Re: Antenna polarization mismatch

David R. Hassall, WA5DJJ
 

Dear Joe,

 

You need a life, but I don’t want to tell you that.   It is true that the nominal difference between horizontal and vertical wave is 20dB.   We use that to advantage in a number of satellite communication channels where we put one RF data signal on one polarization and another on the other.   They end up at the receiving end with vey little interference from the other.

On some really high speed I/Q data channels I is horizontal and Q is vertical.    If you have a line of site to a repeater you can turn your handheld from vertical to horizontal and if you don’t have too much ground reflection. You should be able to hear the null.    I am afraid the normal bread and butter Amateur of today would just shrug his shoulders and say “So What?”   as long as he can key up the repeater he is happy and doesn’t really care how the signal gets there, just as long as it does.   I am not sure that if the signal didn’t get there would spark any kind of experimentation to find out why.   He would just call the repeater guy and tell him his repeater was broke.    Then he would go and find another repeater that wasn’t “broke”.

 

Good job of explaining the radio art.   I knew of the formula as I used it when I was testing TDRSS axial Ratio measurements on the spacecraft to ground links.  I had to test each horizontal antenna and each vertical antenna to make sure they were not damaged when the spacecraft went spinning off during the launch.   It was a really interesting test and it proved that all the antennas on the outside of TDRSS survived the launch in good style.   It taught me a lot of practical stuff about vertical/horizontal polarization. 

 

I have had a really tough week this week.   On Tuesday morning I came down with an ECOLI infection and spent the next three day in the bathroom.  On Wednesday I knew something was bad wrong and my son took me to the Hospital emergency room.  They gave me some antibiotics and sent me home.   I finally stopped shitting early Friday morning.   I think I got it from Denny’s new Restaurant out on hwy 70.   I have not been sick like that in a long time.   But I appear to be over most of it except the weakness and hopefully I can regain some of my strength this week and call it a total recovery.

 

I have a power pole going nuts somewhere in the neighborhood.  It is putting lines all over my spectrum display on my ICOM 7600.   It is attached.   It doesn’t seem to bother the grabber receivers since they are only 200Hz wide but it sure likes to spread havoc on my radio’s spectrum Analyzer.   I am going to call El Paso Electric in the morning.   I was going through the underpass on Missouri and my 2M unsquelched and was just going nuts.  So I think it is in those power lines down there and radiating out into my neighborhood.

 

So, Have fun Joe.   That is about all the excitement in little old Las Cruces for this week.   Of course I may have a mioptic view since I spent most of my time this week on the toilet.

 

73 Dave Hassall WA5DJJ  Las Cruces, New Mexico

Website: http://www.zianet.com/dhassall/

QRSS SUPER GRABBER WEBSITE: http://www.qsl.net/wa5djj/

 

 

 

 

 

 

From: main@MVRC-N5BL.groups.io [mailto:main@MVRC-N5BL.groups.io] On Behalf Of Joseph (Joe) Gray, W5JG
Sent: Sunday, June 10, 2018 7:13 PM
To: MVRC-N5BL@groups.io
Subject: [MVRC-N5BL] Antenna polarization mismatch

 

Just thought I'd share some recent thoughts and discoveries.

 

Do you hold your HT straight up and down when talking through the repeater? Most of us don't. We hold it at an angle. That means that there is some signal loss between our HT and the vertically polarized antenna at the repeater. This applies to your cell phone as well.

 

How much loss is there, and is it worse at certain angles? I have always heard the rule-of-thumb that between a vertical antenna and a horizontal antenna (one antenna oriented 90 degrees to the other), there is 20 dB of loss.

 

I just came across the formula for actually calculating the loss at any angle between the two antennas. Expressed as an Excel formula, it looks like this:

 

=10*LOG(COS(RADIANS(A4))^2)

 

I have attached an Excel spreadsheet, with the calculation for angles from 0 to 90 degrees. The spreadsheet cosine function wants radians, thus the conversion from degrees.

 

This formula only applies to linearly polarized antennas. Circularly polarized antennas have a different formula.

 

Note that these calculations are for "ideal" antennas, and "ideal" RF propagation between them. The real world is a little different.

 

We can see from the spreadsheet that for angles up to 30 degrees, there isn't a significant signal loss between the two antennas. At 45 degrees, we lose half our signal. Being a logarithmic function, things get rapidly worse. At 90 degrees, we should have almost total signal loss between the two antennas.

 

That is theory. In the real world, antennas are not perfectly polarized either vertically or horizontally. We also have some reflection and refraction of part of our signal, which may change the polarization of those components.

 

So, is that rule-of-thumb figure of 20 dB any good for 90 degree, cross polarized antennas? Various internet sources say that in the real world, the loss is 20-30 dB.

 

For those who need to brush up on their Decibels, that means that if you hold your HT horizontal, only 1/100 to 1/1000 of your transmitted signal is reaching the repeater. Keep it under 30 degrees from vertical.

 

This might be a good demonstration for someone to do at a club meeting. You could use anything from a simple field strength meter, to a spectrum analyzer to see the effect. You'd probably have to attenuate the transmitted signal quite a bit, at such close range, however. I'd volunteer, but I live in Carson City, NV now. Too long a drive :-)

 

BTW, since almost all of us have a rubber duck antenna on our HT's, you might be thinking that since the antenna is made from a spiral of wire (a helical antenna) the polarization of a rubber duck is not vertical. It turns out that since both the diameter of the helix and the spacing of the turns is significantly less than 1/4 wavelength, the rubber duck antenna acts like a base-loaded monopole, and has a vertical radiation pattern.

 

Joe Gray

W5JG

 

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