G.3.6.6 Croydon 104.6 MHz

Table G.14

Population coverage from Croydon: 7,067,530

Co-channel: The frequency is currently allocated at Guildford, which must be re-located to 104.3 MHz (see above).

Lower adjacent (104.5 MHz): Bow Brickhill (Milton Keynes) and Heathfield (East Sussex) currently use this frequency, giving rise to a pfs on 104.6 MHz of ~65 dBV/m.  While this might prove tolerable, it is proposed that Bow Brickhill be re-located to 104.0 MHz (see above) and Heathfield to 104.8 MHz (see below).

Upper adjacent (104.7 MHz): No significant interferers.

Lower second adjacent (104.4 MHz): Reading gives pfs values of ~49 dBV/m in Central London.

Upper second adjacent (104.8 MHz): Currently allocated at Burton Down (West Sussex).  Heathfield has been moved to this channel from 104.5 MHz (see above) and Burton Down is consequently moved +100 kHz to reduce CCI.  While the population coverage of Burton Down is reduced by 63%, this loss is sustained almost entirely in Portsmouth, which is outside the target service area.

G.3.6.7 Crystal Palace 105.0 MHz

Table G.15

Population coverage from Crystal Palace: 6,792,654

Co-channel: The allocation is currently unused in South-East England.

Lower adjacent (104.9 MHz): Currently allocated at Crystal Palace for a London‑wide local service.  Under the new scenario, this service would, presumably, transfer to the new assignment at 105.0.

Upper adjacent (105.1 MHz): No significant interferers.

Lower second adjacent (104.8 MHz): Currently allocated at Burton Down (West Sussex).  As noted above, it is suggested that this allocation be moved to 104.9 MHz to maintain the current adjacent-channel relationship with London, and to keep the 200 kHz offset channel clear to accommodate other possible changes.

Upper second adjacent (105.2 MHz): Currently allocated to Chillerton Down (Isle of Wight).

G.3.6.8 105.4 MHz

This frequency is currently allocated at Croydon, for a London-wide service, and no changes are proposed.

G.3.6.9 105.8 MHz

This frequency is currently allocated at Croydon, for a London-wide service, and no changes are proposed.

G.3.6.10 106.2 MHz

This frequency is currently allocated at Croydon, for a London-wide service, and no changes are proposed.

G.3.6.11 Croydon 106.6 MHz

Table G.16

Population coverage from Croydon: 6,650,028

Co-channel: The allocation is currently allocated at Slough, which gives rise to very high pfs values within London.  It is proposed that the allocation be moved to 106.8 MHz, co-channel with Peterborough and Dover, and adjacent to a new service from Hertford. The relationship with the Hertford allocations is unlikely to be tenable.

Lower adjacent (106.5 MHz): No significant interferers.

Upper adjacent (106.7 MHz): No significant interferers(it has been noted that, since the start of this study, a new allocation has been made at Stevenage).

Lower second adjacent (106.4 MHz): Allocated at Mendlesham and Haywards Heath, giving a pfs of less than 40 dBV/m.

Upper second adjacent (106.8 MHz): Newly-moved Slough gives pfs of ~51 dBV/m, and is co-channel with Peterborough and Dover.  The existing small-area allocation at Shooters Hill will be closed (since the start of this study, an allocation has been made at Reading on 106.9 MHz).

G.3.6.12 107.0 MHz, 107.4 MHz and 107.8 MHz

It has been assumed that these frequencies should not be used for the provision of wide-area coverage services, to minimise the possibility of aeronautical interference.

G.3.7 Analysis and conclusions

The scenario described above has assumed that a further nine London-wide local radio services are provided, in addition to the current nine.  It must be emphasised that this scenario is intended only as a means by which any ‘slack’ in the existing use of the spectrum can be explored: it is emphatically not to be viewed as a ‘plan’.

