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Re: Antennas



On Tue, 28 Mar 2000 Mercxx@aol.com wrote:

> Your response was excellent [about the 3/4 wave vertical] a lot of folks
> have said the M2 eggbeater, hmm... Thanks again.

I answered your Email because I too am in search of the ideal omni.  And I
am willing to try and have tried many different antennas.  So I am only
reporting from my experience and would love to hear comparable data from
other experiments...

Yes, the cross polarized eggbeaters are  IDEAL for a general purpose
satellite OMNI antenna.  There is no question.  They can see everywhere
and in both polarizations.  But to get a real "omni" you must give up some
gain somewhere.. If you can afford the loss in gain, then the omni and
cross polarized advantages of the eggbeaters and others are worth it...

But there are many factors involved in a "satellite" omni.

1)  Neglecting satelltie attitude and antenna patterns, the signal will
    be about 10 dB weaker at the horizon than overhead
2)  Satellites will spend most of their time at low elevations
3)  Satellites will spend little time directly overhead 
4)  Most of the existing birds have a vertical, linear antenna

So when I say the 3/4 vertical is a good antenna, it is because I am
willing to give up the low portion of the pass and give up circular
polarity for the advantage of an additional 5 dB gain between 30 to 70
degrees.

Considering the EXISTING satelites, their powers, their near vertical
polarity signals, their linearity and so forth.  I think that the cross
polarization is an advantage only  some of the time.  But the 3 dB loss
all the time in the unused polarity means you will copy less MOST of
the time...

With as much as 8 dBi gain from 30 deg up to about 70 deg, the 3/4 wave
over a large ground plane will be the best in this specific application.
It will NOT hear the brids on the horizon, but it will give the most data
from an OMNI in this specific application.... (In my opinion.... and I am
very open to comarpisons.. I too want to find the ideal omni for thesee
satellites!

See the GAIN plot comparison at the bottom of my web page:

  http://web.usna.navy.mil/~bruninga/astars.html

de WB4APR, Bob




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