[amsat-bb] Re: Arecibo and circular polarization using cheap yagis
kl7uw at acsalaska.net
Thu Apr 15 14:49:59 PDT 2010
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
>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
>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?
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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
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com
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