Furthermore, the scenario has not considered significant planning restrictions such as continental interference and aeronautical protection (though some notes on the latter are included in Annex E).  The population gains are summarised below: it should be noted that the figures quoted in this report are based on 1991 data for the ‘usually-resident population’, and are counted over a 60 x 60 km area that includes the M25.  For comparison, the predicted population coverage (60 dBV/m, protected) of the existing service from Croydon on 95.8 MHz is 7,375,301.

(1) Formerly 95.8 MHz  (2) Formerly 104.9 MHz
Table G.17   Results of Scenario A

The hypothetical assumption of the introduction of nine new stations to the London area naturally has ramifications for the outlying areas.  The overall impact, in terms of population coverage, is summarised in the following table.

Table G.18

* The figures for Peterborough refer to the southern part of the service area only.

In the short timescale available it has not been possible to follow through all the consequences of the changes described above.  However, across the whole of South-East England, a total of at least 3.8 million listeners lose part of their current station choice if an additional nine London-wide services are accommodated.  In practice, if all the consequences were explored, and continental interference taken into account the impact would be greater.

The impact upon the stations themselves varies quite significantly.  Some stations, such as Cambridge on 96.0 MHz and St Albans on 96.6 MHz experiencing only minor losses, or even gains, in the population that they cover.  Others, however, such as Guildford (which shifts from 96.4 MHz to 103.9 MHz) experience a catastrophic loss of coverage, putting their viability into question.  However, it must once again be stressed that this scenario has only been addressed at the highest level and adjustments to the proposal may well prove able to accommodate the worst affected stations at an alternative frequency, mitigating the impact of the change.

Nevertheless, it should also be noted that 28 of the 37 stations affected will experience a change in frequency, with the consequent marketing costs and the potential loss of listenership that such a move is likely to entail.

It would appear that a relaxation in the restrictions applying to the use of frequencies with a 10.7 MHz relationship would give limited gains.  Such ‘taboo’ frequencies are useful in accommodating out-of-area allocations.

It should be borne in mind that the short timescale of this study has allowed the investigation of only a few of the possible re-arrangements of spectrum use.

• Of the eleven frequencies examined, two (95.7 MHz and 105.0 MHz) can be considered as existing allocations, shifted by 100 kHz to fit the raster.
• Of the remaining nine frequencies examined, the majority could only be used to provide additional London stations at the cost of considerable disruption to existing services in the surrounding area.
• The use of at least three of the frequencies would require the closure of existing services at, for example, Southend, and would lead to a dramatic reduction of the service areas of some other stations (e.g. Bedfordshire, Guildford).
• The use of some of the channels, however, appears to have a less drastic penalty in terms of the coverage of existing services.  For instance, the best case under this scenario indicates that it may be possible to accommodate a London-wide service with roughly 25% loss of population coverage to two existing stations (Chelmsford and Reigate).

It should be noted, however, that this study was not required to take account of continental interference – if such interference is considered, it may be found that there is less scope available for the introduction of further services.  Nevertheless, it appears that there is sufficient likelihood that new services might be accommodated as to warrant further detailed investigation.

The scope for use of the BBC National Network sub-bands was limited by the large number of high-power transmitters in this part of the spectrum.  One frequency was identified, which might be used for a further London-wide service at the expense of some 100,000 potential BBC Radio 4 listeners in Surrey.

G.4 Scenario B: Allocations for medium-scale services

This scenario addresses services with a radius of around 7–10 km.  In the London case, this corresponds to service areas of roughly a quarter of the Greater London area.  For the purposes of this hypothetical study, services were assumed to cover the north-east, the north-west, south-east and south-west quadrants of the M25 area.

Given the short timescale of the project, no great effort was made to optimise the choice of sites: for the South London stations, Crystal Palace (or equivalently, Croydon) was assumed as the transmitter site, with directional aerials as appropriate.  For North London, sites at Hampstead Heath and Walthamstow were assumed, as being roughly in the centre of the intended service areas.

Table G.19   Assumed medium-scale services

The services at Crystal Palace are assumed to radiate a greater erp than the omnidirectional sites, as they are located further from the edge of their service area.

