signal polarization, -20db loss, and the details they left out

[towerdog]

There has got to be more to the "up to 20db of loss" rule for 90 degrees difference in polarization. I think they meant, up to 20db or else I dont think I would hear anything with my HT turned sideways, yet, I can still copy the signal, often with little difference, even on weak signals.

I am thinking it might have something to do with smaller wavelenghs the less signal will be received. Cross polarization has been used for decades for microwave links, like having 2 links in one, so there must be around -20db of attenuation here.

 

[W5LZ]

I think that '20 dB' is just a very rough 'average', 'round number', not particularly definite. There certainly can be that much attenuation at times, probably more, or maybe less at other times.
- 'Doc

 

[Captain Kilowatt]

Using an HT is a very poor way to try checkong out polarity differences. The signal can reflect off anything and everything close to the radio including YOU. Connect a real antenna and tnen you will see the real difference. On 2m SSB I worked a station about 50 miles away. He was a solid S9. His buddy a quarter mile down tbe road witb the same antenna and power could not even be heard. Nothing. He was using his beam vertically while the S9 signal was horizontal lime me. In this case there was much more than 20 dB or so difference.

 

[dxhound]

The 20-db degradation is mathematically calculated. We all know that there are variences from science in real world applications. The issue with the hand held not showing much diff. in polarization is because there is no reference to ground. (i.e. counterpoise or ground plane. This means that your transmitted signal is spherical in ALL directions, not just circular in reference to earth ground.

The poor reference to ground is proven by the fact that when you clasp your hands around most handheld rcvers, youll notice weak signals usually clear up and are rcved better. The added capacitance from your hands and body have "coupled" the radio to a better ground.

 

[towerdog]

I still think there is more too it, I am sort of thinking they come up with the 20db from a dish or highly directional antenna and not a simple monopole.

 

[Moleculo]

I have a 2m Gizmotchy antenna that allows me to change polarity with the flip of a switch. I've tested this a million times and the cross polarity can make a huge difference. However, my testing has also shown that at times due to signals bouncing around, it makes less or more difference than at other times. For example, there are a few stations that I cannot talk to directly from my QTH. As a result, we routinely bounce our signals off near-by hills. My experience suggests that when you are relying on bouncing paths, the cross polarity rules of thumb change the equation. Sometimes the cross polarity is worse because of the bounced path; sometimes it is better.

I guess the lesson is YMMV.

 

[Captain Kilowatt]

I still think there is more too it, I am sort of thinking they come up with the 20db from a dish or highly directional antenna and not a simple monopole.

The type of antenna has nothing to do with it. Gain is in no way related to polarity. The difference in signal is not calculated but rather measured in an RF anechoic chamber or an approved antenna test range. Theoretically the difference in polarity would yield infinite attenuation however in practice the difference is between 20 and 30 dB. Think of it like looking through two slots aligned as if the same polarity. When they line up you can see straight thru them. Rotate one of then 90 degrees and now all you can see is a tiny fraction of what you could see before.

Antenna Polarization Vertical Horizontal Circular Polarization | ASTRON WIRELESS

 

[KB3TQO]

The type of antenna has nothing to do with it. Gain is in no way related to polarity. The difference in signal is not calculated but rather measured in an RF anechoic chamber or an approved antenna test range. Theoretically the difference in polarity would yield infinite attenuation however in practice the difference is between 20 and 30 dB. Think of it like looking through two slots aligned as if the same polarity. When they line up you can see straight thru them. Rotate one of then 90 degrees and now all you can see is a tiny fraction of what you could see before.

Antenna Polarization Vertical Horizontal Circular Polarization | ASTRON WIRELESS

I agree!!! The Observer in Physics Double Slit Experiment and Example in Nature - YouTube