Antennas in a mixed polarisation environment

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Julian Hardstone
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Antennas in a mixed polarisation environment

Post by Julian Hardstone » 21 Apr 2016 01:10

Discussions of antennas for FM Radio dx-ing place a high value on narrow forward beams with small side- and rear-lobes. Even the simplest antennas including the simple dipole, show deep nulls at ±90° which can be used to null out a strong local signal, assuming horizontal polarisation of both tx and rx.

When I bought the K6STI 'Small Five' from HS Publications I was looking forward to taking some detailed measurements on my old Triax FM5 before I replace it with the new ae, but I realise I cannot make any meaningful measurements on the HP performance of any aerial because there are no HP tx anywhere within range of my QTH. In the UK only a few low-power relays, in valleys between mountains in Wales and Scotland, now use HP.

All urban British locations find the band heavily occupied with stations using either mixed or vertical polarisation, probably in a ratio of about 3:1. That means every local tx radiates a vertical component. Now, the value of a horizontal antenna with narrow beamwidth and deep nulls off the sides is of remarkably little use in the pursuit of weak signals from outside the area because each and every HP ae will show a peak off the sides for VP signals.

I am sure that rotating the antenna off the horizontal would find a null to the VP signal, which might allow us to null local VPs - maybe Tim can confirm that this is possible. However this will reduce the inherent null to HP signals, so that no null can be found for a MP signal.

This late realisation gives me complete understanding of why I have been finding Band II dx so disappointing for many years. The installation of the largest antenna I could obtain, the enormous Körner 19.3 with quintuple reflector, gave little or no improvement in this situation. The few dx-ers who can regularly resolve weak signals from outside of or within the UK do so entirely by dint of their location, usually close to a coastline, with some screening from mid-range UK tx. Two exceptions to this rule are Nick Gilly beaming straight at the Capital - how does this work! - and Andrew Webster, overlooking Manchester but screened by a hill to signals from the North. In his case, the height ASL seems to give some benefit involving vertical ducting of distant signals.

For the rest of us, the only prospect of eliminating the QRM on almost every channel seems to require a phasing approach, to null the off-beam local signal against a vertical dipole a few wavelengths separate from the beam. Now we need someone to do some more work on this approach.

k6sti
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Re: Antennas in a mixed polarisation environment

Post by k6sti » 21 Apr 2016 02:26

Julian Hardstone wrote:Now, the value of a horizontal antenna with narrow beamwidth and deep nulls off the sides is of remarkably little use in the pursuit of weak signals from outside the area because each and every HP ae will show a peak off the sides for VP signals.
This isn't true in free space unless the antenna has imperfections. But there can be a small response to vertically polarized signals for an antenna over ground due to the ground reflection from below not exactly cancelling the direct wave from above. Both project a nonorthogonal field onto horizontal elements at a grazing angle.
sm.gif
This shows the response of the small 5-element Yagi at 20 feet over average-quality ground to horizontal and vertical signals.
193.gif
This shows the response of the 19.3.

Unless otherwise annotated, the patterns I show are for the total field, which is the magnitude sum of the horizontal and vertical fields. So the patterns always include any incidental vertical response.
tot.gif
This shows the total and horizontal fields for the small Yagi. You can see that the vertical response hardly makes any difference.
193tot.gif
This is the same comparison for the 19.3.

If you're seeing a larger and more troublesome response than calculated patterns predict, make sure the coax connections are as short as possible and install a good current balun at the feedpoint. The coax shield can easily pick up vertically polarized signals far larger than the responses shown above. If that doesn't help, the unwanted response may be due to reflections and outside your control. A vertically polarized signal that reflects off a slanted conductor can generate a horizontally polarized component to which the antenna may respond strongly.

Brian

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Julian Hardstone
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Re: Antennas in a mixed polarisation environment

Post by Julian Hardstone » 21 Apr 2016 15:15

Thanks for doing the analysis of the polar response to VP signals, Brian

To me it seems intuitively sensible to expect a peak off the ends of a dipole when receiving a VP signal: if you start with a vertical dipole in a VP field and start laying it over, say to 45° I expect to find a maximum response when either end points to the source. As we are never in a perfect free-space situation there will normally be some downwards bend on the incoming signal, due both to ground proximity and to atmospheric refraction.

