SM 5 BSZ - Circular vs Linear Polarisation
(Nov 13 2005)

Standard polarisation

There is a standard for polarisation on VHF bands, and it is there with very good reasons.

For long distance work, the polarisation is linear, horisontal. One reason is that it is easier to mount a single yagi horisontal than vertical, and of course that circular polarisation is more difficult to produce than linear. Another reason is that signals with vertical polarisation is strongly attenuated by passing some hundred metres of three tops while horisontally polarised signals are much less affected.

A circularly polarised wave becomes more or less horisontal after having passed through the three tops on its way into the troposphere because the vertical component is attenuated.

For local communication, vertical polarisation is the standard (FM). Of course the reason is that vertical antennas fit nicely on cars and hand held transceivers. Having opposite polarisation between local and DX traffic makes it easier to avoid interference. The hundreds of kilowatt ERP powers from DX stations is less likely to overload a vertically polarised local station (with poor dynamic range). The high levels of noise sidebands from local FM stations is less likely to cause interference to the DX-er.

Arguments why to use circularly polarised antennas

1. I want to work local FM stations (they have vertical polarisation) and DX stations (they use SSB with horizontal polarization)with a single antenna, and I do not mind a 3dB loss.

This argument is not a reason to use circular polarisation. The same performance is easily obtained at lower cost by mounting a linearly polarised antenna at 45 degrees. Very few amateurs use 45 degrees linear polarisation, but it is a very good solution for the beginner to get a real feeling for both the local and the DX work on VHF.

The only problem is that a 45 degree station can not work another 45 degree station if polarisations are opposite. Circular is no better in this respect if you have only one of the circular modes.

2. I live in a poor location, so all signals that reach me are reflected. The reflection changes polarisation, so when the wave reaches my antenna it can equally well be horisontal as vertical despite the fact that the other station is always horisontal. To have a better chance, I should use circular because then I can receive the signal regardless how the polarisation plane is twisted. There is only a 3dB loss (always).

This argument is false.

It is true that reflections may change the polarisation, but reflections do not only twist the polarisation plane. Reflections can equally well produce circular polarisation and the probability of loosing the signal by being cross polarised is no better. If the signal arrives circular, there is a 50% probability that your antenna is the opposite circular polarisation.

Of course, when the reflected signal arrives as a circularly polarised wave, a horisontal antenna works fine with only 3dB loss. (and a vertical too)

Another thing is that while reflections do change the polarisation plane, forward scatter does not!!! Practically this means that two distant stations who point their antennas towards each other will find that polarisation is conserved.

I have tried this a very great number of times, and I have never experienced a single exception. When I measure the polarisation of a DX station, the polarisation is always accurately horisontal (or in a few cases vertical for newcomers who tried a FM antenna). This is when the antennas are pointed towards each other, and it is valid for tropo, tropo scatter, meteor scatter, and iono scatter. The only exception is strong sporadic E. (Also aurora and EME, but then the antennas are not towards each other) There is NEVER any influence on polarisation by the signal path for tropo, tropo scatter, meteor scatter, and iono scatter.

Side scatter is a completely different thing. If I measure the polarisation of a station with my antenna turned well away from the right direction, or if the station at the other end has his antenna well away from the right direction, the polarisation plane is twisted and when there is multi-path propagation the polarisation may be random.

The polarisation of the side scatter signal is elliptic on 144 MHz tropo or groundwave scatter. The horisontal or vertical component is not zero more often than you expect from a random distribution, and the phase angle between the horisontal and vertical components is also randomly distributed, and there is no preference for 90 degrees (circular) or 0 degrees (linear) or any other angle.

The side scatter signal is always weak compared to the forward scatter signal. Turning the antennas properly is the obvious way to improve signals if necessary.

3. I want to work satellites!

Yes of course. This is a perfectly valid reason.

But if you want to hear the really weak signals that the others do not hear, switchable polarisation might give you another 3 dB. I have only listened to OSCAR 13, and it was linearly polarised, so for that particular satellite, and at that particular time, switchable was clearly an advantage.

From Ross W1HBQ (Nov 11 2005)
4 If both the transmitting antenna and the receiving antenna are cp, multipath is reduced. This assumes, of course, that at least some of the received signal is direct. ATV should benefit from this.

Yes, in situations when the direct wave is strong enough it is sometimes a very good idea to suppress reflected signals by use of circular polarisation.

From Ross W1HBQ (Nov 11 2005)
5 A repeater that is circularly (or slant) polarized can be accessed by (with a 3dB loss) by either horizontal or vertical polarization. This would improve the versatility of repeater stations, allowing access by nearby vertically polarized stations or distant horizontally polarized stations. Nearly all commercial fm broadcasting stations broadcast cp (actually RHCP), presumably because of this reason.

This argument, circular matches both horizontal and vertical antennas pretty well (with only a 3dB loss) is the same as reason 1. Personally I think the polarisation orthogonality between (local) FM and long distance weak signal communication is a good thing but in areas with a low amateur population density circularly polarised repeaters could be a good thing(?).

If you have another reason to use a circularly polarised antenna without polarisation switching, please send an E-mail to me.

I will place your reason as number 6 here, and I will tell if it is a valid reason or not if I know. Otherwise, and much more interesting, I will leave the question open.

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