[amsat-bb] Re: Arecibo and circular polarization using cheap yagis

[Edward Cole]

At 12:16 PM 4/15/2010, Douglas Quagliana wrote:
>I heard Arecibo would be using circular polarization, but I haven't heard
>if it will be left or right-handed circular polarization.
>
>I also heard that there's a way to do circular polarization with the popular
>"cheap yagis."  Apparently the details are in an article in the April 1999
>issue of CQ VHF "Cheap Circular Polarization? It Can Be Done" on pages
>66-69.
>
>Is there anyone on the list that has this back issue that can tell me what
>the method is that is described in this article? Is it a physical
>quarter-wave
>displacement on the same boom? Two booms with a quarter-wave coax delay?
>Something else? How is the circular polarization done with the cheap yagis?
>
>Douglas KA2UPW/5
>
>
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You are running out of time to build a CP antenna system for the 
Arecibo eme event, aren't you?  I would not worry about it and just 
use a simple yagi in any polarity.  Yes, you will lose 3-dB of the 
signal from Arecibo, but it probably won't matter much if your 
antenna is big enough.  Arecibo will be transmitting an effective 
power of 441 Mw.

But if you want to make a CP antenna from a yagi, you need two sets 
of elements perpendicular to each other (i.e. two antennas mounted on 
a single boom or two separate antennas one rotated 90-degrees in 
polarity from the other.  If both are mounted so the antennas are 
in-phase you need to split the feedline and add 1/4 wavelength of 
cable to one antenna.  Or you can split the feedline equally if one 
antenna is spaced 1/4 wavelength ahead of the other.  The relation of 
the fed elements determine whether you get RH or LH CP.  The center 
conductor is connected to one side of the fed element (this is called 
the + side).  If the antenna to the rear (or not with extra feedline) 
is vertical with "+" straight up and the other antenna has its "+" 
element pointing to the right, you get RHCP.  Reverse it and you get 
LHCP.  For antennas in the "X" configuration (back antenna "+" up and 
to the left, front antenna "=" up and to the right for RHCP.

Caveat:  I f you have the antenna configured backwards for Arecibo 
then you will hear nothing.  Little understood fact of eme:  RHCP 
signals are reflected by the Moon as LHCP signals back to earth, and 
vice versa.  We have no info on Arecibo, but for hams the convention 
is to Tx - RHCP and RX - LHCP for eme.  So if you build a CP antenna 
and you plan to transmit, also, you will need a polarity switching 
system between Tx and Rx.

I am building a simple two-element Quagi (linear pol) to feed my dish 
(24.5 dBi).  VK3UM spreadsheet says Arecibo will hear me with a +20 
dB signal on SSB.



73, Ed - KL7UW, WD2XSH/45
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