I am encouraged by your analysis, and reviewing my recent measurements I see a close agreement with my measured figures on a very local VP station. I have noticed this 90° peak particularly on the Irish VP signals, when they are strong in a slight tropospheric 'lift' from about 300km away. Under these condx there will be a significant elevation above the horizon of the virtual source. I had always assumed it would apply to all VP signals including the locals, glad to learn that there is hope!

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Re: Antennas in a mixed polarisation environment

Post by Nick Gilly » 21 Apr 2016 17:50

Julian Hardstone wrote:Discussions of antennas for FM Radio dx-ing place a high value on narrow forward beams with small side- and rear-lobes. Even the simplest antennas including the simple dipole, show deep nulls at ±90° which can be used to null out a strong local signal, assuming horizontal polarisation of both tx and rx.

When I bought the K6STI 'Small Five' from HS Publications I was looking forward to taking some detailed measurements on my old Triax FM5 before I replace it with the new ae, but I realise I cannot make any meaningful measurements on the HP performance of any aerial because there are no HP tx anywhere within range of my QTH. In the UK only a few low-power relays, in valleys between mountains in Wales and Scotland, now use HP.

All urban British locations find the band heavily occupied with stations using either mixed or vertical polarisation, probably in a ratio of about 3:1. That means every local tx radiates a vertical component. Now, the value of a horizontal antenna with narrow beamwidth and deep nulls off the sides is of remarkably little use in the pursuit of weak signals from outside the area because each and every HP ae will show a peak off the sides for VP signals.

I am sure that rotating the antenna off the horizontal would find a null to the VP signal, which might allow us to null local VPs - maybe Tim can confirm that this is possible. However this will reduce the inherent null to HP signals, so that no null can be found for a MP signal.

This late realisation gives me complete understanding of why I have been finding Band II dx so disappointing for many years. The installation of the largest antenna I could obtain, the enormous Körner 19.3 with quintuple reflector, gave little or no improvement in this situation. The few dx-ers who can regularly resolve weak signals from outside of or within the UK do so entirely by dint of their location, usually close to a coastline, with some screening from mid-range UK tx. Two exceptions to this rule are Nick Gilly beaming straight at the Capital - how does this work! - and Andrew Webster, overlooking Manchester but screened by a hill to signals from the North. In his case, the height ASL seems to give some benefit involving vertical ducting of distant signals.

For the rest of us, the only prospect of eliminating the QRM on almost every channel seems to require a phasing approach, to null the off-beam local signal against a vertical dipole a few wavelengths separate from the beam. Now we need someone to do some more work on this approach.
Hi Julian. The main London stations are not that far from here. Crystal Palace is 90 km and Croydon is 89 km. In fact, beaming east the noise floor is considerably higher as this is looking at the centre of Whitchurch with a concentration of shops and potential sources of QRM. However, DX does get through quite easily although the very weakest signals are probably being partly buried.

The only really strong locals here are Rowridge, Hannington and Heart 96.7/Breeze 107.2. The best and quietest direction for DX is probably SSE as signals into France at over 400 km distant are received via scatter much of the time e.g. Le Mans.

One odd thing I have noticed relating to this discussion is the fact that to receive the best signals from Egem and Boulogne when they are in via tropo, I have to beam west! This is pointing at the railway embankment opposite, so perhaps the signals are bouncing off it. This also occurred with Lopik on 100.7. Interesting that these txes are vertical though. I've never noted anything like this happening with mixed or horizontally polarised signals. Can Julian or someone else explain what might be happening here please?
Good DX.

Nick, Whitchurch, Hampshire.

BW Broadcast RBRX Encore, Kenwood L-1000T, Yamaha T-2, Denon TU-800L, Kenwood KT-1100SD (modified), Yamaha T-85, Sony XDR-F1HD tuners, horizontal Körner 9.2 beam (Antennenland version), Yaesu G-450C rotator

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Julian Hardstone
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Re: Antennas in a mixed polarisation environment

Post by Julian Hardstone » 21 Apr 2016 21:22

I couldn't begin to explain your unusual observations re VP signals, Nick. I have however observed resonance effects in the overhead power lines for the railway which borders my garden. The qrn from the power lines seemed to peak at broadly 25-30MHz and odd harmonics, and I put this down to resonant lengths in the vertical poles supporting the HV insulators. HP was blissfully noise-free. The insulators were replaced some years ago and remain relatively quiet :)

Concerning your success with signals from your east, I put that down to the luck of living in the tropo sweet-spot that is the SE of England. The Hepburn charts often show lift condx from that corner of England into the near Continental countries

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