APRS BASE STATION ANTENNA REQUIREMENTS                       22 Mar 09
----------------------------------------------------------------------

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 usually have plenty of dB to play with.

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.  Remember, the design of APRS digis is that they 
are all supposed to key up on the same packet at the SAME time to 
minimize channel loading.  If you have not taken this fundamental 
antenna arrangement, and hear 2 or more digis within a few dB of each 
other then you are missing a LOT of packets!

You can tell if you are seeing collisions, by watching the S meter and 
your TNC.  First, by ear, you can usually hear them as jumbled audio.
If you see strong RF signals and do NOT deccode, then you are hearing
collisions and 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