Verticals and EMC (Electro-magnetic Compatibility)

Whilst we have all heard that vertical antennas cause interference problems, I suspect that most people don’t fully appreciate why. I certainly didn’t, until I investigated the TVI problems that I had with my original 5 band vertical design. In fact I always used to say that verticals cause TVI because they put out such good signals at the low angles that we need for DX communication! It was only when I discovered that my vertical was causing lots of EMC problems with my neighbours, whilst my horizontal half wave dipole antenna at 30 feet was not causing any problems at all, yet giving a better signal in Australia, that I realised that it was the vertical polarisation that was to blame.

It is obvious that vertical antennas will couple more power into nearby vertical conductors than horizontal antennas will. This causes much loss of radiated signal and also causes many EMC problems. If more RF energy is being coupled into a vertical TV feeder co-ax cable, then TVI is going to be more lightly. Interference will also be picked up from the TV, producing an increased background noise in your receiver.

However I was not convinced that this was the whole story. My neighbours portable radio was also clobbered by the vertical but completely clear when I used my horizontal! The portable radio could be oriented in any plane, with no effect on the amount of interference received.

Tests with a calibrated field strength meter showed that the field strength levels near the ground were very low from the horizontal antenna, and very high from the vertical antenna! The field strength meter needed to be up at a height of  about 20 feet before the field strength readings from the horizontal antenna were similar to those from the vertical. Reports from Australia showed the horizontal to be 3 dB stronger though!!

The susceptible portable radio was substituted for the field strength meter. It became obvious that the ground was effectively “shorting out” horizontally polarised signals. It was found that at low heights the power fed into the horizontal antenna needed to be about 100 times (i.e. 20 dB) greater than that fed into the vertical antenna, for the same interference levels at a given distance.

It was realised that if any electronic device with an EMC problem is mounted close to the ground (in terms of wavelength) then it will not receive horizontal polarised signals very effectively. Up to a height of about a quarter of a wavelength the ground will act as a reflector, so that horizontally polarised signals will only be received from higher angles. Below a height of about an eighth of a wavelength the ground will also provide a lossy dielectric path for the horizontal electric fields, and thus tend to short them out and/or dissipate them. Thus the ground will protect electronic devices from horizontally polarised electromagnetic fields. Vertically polarised fields will however be received very well, even if the susceptible equipment is actually sat on the ground.

I decided to concentrate all my efforts on the design of horizontal antennas, to avoid all the hassle of  TVI, HI-FI and telephone breakthrough etc. I thought that it would be very anti-social of me to market a vertical antenna that would subject lots of radio amateurs, and their innocent neighbours, to EMC problems that could be avoided by the use of the correct polarisation.  Thus the CobWebb concept was born!!

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