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Re: Speaking of antennas...howbout a Lindenblad?




> I've been reading from the ARRL Satellite Handbook about the Lindenblad 
> It looks to me like this would be a very good downlink or even
> uplink antenna for some of the LEOs and requires no rotors

There are many excellent circularly polarized omni antennas, such as the
lindenblad, quadrifilar Helix, Egg beaters and more.  But nothing can
violate the laws of physics...  To get gain anywhere, you have to give it
up somewhere else.

The current crop of satellites (except SO35) do not radiate enough power
to be heard by even a theoretically perfect antenna from horizon to
horizon.  The path loss when the satellite is on the horizon is
typically 6 dB more than it is say above 20 degrees.  That 6 dB is a lot
of "free gain".

So, if you want horizon to horizon coverage, you must use tracking gain
antennas.  If you are willing to accept the downlink above say about 20 to
30 degrees (and give up 50% of all operating time) then an OMNI is for
you.  Now then the quesiton about polarization comes up.  Most of the
current stellites have some form of attitude control and the strive for
predominatnly vertical polarization.  

You can decide if you want to give up 3 dB of performance most of the time
to gain a little bit of performance when faraday rotation is bad or not.
In most cases, giving up the 3dB to pick up some horizontal components
will lose you more than you will ever gain.

Next, since you won't hear any satelites below say 20 to 30 degrees
anyway, then your antenna should not waste any gain down there either.
That is why I have found that the best omni antenna for the current
satllites that are mostly vertical is just a 3/4 wave vertical whip over a
ground plane.  It exhibits over 7 dBi of gain above 30 degrees.  that
combined with the 6dB reduction in path loss gives you a wopping 13 dB of
gain in a range from about 30 to 60 degrees compared to a normal omni on
the horizon.

With this 20.25" whip antenna over a ground plane, I copy 7 of the
existing satllites with my HT.. (while they are above about 20 to 30
degrees).  But hey, its no-fus, no muss.

In my opinion. None of the circularly polarized antennas will approach
this performance under these limited circumstances...

de WB4APR, Bob


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