RADIO AMATEURS GUIDE TO EMC (ELECTROMAGNETIC COMPATIBILITY) Robin Page-Jones Radio Society Great Britain 2004 (pg 86)
VHF antennas should be mounted as far from house wiring as possible. Very high field strengths can occur when high power is fed to high gain beams.
PASSIVE INTERMODULATION PRODUCTS
All circuits use non-linear elements (diodes) to distort the current waveform, in order to generate the required frequency components. For a single transmitter the effect causes excessive harmonic radiation, but if two or more transmitters are operating in close proximity the result can be quite spectacular intermodulation product generation.
HAM GENERATED EFFECTS
Severe breakthrough unmodulated signal can occur if the antenna is close to mains supply cables, or telephone wires which may act as transmission lines for the RF signal. Unterminated cables will cause unwanted resonant circuits.
Those near enough the desired frequency to be radiated efficiently by the antenna, either through direct instability at the signal frequency OR by a relatively low parasitic oscillation which ‘beats’ with the output frequency will give a ‘comb’ of spurious outputs.
Peak power emitted from transmitters can be up to 1000 times greater than mean power. Further VLF signals have a very unusual ability to exhibit nonlinearity when a masthead amplifier causes cross-modulation.
This will vary quite markedly over small distances as main and reflected signal will add and subtract and re-radiation from resonant conductors takes place.
The main problem with HF is that large beams can cause very high field strengths. For instance 100W fed to an isotropic transmitting antenna in free space would give a field strength of about 3.6V/m at a distance of 15m. The same transmitter into a beam with a gain of 20dB would give a field strength, in the direction of the beam, of 36V/m the same distance away. Again, it comes down to the fact that if you want to run high power to a high-gain beam, the antenna must be kept as far from neighbouring houses as possible, and of course, as high as practical.