[amsat-bb] Re: Arrow and EME?
tosca005 at umn.edu
Thu Apr 1 14:11:59 PDT 2010
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?
>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
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
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! :)
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