G.4.1 Choice of frequency

The selection of frequencies differs little from that for the London-wide coverage sites, as any of the frequencies chosen above for the London-wide services will be equally usable for the medium-scale stations, though the cost/benefit implications would be less attractive.

One important difference is that the option exists to limit outgoing interference by appropriate choice of transmitter site and antenna directionality.  There are also a few cases where the pfs due to a local interferer differs greatly from one part of London to another.

For example, the incoming interference from the local radio allocation at Rowridge (see Figure G.13below) is sufficiently low in the south-east quadrant of London to allow its use for a local service with a pfs of ~70 dBV/m.

Figure G.13   Field strength in London from Rowridge (96.1 MHz)

Unfortunately, in this case, the frequency is only separated by 300 kHz from an existing allocation 95.8 MHz at Croydon.  In practice, therefore, the channel would be unusable within the Croydon service area.

In the limited time available in the current study, a somewhat arbitrary choice of frequencies was made, three of which fall within the current National Network bands.

Table G.20   Allocations assumed in study

The results achieved using these allocations are illustrated in the plots of Figure G.14 to Figure G.17 below: it can be seen that none of the services is particularly satisfactory.

The site chosen for the north-west station is not ideal, in that the coverage extends over a much greater area than is sought.  This service is even more strongly interference-limited, by co-channel interference from Tacolneston which gives pfs levels in the target service area of around 78 dBV/m, which field strength is exceeded only over a very small area.

The north-east station achieves a reasonable coverage in most of the target area, but is limited by adjacent-channel interference around the Thames estuary.

The south-east service from Crystal Palace provides a reasonable, though interference-limited service in the target area.  The south-west service from the same site is limited by both terrain and interference, and achieves more successful coverage to the north and east!

Figure G.14   Coverage of London NW (89.7 MHz)

Figure G.15   Coverage of London NE (103.7 MHz)

Figure G.16   Coverage of London SE (94.3 MHz)

Figure G.17   Coverage of London SW (90.0 MHz)

G.4.2 Analysis

At first glance, Scenario B for the London areas offers a potential gain of four, new, 9 km radius stations, centred on Hampstead, Walthamstow and Crystal Palace.  In total, these allocations provide new services to over 4.5 million potential listeners.

In practice, the frequencies chosen offer poor coverage in their intended areas.  It has not (in the time available) proved possible to identify any frequencies suitable for services of this type that would not also be suitable for wider-area services.

G.4.3 Conclusions

In the limited time available, it has not been found possible to identify frequencies that would be suitable for services of this scale that would not also be suitable for London-wide coverage.  It appears that, from a planning perspective, the two scales of service will require similar treatment.  One important difference is likely to be the ability to limit outgoing interference by appropriate choice of transmitter location with respect to the target service area.

G.5 Scenario C: Allocations for small-scale services

This scenario attempts to accommodate a number of ‘community’ type services, with service area radii of some 3–5 km, limited primarily by interference.  The intention is to accommodate these new services with a minimum of disruption to the existing services.

G.5.1 Assumed characteristics of services

It is assumed that transmitters will have an erp in the order of 50 W, will be located on an existing building near the centre of the service area, and will employ an omnidirectional transmit aerial.

A largely arbitrary choice of service areas has been made, though aiming to represent a variety of areas from dense urban to suburban.

The areas for which allocations have been sought are listed in Table G.21 below.

Table G.21   Assumed small-scale services

G.5.2 Potentially available assignments

The assumption has been made that the new services will be interference-limited; with the low power that it is intended to use, there will be minimal outgoing interference beyond the London area.

One option, therefore, would seem to be the use of channels on which relatively high pfs levels exist due to interference from distant, high-power stations, such as Oxford (seeFigure G.18below).

Figure G.18   Field strength in London from Oxford (95.2 MHz)

To provide a London-wide (~25 km radius) service on such a channel would require the use of a high-power transmitter, giving rise to substantial outgoing interference.  The proposed local services, however, would give rise to minimal outgoing interference, though the service area boundary will be sharply defined (and time-varying), and may well be very irregular due to local clutter.

