[aprssig] Solar Powered Digi

Robert Bruninga bruninga at usna.edu
Fri Dec 24 10:44:18 EST 2004

>>> Ryan Tourge <k2rrt.lists at gmail.com> 12/23/04 4:07 PM >>>
>If it listens on 144.99 and talks on 144.39, won't it QRM everything
>on 39 because it won't know there is traffic there?

Ideally an alt-input APRS reeciver on 144.99 should be colocated
at a normal digi site which has its own receiver on 144.39.  That
way the TNC's external XCD can be cross connected to that
there are no collisions.

But remember that when a mobile or low power tracker transmits
down on the ground normally on 144.39, he cannot hear any of the
other 97% of packets that the local digi is hearing either.  So 
his chance of colliding with any other distant packets is a given,
a fact of life, and something over which he has no control.

Putting up an alt-input receiver on 144.99 that is cross conneected 
to a 144.39 transmitter can simply be thought of as a remote
base for those small low-powered trackers to let them share the
144.39 transmitter.  The statistics of a collision do not change.
The only difference now is that his 1/4W transmitter is now
magically transformed into a higher power transmitter that will
still experience the same collision that a 50W mobile would 
experience, but now he has a better chance of being heard.

His packet will collide with distant packets exactly the
same as if he was running a mobile 50 watt station, but now
he has local priority.  Remember, this key point:

The measure of the quality of  your local APRS network is how well
your local stations can reliably communicate, not how much DX
and QRM your network can bring in from 500 miles around.

Is giving locals priority a problem?  No.  Back east, where there
are hundreds of digipeaters and hundreds of users, any one digi
only has say a dozen or so "local" users.  Their statistics are 
such that their packetrs are only using that digi less than 4%
of the time.

Similarly out west, where one 10,000 foot mountain digi covers
all of Denver and has nearly a 100% full user base, still is it
not better to give those locals priority to the digi output than
DX packets hitting it from 500 miles away?

As has been stressed over and over on this SIG, APRS is full
of collisions.  It is a fact of life.  What we can do about it is:

1) educate people to use shorter paths to stay within their
Aloha range

2) Set up the network (new n-N paradigm) to limit by design,
how far abusive N paths can go

3) Encourage more local digis andfind other uses besides
QRM generators for the big tall digis that hear too much
(ie, put their outputs over on UHF to double its throughput
and *halve* the QRM on the network to everyone else)

4) Put up alt-input digis where small low power trackers
are expected to be to give them a chance to get into the net.



On Thu, 23 Dec 2004 12:42:40 -0500 (GMT-05:00), Eric Christensen
<kf4otn at earthlink.net> wrote:
> Mark,
> You are missing the point.  The digipeater that Bob is describing
would only listen to 144.990 and thus would not be listening to out of
area traffic.  Thus, only 2-5%.
> Eric KF4OTN
> -----Original Message-----
> From: "Mark A. Lewis" <mark at siliconjunkie.net>
> Sent: Dec 23, 2004 10:59 AM
> To: TAPR APRS Mailing List <aprssig at lists.tapr.org>, jj at aprsworld.net
> Subject: RE: [aprssig] Solar Powered Digi
> There are some big assumptions in that. Our local digi on 144.39 does
> not hear much non local traffic. Saying that TX load would only be
> because it is local only is very dependent on geography and if it can
> hear other digis from where it is.
> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Robert Bruninga
> Sent: Wednesday, December 22, 2004 11:16 AM
> To: jj at aprsworld.net; aprssig at lists.tapr.org
> Subject: Re: [aprssig] Solar Powered Digi
> A real power-saver on SOlar Powered Digis:
> If you need a digi in an area, but cannot afford the huge solar power
> system to keep it on the air most of the time, consider making it an
> ALT-INPUT digi on 144.99 instead of a full service digi on 144.39.
> The TX load will be 2 to 5% of what it would be if the input was on
> 144.39 because it will only digipeat the locals that need it, and not
> the 98% of other digipeater traffic coming at it from all directions.
> Your digi will listen on 144.99 for locals and low power trackers
> (probably fewer than a dozen) but will still digipeat them over to
> 144.39 to join the network.
> Just a thought for some special applications...
> bob
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Ryan Tourge - K2RRT
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