Radiocommunications Agency  EMC Awareness

Preventing intermodulation

What this technique is used for

Intermodulation prevention techniques in installations are used to help prevent spurious noise frequencies from being caused by two or more frequencies being applied to a non-linear electrical bond.

How this technique is used

In electrical bonding, careful control of materials, contact pressure, and protection from corrosion is used to reduce the non-linearities in the bond, hence reducing the amount of intermodulation.

Key issues in employing this technique

Non-linearities and intermodulation

A non-linear bond has an impedance that varies according to the magnitude of the voltage or current it is exposed to. When signals at two different frequencies (f1 and f2) pass through a non-linearity they create signals at their sum and difference frequencies (f1 - f2 and f1 + f2). These are known as ‘intermodulation products’.

When three signals pass through a non-linearity they create signals at the sum and difference frequencies of each pair of frequencies, plus frequencies corresponding to a number of other sum and difference relationships between them – 6 intermodulation products in total.

The radio spectrum is very carefully controlled so that different services operate in well-defined frequency channels. Intermodulation creates frequencies that can be difficult to control.

Intermodulation products always have a lower amplitude than the signals that gave rise to them, and in many cases are only of concern to very sensitive radio receivers operating on weak signals. However, where a radio transmitting antenna is transmitting on more than one frequency at once, or when two or more radio transmitting antennas are in close proximity (so that they each carry high levels of the signals radiated by the others), the intermodulation products that can be created, say, in the connector at the base of the antenna can radiate at sufficiently high levels to cause the transmitter to fail its compliance tests. This can be a big problem in the radio transmitter industry, e.g. for cellphone base-stations.

Electrical joints and connections are non-linear

Intermodulation caused by two or more frequencies being present at the same time in a non-linearity in an electrical joint (connection) or similar situation is sometimes called ‘passive intermodulation”.

Metals are always covered by an oxide film, due to the metal reacting chemically with the oxygen in the air. Metal oxides are semiconductors, to some degree (e.g. copper oxide used to be used to make power rectifiers), so any oxide film between two mating electrical contacts creates a non-linearity. When amps of numerous different RF currents are flowing through this non-linearity, significant levels of intermodulation product can be generated.

As well as the oxide film, there are other metal salts created by acid rain (sulphurous, sulphuric, nitrous, nitric, carbonic, etc.), salt spray, and other environmental exposures to chemicals (such sweaty human fingerprints, cleaning chemicals, etc.) and some of them will be more or less non-linear as well.

As well as metal salts created by exposure of the outer surface to the environment, there are also a variety of non-linear metal variety of salts from the harsh chemicals used in plating processes inside a plated metal part. In addition, plating layers that are thinner than the skin depth at the frequencies concerned, and ferromagnetic metals such as nickel or steel, can cause passive intermodulation.

Careful design of the metal pieces used in antenna connectors is required to keep passive intermodulation down to acceptable levels. The best metal connectors are made of non-ferrous metals that do not need to be plated, and are compatible with each other and with their environment.

Besides intermodulation in non-linear electrical joints in connectors, intermodulation can occur in any metal joints, since all metal items can act as ‘unintentional antennas’. So metal railings, chains, floors, wall supports, etc., can all pick-up RF signals from their environment, intermodulate them at metal joints, and re-radiate the intermodulation products as interfering signals.

Normally the RF signals in the environment are not very powerful, so the intermodulation products are even less powerful and don’t cause any problems. But in environments where high levels of RF are present, the re-radiated intermodulation products could be powerful enough to interfere with other radio channels.

In such situations it is important to ensure that RF voltages cannot be generated across joints despite the RF currents flowing in them, or preventing the RF currents from flowing at all. This is done by either by correctly RF bonding the joint (e.g. by welding it), by protecting it from corrosion, or by insulating it.

Badly designed or poorly protected metal joints and connections can be pushed apart by the corrosion products, leading to extremely non-linear behaviour. This has been such a problem in the past that passive intermodulation is commonly referred to as ‘the rusty bolt effect’, commonly seen in the metal structures of RF transmitting towers, as well as in metal structures that are near to transmitters.