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[aprssig] OMNI Antenna not best for APRS

Robert Bruninga bruninga at usna.edu
Sun Mar 22 14:15:36 UTC 2009


> I am considering setting up a digipeater to...
> ... "repeat" the traffic in my area at a high 
> enough power to overcome the noise in our area
> and reach a i-gate....

One thing that many people overlook both in their Home station
and in Igates is the value of NOT using an omni antenna at their
base station if they want to get maximum decoding from all digis
in the area.

It is counter intuitive, but if your home station or Igate can
hear two or more digis, then you do NOT want to have an omni
antenna that can hear them all equally.  If you can only decode
two digis direct, then set your antenna on one side of your
house so that it hears one of those digis at least 10 dB
stronger than the other digi.

If you can decode 3, then set your antenna such that it hears
the best one 10 dB better than the second one, and 20 dB better
than the third one.

You do this, because in a collision (happens all the time in
APRS by design), you want ONE copy to always be 100% decoded and
not destroyed by the other digis.  Yet, you still hear all three
digis if they do not collide.

You can tell if you are seeing collisions, by watching the S
meter and your TNC.  If you see strong RF signals and do NOT
deccode, then you are losing most of your packets due to
collisions!

You should certainly fix this problem by taking the necessary
effort to assure this anti-collision arrangement of your
antenna.  Remember, HIGHEST and BEST is not the optimum for
APRS.  APRS is a network with a contention based delivery
system.  Set up  your antenna to match that network
characteristic and you will double or triple your station
performance.

This may mean an antenna below the roof line, or on a specific
side of your tower, or a small beam, or any combination.  Or
just sticking up a 39" reflector rod somewhere within a foot of
your existing antenna to change the pattern a bit..

All it takes is 10 dB difference, and that is not much at RF.
Think of it as 2 "S" units..

If your 2 or 3 digis all "pin your S meter" so you cannot see
the difference, then simply temporarily insert an attenuator so
you can get ALL the levels down into the range of  your "S"
meter.  Then assess how the 2 or 3 digis compare.  Make your
antenna adjustments to get the 2 "S" unit difference between all
of them... Then when you are satisfied, remove the atennuator
and you are all set.

This is Ham radio.  We work in RF.
RF is not plug-n-play...

Good luck.

Bob, Wb4APR

Bob, WB4APR




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