These limitations may not be a significant problem for a new service, as the listeners will be conditioned to tolerate such performance from start-up.

The other approach that might be used in seeking suitable frequencies to provide these services is to make use of fairly local assignments from which the interfering power is excessive only in parts of Greater London.  An example might be that of Guildford, the interfering power of which, in the London area, is illustrated below.

Figure G.19   Field strength in London from Guildford (96.4 MHz)

A ‘trawl’ through the spectrum (assuming the current pattern of spectrum use) produced the following results.

G.5.3 National network sub-band

Field strengths from Guildford are sufficient to preclude the use of frequencies offset by less than 200 kHz anywhere in Greater London.  A 200 kHz offset might be usable in East London, giving a pfs of ~60–70dBV/m.  The 300 kHz offset would be usable throughout London, but is currently allocated to the low-power ‘filler’ site at Kenley.

Rowridge lies at –300 kHz with respect to Crystal Palace, and it might be possible to use the channel in areas where the directionality of the Crystal Palace aerial offers sufficient protection.  However, fields from Rowridge give a co-channel pfs in East London of up to 85 dBV/m. It is not likely to be possible to use an offset closer to Crystal Palace.

Frequencies from Crystal Palace–200 kHz to Wrotham+300 kHz will be unusable.

Oxford gives rise to a pfs in North-East London of ~79 dBV/m at 100 kHz offset, rising to some 90 dBV/m in West London.

At an offset of 100 kHz from Tacolneston, the required pfs is less than 66 dBV/m in West London.  It might also be noted that the Tacolneston assignments in the ‘Radio 2’ and ‘Radio 3’ sub-bands are also assigned in the Geneva Plan to a site in ‘North London’, which has never been used.

The frequencies co-channel with Wenvoe offer a relatively low pfs in Central London, except on 89.9 and 92.1 MHz where tentative local allocations for Harlow and Welwyn exist in the Geneva Plan.

Some 10.7 MHz relationships also exist that must be considered.  For instance, while 89.8 MHz, at 100 kHz offset from Tacolneston, might seem an appropriate choice for use in the Epsom and Clapham areas, these areas are in the primary service area of Crystal Palace, and a 10.7 MHz relationship exists with the 100.6 MHz assignment at this site.  The same restriction does not apply to the relative frequencies in the other sub-bands (i.e. 92.0 MHz, 94.2 MHz and 99.4 MHz).

G.5.4 Local sub-bands

A very large number of potential frequencies is available in these sub-bands.  The following frequencies were identified in a brief search:

• 103.7 MHz: This frequency lies 400 kHz above the existing low-power allocation at Alexandra Palace.  The highest interference is 37 dBV/m from Zouches Farm (Luton) on 103.8 MHz, giving pfs of ~70 dBV/m.  The frequency might be suitable for a service area in the low-lying land to the east of London.
• 96.2 MHz: This frequency is co-channel with the local assignment at Rowridge, which gives low median values of pfs.  The channel is 400 kHz above an existing allocation at Croydon.

G.5.5 Implementation of the scenario

For illustration, the following frequencies have been chosen from the wide range of options available.

Table G.22

The implementation of these small-scale stations should not involve any loss of coverage to existing services.  The coverage areas achieved are indicated in the map inFigure G.20 below.

Figure G.20   Coverage areas of small-scale stations

The table below indicates the population coverage achieved in each area, and the significant sources of interference by which the service areas are limited.

Table G.23   Local coverages achieved

The sensitivity of the service area to the choice of frequency is illustrated in Figure G.21 below, for the case of the Epsom transmitter.  At 89.5 MHz, the service is co-channel with Oxford, and the coverage is therefore minimal.  At 90.1 MHz, the co-channel interferer, Peterborough, gives rise to a much lower pfs and the service area expands to some 20 km².

