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




Hello Bob.

03 Mar 00 17:24, you wrote to ALL:

 BB> The current crop of satellites (except SO35) do not radiate enough
 BB> power to be heard by even a theoretically perfect antenna from horizon
 BB> to horizon.  The path loss when the satellite is on the horizon
 BB> is typically 6 dB more than it is say above 20 degrees.  That 6 dB is
 BB> a lot of "free gain".
 BB>
 BB> So, if you want horizon to horizon coverage, you must use tracking
 BB> gain antennas.  If you are willing to accept the downlink above say
 BB> about 20 to 30 degrees (and give up 50% of all operating time) then an
 BB> OMNI is for you.  Now then the quesiton about polarization comes up.
 BB> Most of the current stellites have some form of attitude control and
 BB> the strive for predominatnly vertical polarization.

Considering that many passes do not exceed 20 degrees (often, 2 passes in a set
of 3 for a typical LEO), this is a huge compromise.  Also, it _is_ possible to
work down to less than 10 degrees elevation with simple antennas - the trick is
that the antennas need to be moveable, and close to the Rx (on top of a HT is
an ideal place!).  I regularly work UO-14 down to 5 degrees or less this way.
However, I am building a portable directional antenna for the 2m and 70cm bands
to try and get closer to the horizon (for those 3 degree passes over ZL, which
I can't quite hear on the 1/2 wave).

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

UO-14 sometimes drifts all over the place (polarisation wise), though it's
probably mostly vertical 80-90% of the time.  I find SO-35 works best vertical,
most of the time.

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

Might be a good choice for the higher passes.  There's another idea to try.
I'll also look into it for portable club activities, where the beams we used
last time had poor performance at high elevation (but excellent at low
elevation), the 3/4 wave trick might fill the hole perfectly, with a manual
coax switch to choose between Yagi and vertical.

 BB> With this 20.25" whip antenna over a ground plane, I copy 7 of the
 BB> existing satllites with my HT.. (while they are above about 20 to 30
 BB> degrees).  But hey, its no-fus, no muss.
 BB>
 BB> In my opinion. None of the circularly polarized antennas will approach
 BB> this performance under these limited circumstances...

Interesting approach, with potential application for those who don't have
elevation rotators as well.

Tony

.. at the corner of Walk and Don't Walk."
--
|Fidonet:  Tony Langdon 3:633/284.18
|Internet: tlang@freeway.apana.org.au
|
| Standard disclaimer: The views of this user are strictly his own.


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