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Re: Eggbeaters



At 08:56 AM 6/17/2005 -0400, Robert Bruninga wrote:
>There are 3 things to think about when using OMNI 
>antennas for Satellites.
>1) Satellites spend 67% of all in-view times below 20 deg
>2) Satellites below 20 deg are 6 to 10 dB further away
>3) You cannot afford ANY coax loss (use a preamp)
>
>Thus, assuming you have a preamp,  a good base 
>station antenna will work well for about 50% of the
>time when the satellite is  below about 15 deg IF THE
>SATELLITE IS STRONG ENOUGH  TO BE HEARD IN THE
>FIRST PLACE.  But the main lobe of such antennas
>is only on the horizon and there is a big null from about
>15 deg up to about 45 deg.   Thus if the signal
>is too weak on the horizon to be heard anyway,
>then no only do you not hear it there, but now you
>have null higher up, where it is closer in and you
>would be able to hear it on a 1/4 wave low gain
>whip.
>
>Conversly, a hemispherical antenna has equal
>gain everywhere (usually inadequate on low
>elevations where the bird is 6 to 10 dB farther
>away and also where it spends 67% of its time.
>But when the bird gets closer (higher) then
>this antenna gives solid performance (but for
>less than 1/3rd of the pass.
>
>So it all goes back to one thing.  You cannot have
>high gain on the horizon witout having a null higher
>up, and if you have gain higher up, then you miss
>most of the time on the horizon.  BUT IF THE 
>SATELLITE IS SUCH THAT IT CANNOT BE HEARD
>ON THE HORIZON ANYWAY without gain, then
>dont bother.  Go with a omni such as a 1/4 whip
>over a gorund plane and just plan on only hearing
>the 30% of all pass times when the satellite is
>closer in (and above the trees) and hear it well.
>
>But always use a preamp.
>
>I dont think there are any amateur satellites other
>than the ISS that fall into the category of workable
>at the horizon on an omni.
>
>de WB4APR, Bob

Bob, Dale, and John:

MY guess is the ground-plane on the EB pulls the pattern down more to the
horizon but still it mainly radiates upward and not to low elevation
angles, thus as Bob states, it is best for the middle of a pass but not so
great near AOS or LOS.

K5OE (do I have the correct callsign?) finally tipped his potato masher
antennas (rectangular variation of the EB) to point nearer to the horizon
and used azimuth rotation to track (kind of defeats the "omni" nature of
the antenna, but solves the radiation at low elevations).

Initially, at AO-51's launch I had no cables to my satellite beams so I
merely placed a 1/4-WL 2m mag-mount whip on a square of steel plate on my
roof.  On 435-MHz my radio was deaf and didn't hear AO-51 hardly.  Addition
of a 432 preamp on the roof at the antenna fixed that FB with excellent
telemetry copy of the 1/2 w signal from AO-51 throughout a pass (well at
least to about 5-deg elevation.


73's,
Ed - KL7UW 
===================================
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
Amsat #3212
Modes: V - U - L - S
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