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Author Topic: Sat com's, antennas and polarization questions  (Read 1430 times)
GRANDKODIAK
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Posts: 85




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« on: January 07, 2013, 02:16:39 PM »

Hey all, I've seen a few videos online of people using the HAM satelites and I'm really intrigued! I see they almost all have been using hand held yagis that they rotate back and fourth from horizontal to verticale position to better match the polarization of the satelite during its rise and fall in the sky, and second, they slowly decrease then increase the recieve frequency to match the doppler affect. (ps. I'm not to great of an antena person just yet, my ARRL books are on the way!)

Now, I've seen pictures of military antennas and even some "civilian" antennas such as these:

http://images.monstermarketplace.com/1-stop-tactical-gear/trivec-av2055-3-miniature-coat-pocket-high-gain-satcom-antenna-300x300.jpg

or

http://olive-drab.com/images/elect_anpsc5_375.jpg

Now, the first one looks like a yagi that has both vertical and horizontal pieces, and a circular piece at the rear... and the other has a single center cross and the circular piece at the rear.

Could someone explain to me what makes these different compared the a handheld yagi? Am I right in assuming that they are basically 3 antennas in one, being 2 yagi stle but one in each polarization, and then a single circular one at the rear? If so, how about that single piece cross... is that just like having 2 dipols and a ciruclar at the rear?

Would either of these types be better to build/buy for ham sat use, since they are set to already be circularly polarized?
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WB6BYU
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« Reply #1 on: January 07, 2013, 04:09:01 PM »

The "circular thing" on the back in both cases is a reflector.  My guess is that it is
some sort of conductive screen or mesh, though it would also be possible to
dimension the loop and supporting rods to be resonant.  It could just as well be
a pair of reflector elements on the backs of the yagis.

Circular polarization is great when a satellite is spinning, and it makes operation
easier.  But the common ham hand-held yagi has a 2m yagi at right angles to a
70cm yagi because the uplink and downlink frequencies are on different bands.
(Interlacing the two doesn't work as well because the elements interact, though
there are other possible ways to do it.)  That means that using two yagis for
each band at right angles to each other on the same boom isn't practical in that
case.
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GRANDKODIAK
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Posts: 85




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« Reply #2 on: January 08, 2013, 01:45:47 PM »

Why would there be an extra reflector at the end and be a circle? couldnt they get away with just having a single pole at the end of the 4 branchs already on the antenna, or does that enable the cicular polarization sensitivity?
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« Reply #3 on: January 08, 2013, 02:28:59 PM »

Quote from: GRANDKODIAK
Why would there be an extra reflector at the end and be a circle? couldnt they get away with just having a single pole at the end of the 4 branchs already on the antenna, or does that enable the cicular polarization sensitivity?

You could add an extra yagi reflector to each of the two beams instead of the
screen reflector.  The screen works over a wide bandwidth (as long as it is big
enough) and is effective regardless of the polarization.  Similarly, a loop tuned
as a reflector will work for both polarizations, though it will be more limited in
bandwidth.  In fact, because loop elements will work for any polarization you
can use a quad antenna and feed the driven element in two places to get
circular polarization.  Like this:  http://www.homingin.com/dualfeed.html#toc

Why would they choose to use a reflector instead of the other options?  Might
make a slight difference in performance.  Might be required to meet some sort
of F/B ratio requirement.  Might enable them to sell it for a higher price to the
military because it looks more complicated.

That's true of most antennas:  we look at a lot of options and then choose one,
possibly on the basis of some parameter (and the assumption that the specifications
are accurate).  Sometimes just because we like the look of it, or we get a good
deal on one, or it is within our ability to construct, when there are many other
options that would work just as well.
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GRANDKODIAK
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Posts: 85




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« Reply #4 on: January 10, 2013, 02:51:34 PM »

Thank you for the replys! Makes sense... I'm still waiting on my antenna books, I have $2.15 in my account right now so they have to wait till payday... I did in the meantime find some pdf Army field communications manual that has some very good info on such things, pretty easy read too!
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KB4QAA
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« Reply #5 on: January 11, 2013, 10:11:45 AM »

Keep in mind that this military satellite antenna can be so small because the military satellites have relatively huge antennas with thousands of watts of transmitter power, and are geostationary (fixed position in the sky).

Ham satellites are truly small, some no bigger than a shoe box, with just a few watts of power. Unable to afford the huge expense of high altitude positions, they are at low altitude and constantly changing position.    This typically requires larger, more precisely built antennas with the ability to track the predicted position.   Still this is well within the ability of the average ham.

Check out the http:www.amsat.org  website, and I encourage you to join or donate! 

bill
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GRANDKODIAK
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« Reply #6 on: January 11, 2013, 01:10:32 PM »

Very cool! Anything space/rockets/nasa related always blew my mind as a kid, to be able to interact on any scale with something in that realm was always a childhood goal! Kinda sparkin old childhood dreams... but if a hobby like ham can keep holding your interest with new avenues and goals to achieve, its a good hobby!
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