Horizontal Polarization for VHF/UHF DX


Daniel Fay
 

Hi Everyone,

Whenever people do weak-signal work on VHF and UHF frequencies, it's always done with horizontal polarization. I'm trying to understand exactly why in the context of some long distance packet radio experiments I'm currently performing. Looking around the Internets, I find answers ranging from "you use horizontal polarization for weak signal VHF/UHF because that's what everyone else is doing" to various justifications for its superiority. Note that I'm discussing weak-signal work via e.g. troposcatter and diffraction, and not Es, meteor scatter, or EME.

Here are some of the answers that I've seen:
  1. It doesn't really matter. ISTR one paper discussing troposcatter stations in the North Atlantic that didn't see much difference between horizontal and vertical polarization. Similarly, it appears some commercial troposcatter systems communicate using both horizontal and vertical polarization as a form of diversity.
  2. Ground gain. Horizontal polarization gives you a few db (up to ~5dB) of ground gain over vertical polarization.
  3. Lower takeoff angle. I've seen some claims about this, but I'm not at all sure if this is the case.
  4. Better diffraction. Horizontal polarization may diffract better around obstacles like hills, mountains, etc. I don't know if this is analogous to how sunglasses are often vertically polarized because glare off of objects (e.g. water) tends to be horizontally polarized.
  5. Less QRM. Apparently interference (both natural as well as man-made) on some frequencies such as 2m is vertically polarized, so horizontal polarization avoids reception of it.
I was wondering if anyone who was familiar enough with the theory behind troposcatter/diffraction communications could key me in on how much better horizontal polarization is vs. vertical polarization and whether the info I've seen is correct.

Thanks,

Dan Fay KG5VBY in ABQ


Jonesy W3DHJ
 

On Sat, 26 May 2018, daniel.fay@gmail.com wrote:

... find answers ranging from "you use horizontal polarization for
weak signal VHF/UHF because that's what everyone else is doing"
Well, there you go! :-)

Jonesy
--
<pre> Marvin L Jones | W3DHJ | W3DHJ | https://W3DHJ.net/
Pueblo, Colorado | @ | Jonesy | __ linux FreeBSD
38.238N 104.547W | jonz.net | DM78rf | 73 SK


Bill
 

Hi Dan,

I'll add one more reason why horizontal polarization is preferred: 

As a practical matter it's much easier to stack horizontally polarized yagis on a single mast. It's important to (co-aim) each antenna such that peaking an antenna bearing on one band results in a peaked signal for all other bands on the same mast. Stacking those same antennas with vertical polarization would require more unsupported mast length. This vertical mast dimension is often at a premium.  Mechanically one usually prefers to minimize unsupported mast above the tower's thrust bearing to minimize the cantilevered wind loads.

Answer number 1; It Really Doesn't Matter is one I disagree with based on my own on-air experience.  It probably matters less if one assumes each station's polarization is consistent with others, but communication effectiveness is greatly affected by cross polarization between stations...... and not in a good way. The comment about commercial stations using both vertical and horizontal polarization does not apply to weak signal operation because commercial communication is designed to maintain an effective link margin. Positive link margin may be maintained by adding transmit power, increasing receive S/N ratio or managing the path loss and distance, the latter being the variable that weak signal DXers do not manage.

Keep us posted on your conclusions as you work through this question.

73 Bill W7QQ

On Sat, May 26, 2018 at 6:12 PM, <daniel.fay@...> wrote:
Hi Everyone,

Whenever people do weak-signal work on VHF and UHF frequencies, it's always done with horizontal polarization. I'm trying to understand exactly why in the context of some long distance packet radio experiments I'm currently performing. Looking around the Internets, I find answers ranging from "you use horizontal polarization for weak signal VHF/UHF because that's what everyone else is doing" to various justifications for its superiority. Note that I'm discussing weak-signal work via e.g. troposcatter and diffraction, and not Es, meteor scatter, or EME.

Here are some of the answers that I've seen:
  1. It doesn't really matter. ISTR one paper discussing troposcatter stations in the North Atlantic that didn't see much difference between horizontal and vertical polarization. Similarly, it appears some commercial troposcatter systems communicate using both horizontal and vertical polarization as a form of diversity.
  2. Ground gain. Horizontal polarization gives you a few db (up to ~5dB) of ground gain over vertical polarization.
  3. Lower takeoff angle. I've seen some claims about this, but I'm not at all sure if this is the case.
  4. Better diffraction. Horizontal polarization may diffract better around obstacles like hills, mountains, etc. I don't know if this is analogous to how sunglasses are often vertically polarized because glare off of objects (e.g. water) tends to be horizontally polarized.
  5. Less QRM. Apparently interference (both natural as well as man-made) on some frequencies such as 2m is vertically polarized, so horizontal polarization avoids reception of it.
I was wondering if anyone who was familiar enough with the theory behind troposcatter/diffraction communications could key me in on how much better horizontal polarization is vs. vertical polarization and whether the info I've seen is correct.

Thanks,

Dan Fay KG5VBY in ABQ



Daniel Fay
 

Bill,

Thanks for your response. I think I will test out both horizontal and vertical polarization when I get the chance. Right now, I'm trying to finish up the basic long-distance tests so that I can submit a paper to the ARRL/TAPR DCC conference (it's in ABQ September 14-16, and the paper deadline is at the end of July). I also have an STA from the FCC to do up to 50W on 70cm for this work (the modulation technique is a form of spread spectrum, and amateur spread spectrum communications are limited to 10W by the FCC). Unfortunately, the modules I'm currently using don't let me access the RSSI information on the transceiver chips, so I'm not going to be able to get precise results for horizontal vs. vertical polarization. Eventually I hope to try a different module that gives me low-level access to the transceiver chip.

Hopefully, I'll at least make some progress and put up my 15-element 70cm Yagi this weekend.

-Dan Fay KG5VBY in ABQ


Bill
 

Nice work Dan! I put the TAPR conference on my calendar.  Thanks.

Bill W7QQ

On Sun, May 27, 2018 at 9:01 AM, <daniel.fay@...> wrote:
Bill,

Thanks for your response. I think I will test out both horizontal and vertical polarization when I get the chance. Right now, I'm trying to finish up the basic long-distance tests so that I can submit a paper to the ARRL/TAPR DCC conference (it's in ABQ September 14-16, and the paper deadline is at the end of July). I also have an STA from the FCC to do up to 50W on 70cm for this work (the modulation technique is a form of spread spectrum, and amateur spread spectrum communications are limited to 10W by the FCC). Unfortunately, the modules I'm currently using don't let me access the RSSI information on the transceiver chips, so I'm not going to be able to get precise results for horizontal vs. vertical polarization. Eventually I hope to try a different module that gives me low-level access to the transceiver chip.

Hopefully, I'll at least make some progress and put up my 15-element 70cm Yagi this weekend.

-Dan Fay KG5VBY in ABQ