[amsat-bb] Re: Arrow and EME?

[tosca005@umn.edu]

On Apr 1 2010, Idle-Tyme wrote:

>the best was neither vert or horizontal, and continously changing,  well 
>slowly changing faraday rotation.  plus why would you want to aste n 
>extra 3 db if you have it to be used?
>
>Joe WB9SBD
>On 4/1/2010 2:27 PM, Mark Lunday wrote:
>> Would this also be accomplished in "poor man's design" by simply having 
>> one yagi vertical and one horizontal?
>>
>> Mark Lunday
>> WD4ELG

Sure, with an Arrow or similar antenna, you simply twist the beast until 
the signal is the loudest, and you have your best polarity. On a fixed 
station, getting continuously-adjustable polarization is not a trivial 
task.

With one yagi vertical, one yagi horizontal, and appropriate phasing 
harnesses, one could accomplish a variety of specific polarizations. The 
number of different polarizations would be a function of the complexity of 
the phasing harnesses. If you use no phasing harness at all (or two random 
but equal lengths of harness) you can still select a phase by moving one 
antenna a fraction of a wavelength forward or backward relative to the 
other. But at best, if you had one vertical and one horizontal yagi, with 
equal cable lengths to the impedance transformer, and the driven elements 
lined up with one another, you would only end up with the equivalent of a 
single yagi oriented diagonally, so no big improvement. You would always be 
-3 dB from any signal that was polarized either vertically or horizontally, 
but could still be way down (-20 dB or more) from a signal that started out 
in one of those polarizations but got rotated by the Faraday effect.

With two crossed yagis, and two phasing lines, you could set up either 
right or left-hand circular polarization. With two crossed yagis, three 
phasing lines, and a SPDT coaxial relay, you could have switchable 
left-hand/right-hand circular polarization.

Another step up (in both complexity and in the number and quality of 
phasing choices) would be one yagi at upper left to lower right diagonal, 
and one yagi at upper right to lower left diagonal, four SPDT coaxial 
relays, and 4 specific lengths of phasing harness. This would give the 
following four choices of polarity, depending on the choice of one of two 
phasing lengths to one of the diagonals and one of two phasing lengths to 
the other of the two diagonals:

  Diagonal One    Diagonal Two    Net Phase    Net Polarity
    1/4 WL          1/4 WL             0         horizontal
    1/4 WL          3/4 WL           180         vertical
    1/2 WL          1/4 WL           -90         right circular
    1/2 WL          3/4 WL           +90         left circular

Note that at a net phasing of 0 and 180 degrees, the polarization is 
halfway between the two yagis, so if the yagis are oriented diagonally, 
that gives you horizontal or vertical choices with that degree of phasing.

This, of course, does not give continuously-adjustable angular 
polarization, only a choice of 4 specific polarities. But it is a nice 
choice of 4: horizontal for terrestrial SSB/CW, vertical for terrestrial 
FM, PLUS both right circular and left circular for satellites and EME.

If the other station is running circular polarization, in theory, ANY 
linear polarization will be -3dB from optimal, and there is no advantage to 
any in-between adjustability.

If the other station is running linear polarization, in theory, any random 
linear polarization on your end will be anywhere from 0dB (perfect) to 
worse than -20dB (horrible) depending on the amount of polarity mis-match; 
and because of Faraday rotation, his horizontal signal (for example) may 
not reflect back to you as horizontal, but rather somewhere else (probably 
NOT vertical either!).

If both stations run circular polarization, and at least one of them is 
switchable between right and left-hand circular, then you can always have 
an ideal polarization match no matter what the degree of Faraday rotation. 
So, that might be the best option if you have it available to you. Remember 
that if you expect to hear your own echoes, the reflection from the moon is 
the opposite circularity from what you transmit...

Of course, if you were trying to out-gun W5UN and build an array of 128 
circularly-polarized yagis, that's a helluva lot more work than building 
128 linearly-polarized yagis! OTOH, building 128 of ANY sort of antenna is 
an enormous undertaking, so there! :)

W0JT