[amsat-bb] Re: Arecibo and circular polarization using cheap yagis
kl7uw at acsalaska.net
Fri Apr 16 11:20:17 PDT 2010
At 08:25 AM 4/16/2010, Rick - WA4NVM wrote:
>A picture is worth a thousand words.........
>Maybe this will help......
>73, Rick WA4NVM
>Subject: [amsat-bb] Re: Arecibo and circular polarization using cheap yagis
>>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
>>73, Ed - KL7UW, WD2XSH/45
>Hi Ed, KL7UW
>Please find here a necessary amendment to your statement:
>We assume that you are looking from the rear of the antenna in direction
>of propagation and one antenna is spaced 1/4 wavelenght ahead of the other.
>If the dipole of the rear antenna is vertical with "+" straight up and if
>the dipole of the front antenna is horizontal and has its "+" element
>pointing to the left then you get RHCP. Reverse it and you get LHCP
>In a separate email I have sent to you a drawing showing how two linearly
>polarized components shifted 90° one to the other adds togheter to generate
>a circularly polarized wave but I can send the same drawing to everybody
>>For antennas in the "X" configuration (back antenna "+" up and
>>to the left, front antenna "=" up and to the right for RHCP.
>If the signe "=" is a typing mistake and you means " - " (minus)
>then your statement is correct.
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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OK, that seems counterintuitive, but I guess I
was not accounting for both driven elements to be
fed in-phase. The electrical vector must rotate
clockwise to produce RHCP, and so it does in
Domenico's drawing. Dom's drawing actually shows
the rear driven element horizontal and the
forward driven element vertical, but its the
relationship that produces the rotation so does
not matter (i.e. they both could be angled +45
and -45 degrees in the "X" configuration). I
guess my confusion was not accounting for both elements being driven in-phase.
My M2 436CP42 looks just like Dom's drawing but
it has a 1/2 wavelength longer phasing line
between the rear and front driven elements. They
say this is the setup for RHCP? It would seem
the M2 system would have the front antenna
180-degrees out of phase? So would that not
produce LHCP? It is actually easier to visualize
with both elements at the same point on the boom
and one fed with a 1/4 wave longer feed line.
Nice thing about helical antennas is the
corkscrew turns in the direction of the rotating wave.
73, Ed - KL7UW, WD2XSH/45
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
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