[aprssig] 9600 APRS net experiment in Central NJ

Bill V WA7NWP wa7nwp at gmail.com
Sat Feb 26 12:00:18 EST 2011

Some thoughts from a long time alternate-channel 9600 baud user...

Number 1 and most important - YES!  An alternate channel is the way to
go.   It makes an incredible difference.   It''s trivial to set up as
the only requirement is an IGate on the new frequency.  Add a couple
digipeaters and a back up IGate and you have a really useful LAN.

> Here in the Pacific NW we are experimenting with 9600 baud on both 440.80
> and also on 144.35.  I am using 144.35 at 9600 baud and in my area it works
> better for my trackers than the standard 144.39 at 1200 baud.

The "trick" of running 9k6 on a channel near 144.39 is ingenious - for
locations where transmitters are restricted and controlled.  There is
still an unknown quantity of lost packets due to desense and
overlapped transmissions..   This isn't an issue with an alternate
channel on UHF.    Herb's success even with the desense issues shows
how well the alternate channel concept works.

For this alternate channel APRS - I'd argue against going to 9600
baud...   IF your goal is simply a personal LAN and you have the
radios - that's great.  If you want to open it up to others and take
some of the load from a busy 144.39 - then go with 1200 baud..  Go
with 1200 because a) at 9600 none of the proliferation of trackers
will be be able to leave 144.39 and use the new channel.  b) 1200 is
fast enough to fill the display on a D7?0 anyway.   The higher speed
isn't needed.

> So this is a fairly simple way to set up a duel 9600 baud and 1200 baud
> digi.  The only thing is you have to sacrifice a D700 or D710 radio.  Not
> sure if you could use a D7A(G) or D72A since the external TNC in the above
> set up is connected through the 6 pin data port on the radio.
> We have a number of similar mountain top digipeaters set up like this and
> are even doing some fancy computer stuff with a mountain top internet feeds
> to a home computer and are blending to two systems so that what one hears it
> is also transmitted out on RF on the other side.  On our 440.800 network, we
> also do similar blending between it and the 144.39 system.

I'd say the jury is still out on the value of 'blending'.   The big
problem with alternate channels is that the mobiles on one channel
don't see the mobiles on the other.   Blending (digipeating) the
packets between channels was an attempted solution.  For now we're
probably generating a marginally unhealthy amount of traffic on the
main channel from the alternate channels.  Smarter digipeating may be
the answer but that software technology is currently not available.

> We can send RF APRS messages between all three system.

Note the messaging comes from the IGates - not cross band digipeaters.

> Pretty slick stuff...  No, I am not
> the one who came up with these ideas but once I saw it in operation, I was
> willing to try it and liked it.....

Herb's too modest.   :)

> Herb, KB7UVC
> NW APRS Group, West Sound Coordinator
> Our WEB Site:  http://www.nwaprs.info

Bill - WA7NWP

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