[aprssig] Optimum APRS Antenna Tip:

Robert Bruninga bruninga at usna.edu
Mon Nov 21 19:42:53 CST 2005

APRS Antenna Placement Tip:

If you want maximum APRS reliability for your uidigi at yahoogroups.com
station, take care to set your antenna such that you 
hear/hit your best digi at least 10 dB better than
any other digi.  Same goes for digis too.

Sometimes this only means moving the antenna  a 
few feet, or changing the leg of the tower, or lowering
it or positioning it so that it is partially blocked.

The reason for this, is that you want ONE digi to
always WIN whenever there is a collision, not both
of them to block each other.  its nice to have
enough height so that you do hear other digis,
for when your digi is quiet, but you just want 
to not set yourself up to have equal signals 
from multiple digis.

Remember, it only takes 10 dB of difference for one
signal to capture your FM receiver and give you 
a good packet.  So it does not take much to
give you that kind of differentation between
distant digis.

Here is the full explaination.

APRS is intended to flood a local area network so that
everyone gets a good chance to get the packet, but
also to minimize the air time consumed.  The concept
is that packets are supposed to radiate-outward
from the first tier to the next tier and so forth. 

But in most cases, each digi can hear 4 or
more others.  Thus, a single packet going 3 hops
can generate as many as 36 different packets.
If all those digis operated courteously and waited
for the channel to clear, then a single one-second
packet could tie up the channel for 36 seconds!

Instead, we want the 3 hop packet to be DONE in
3 seconds.  That is, we want each tier of digis
to all key up at the same time so that only a total
of 3 digipeat periods are used by these 36 digis.

This is not a problem in most cases, because the
collisions at each TIER are with other digis in
the same tier but the area before them and after
them heard it on the previous or the next hop.

Yes, there are some zones exactly equi-signal
between pairs of digis, that hear a collision but
since FM only needs a 10 dB difference in signal
for one to capture, these zones are very narrow
and changing with terrain. 

Thus the design of APRS chose to minimize channel
load by having tier-by-tier fratracide of packets
at the expense of occassional lost packets.

The receommendation has always been for anyone
that hears two digis, to so situate their antenna so
that one digi will always win in the case of a 
collision.  This way, one will always hears a clean
copy of everything.


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