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Re: Polarization question to go along with previous email



Jacob,
Tony's explanation is right on the mark.  The quagi's you referenced were 
offset on the booms by 90 deg (1/4 wl, about 6-1/2" as I recall) and then fed 
in parallel (in-phase, no phasing line).  I used 50 Ohm feed to a coax tee 
and a matching section of two parallel 75 Ohm 1/4 wl coax sections to get 
back to 50 Ohms after the tee.  I could not switch polarity with that setup.  
If you wanted to switch polarity with a similar setup, do just as Tony 
advised and use a 1/2 wl matching section in one of the antenna feed lines 
(doesn't matter which one).

Polarization sense is determined by looking from the back of the antenna.  
Imagine the RF wave like a Slinky coming at you:  if you have the center 
conductor of the coax connected to the "right" side of the driven element on 
the forward antenna and to the "bottom" side of the rearward driven element, 
you will have RHCP.  By changing the rearward coax feed so the center 
conductor is connected to the "top" side, you would have LHCP.  Follow?

While I certainly can't speak for P3D, I can say without a doubt working 
AO-10 (our only other P3 bird) without polarity switching is very limiting.  
That bird changes both uplink and downlink polarity constantly during a pass 
(much more so on perigee than at apogee or so it appears to me).  The 
polarity shifts can be almost perfect--making a signal disappear either on 
uplink or downlink.  If your setup has the wrong fixed polarity, you are just 
out of luck for hearing or getting into the bird for 50 % of the time.  

My experience strongly suggests you are better off with a pair of linear 
antennas fed in phase than with fixed circular polarity for most of the LEO's 
(RS-13 being the exception).  This is sometimes referred to as "polarity 
diversity," meaning you can hear either one, but could be as much as 3 dB 
down from being perfectly oriented.  The theoretical gain is the same when 
the target is CP, but the downside is your beamwidth will be much narrower, 
making tracking much more critical than with CP.  If you track manually like 
I do, that is of no small concern.  

Good luck with the project,
Jerry, K5OE 

> >OR am I totally mistaken and don't need the phasing delay line segment at
>  >all??? Since the antennas are already 90degrees out of phase since they 
are
>  >ofset 90degrees/ 1/4wave seperation on the boom already?
>  
>  Precisely.
>  
>  You can place one of antennas 90 degrees (1/4 wavelength in air) ahead of 
>  the other and drive them in phase, OR you can place the two antennas 
>  together, and put the 90 degree (1/4 wave in coax) delay in series with 
one 
>  antenna.
>  
>  If you place the antennas with one 90 degrees ahead, and drive them with 
>  electrically in phase signals, then to switch to the other sense of 
>  circular polarization (from right to left or vice versa) you would need to 
>  switch in a 180 degree electrical delay in series with one antenna.  That 
>  is sometimes a convenient thing to do.  That works because 90+180=270 
which 
>  is like -90.
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