[aprssig] Re: Fill-in Digis

Robert Bruninga bruninga at usna.edu
Fri Mar 24 16:23:42 CST 2006

Regarding Mobiles hearing digis better than getting
in due to the DIGI's high site vulnerability to QRM:

>What we have experienced is exactly the opposite,
> I can get into the  Digi, but not be able to decode 
>the forwarded on packet (can hear it/ see the S meter 
>rise though).

There are two very common reasons for that:

1) THe digi audio was never properly set up, so that the
tones are properly balanced, or the digi has too much 

2) If  two digis hear your mobile, both will digipeat it
as they should, and quite often you will "see" 
a collision and not decode it.  This is normal.

It does not mean that you are not hearing the digi.
You will still hear packets coming in from one digi
or the other, just not at the same time.  Also
anyone else near either digi will hear you fine too.

For mobiles this rarely makes any difernece because
all the variables mean half the time you will hear one
digi 10 dB better than the other.  The place where
it really gets you is a FIXED location.  There, if you
can hit 2 digis and live half way inbetween, it might
be very rare for you to every decode your own
digiepeates because you will always hear a collsion
unless one is held off by some other QRM..., but 
everyone else will hear you fine.

For fixed stations, one should always arrange their
antenna such that ONE digi is 10 dB stronger than
all others.  This way, you WILL always deocde a
digi collision.

THis is fundamental to APRS FIXED station set up.  Make sure
you get one digi at least 10 dB stronger if you want MAXIMUM
decodeability on every packet.  Use a small beam or
shading pole if you have to.

de Wb4APR, Bob

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