Figure G.21   The impact of interference on service area

The question of the receiver aerial directivity assumed in planning is of particular importance in the case of services such as these, where the coverage is determined by incoming interference.  Services are traditionally planned on the assumption that some rejection of interference is obtained by the use of directional receive aerials; the implications of this assumption are illustrated in Figure G.22 below, again for the case of Epsom (90.1 MHz).

Figure G.22   The effect of aerial directivity on service area

It can be seen that, where the use of omnidirectional receiving aerials is assumed, the coverage obtained shrinks, particularly in the north of the service area, where the maximum antenna discrimination of interference from Peterborough had been available.

G.5.6 Analysis

This scenario has examined the impact of the introduction of six, new, low-power stations to the London area.  The choice of six stations brings additional choice to around 0.5 million listeners, with little, if any impact upon existing stations.

The costs associated with the scenario are, as might be expected, fairly low.  The six new stations would each require either new equipment for a 50 W transmitter, costing £5,000 to £10,000 or a Total Broadcast Contract, the cost of which will be dependent upon the level of service required and the supplier selected.  It might be anticipated that the typical small station will purchase its own equipment (possibly through lease-purchase or similar agreement to spread the cost) with maintenance and support being purchased through a basic annual contract with additional call-out charges for emergency events.  Such an arrangement might be expected to cost around £1000 to £2000 per annum.  Existing stations will not incur any equipment-related costs under this scenario.

The marketing costs will only be incurred by new entrants in launching the stations.  Whilst for larger stations a launch budget of around 10% of turnover has been assumed, for these very small stations a slightly larger budget would be required to undertake any realistic type of marketing activity.  However, given that these stations are likely to be non-profit-maximising, launch costs of a few thousand pounds per station, to introduce the station to the local community, would seem reasonable.

This scenario sees the addition of multiple, community-focussed, radio stations, with the potential to provide services targeted at specific community groups.  The scenario has no technical impact upon existing stations (i.e. no frequency shifts) and, given the low level of forecast weekly listener hours, seems likely to have a negligible impact upon the audiences for the London-wide stations.  If it is assumed that each station will address currently unserved communities, it is unlikely that the 7-10 km radius stations would suffer loss of market share to the new stations.

G.5.7 Conclusions

This scenario has examined the impact of the introduction of new, low-power stations to the London area.  An arbitrarily chosen number of stations has been examined, in this instance six, and the work indicates that these can be introduced with little, if any, impact upon existing stations.  It is interesting to note that it may prove possible to introduce still more low-power stations without impact; however, the time constraints of the study prevented the detailed analysis that will be required before an upper limit can be estimated.

The introduction of six stations will bring additional choice to 0.5 million listeners in pockets across London.  Again, it should be noted that the figure is related to this specific scenario and is intended only to be indicative of the general size of additional listenership that could be achieved with new, low-power stations.  Additional stations would tend to increase the choice and listenership achieved.

The expansion of restricted-coverage radio stations within the London area could be seen to offer a major new resource to the capital.  It offers the potential to give a voice to the rich and diverse range of communities and interest groups that exist within London.  Drawing upon the experience of the USA, it might be forecast that such stations would prove attractive to civic organisations, the churches, schools, expatriate communities, ethnic and linguistic groups, charitable organisations and others.  Foreseen uses might include entertainment, education, public safety and local information including news, traffic and weather forecasting.

This scenario highlights the potential for increasing choice to many London listeners, but there are two points important to note.  Firstly, irrespective of the number of low-power stations introduced, the service areas of these new stations will be determined by incoming interference rather than by terrain; as the incoming interference will vary, generally in an unpredictable fashion, so too will the quality of service achieved.  Secondly, it seems likely that the potential revenue generated by such stations will be nominal; with the low audience figures and varying service quality levels, it would be difficult to see such stations being financially viable as commercial operations.

In summary, it can be seen that there exists the potential to introduce a number of new, low-power stations in the London area, without impact upon existing stations.  The potential significance of this opportunity, in terms of the important social resource that it suggests for the capital, merits additional investigation.

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