Development of a portable measuring system

Purpose of this document

This document is the draft final report (Deliverable 4) under a contract awarded to the University of Hertfordshire by the Radiocommunications Agency, Ref. AY3430 (510002080). The contract was for the development of a portable measuring system for LF and HF field strength measurements. This report incorporates the contents of Interim Report No. 1 which described the theoretical design of the antennas and Interim Report No. 2 which described the practical design.

The solution adopted is a set of measuring loop antennas with integral pre-amplifiers for use with a conventional EMC measuring receiver or spectrum analyser. In order to cover the required frequency range of 100 kHz - 30 MHz, which is a 300:1 ratio, it is necessary to use three separate antennas.

The target sensitivity was to be able to measure field strengths of 1V/m (0 dB(V/m)) or less in 9kHz bandwidth over a frequency range of 100 kHz - 30 MHz. The Invitation To Tender (ITT) acknowledged that such an ideal was unlikely to be achievable in practice and that the design should aim to meet these objectives as is feasible and acceptable to the Project Manager. Some compromises were necessary as follows:

(A) The system measures the 'H' field strength with a noise floor of approximately -51.5 dB(A/m) from 400 kHz - 30 MHz. This is equivalent to an 'E' field strength of 0 dB(V/m) under far field conditions, i.e. where E/H = 120. This level is achieved with 9 kHz receiver bandwidth and a CISPR 16 average responding detector (the ITT did not specify the type of detector). Thus the minimum measurable signal would be approximately 4 dB higher than the noise floor at -47.5 dB(A/m) or +4 dB(V/m) with average detection and -43.5 dB(A/m) or +8 dB(V/m) with Quasi-Peak (QP) detection.

(B) Below 400 kHz, the sensitivity is reduced for reasons explained in 3.5 below. The sensitivity is 6 dB lower at 200 kHz and 12 dB lower at 100kHz. Development work has shown that the above design goals can be met or exceeded. In particular, a lower noise floor has been achieved in practice.

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