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Re: Topband: circular polarization on 160m

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Subject: Re: Topband: circular polarization on 160m
From: "Tom W8JI" <>
Reply-to: Tom W8JI <>
Date: Wed, 5 Feb 2014 12:38:06 -0500
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In producing a good satellite AMSAT antenna KLM uses the method of quarter wave stagger of two yagis. One is about a quarter wave ahead of the other and fed with a 1/4 wave delay line.

To be circular, one is either staggered 1/4 wave forward and they are fed in phase, or they are even without stagger and fed in quadrature.

Polar plots of this antenna suggest that they are not really producing a screw sense CP antenna but rather an Axial mode antenna that receives both vertical and horizontal components of the arriving space signal as they occur.

That cannot be done. If two antennas are combined without spatial 90 degree stagger **or** phase 90 degree stagger, they are simply a tilted linear polarization.

Many people tilt polarization and think it is both V and H, and think it somehow eliminates polarization rotation fading. All they do is tilt the polarization, and 90 degrees from that tilt is a new null. The confusion is because people and programs express polarization from only tow references, V and H. If I tilted a vertical the right amount it would look like a perfect mix of V and H, but it really would be a single polarization tilted at a 45 degree angle. 90 degrees tilt from that angle, say at -45 degrees, would be a null. With different waves and a "left" tilt we would have:

1.) circular polarized =  no improvement at all

2.) slow lazy fading rotation (this is NOT circular) = no improvement at all

3.)  polarization tilted at left 45 = a peak response

4.) polarization tilted an -45 degrees = a deep null.

To be circularly polarized the wave has to be rotating fast, at the frequency of the wave, so the wave makes one rotation every wave period. This would NOT be a slow fade anyone would hear, it would just be a few dB signal loss.

If the wave were slowly rotating, such as to produce a slow fade, the SENSE of the antennas would not matter one bit. You never get the 3 dB back. You would stop the fade from cross polarization, but would also pick up some significant amount of additional noise.

I'm not sure how well thought-out or properly conveyed any of this has been, so I'm enjoying the brain exercise.

A circularly polarized antenna on 1 MHz cycles through the entire polarization shift in 1uS. A circularly polarized antenna on 1 MHz cycles through one electrical rotation in 1 uS.

Anyone here having fading at a 0.5464481 uS rate? If so, the CP antenna will either make fading near infinite or near zero. :)

The way I see it is if the rate is not 0.546 uS or so, you do not have circular polarization.You have a slowly rotating wave, and the sense of the RX antenna would be meaningless unless you could time-sync rotation at that slow fading rate.

Someone correct me if I am wrong.

73 Tom
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