[aprssig] Use of the 144.99 alt-input channel

Bob Bruninga bruninga at usna.edu
Sun May 1 10:32:27 EDT 2011

A few thoughts on the proper use of 144.39 +600 alternate input channel:

We were planning on using the 144.39 +600 input channel for our special event in the Maryland Mountains so that our system can hear the weak signals from the 5W walkie-talkies along the 40 mile trail on 7 May.

When we configured our split-freq temporary digipeater (RX on 144.99 and TX on 144.39) we were surprized to hear a busy channel with dozens of packets on 144.99 from over 20 Weather stations in another state, complete with digipeaters on that frequency.

Maybe it is a good time to remind people about the intent of the 144.99 alt-input channel for APRS.  

It only works properly (to greatly improve weak signal reliability) if there are no digipeater OUTPUTS on that frequency, only digipater INPUTS.  This way, each signal source only has to contend with extremely low peer-to-peer collision probabilities and not the 70% or higher collisions with all the other digipeater output packets guaranteed on 144.39.  But if outputs of multiple, multi-hop digipeaters are used on that same 144.99 frequency, all the split-frequency advantages of an alternate input channel are lost.  

Such a separate multi-digi channel then becomes just another parallel system with its own output collision issues blocking other source inputs.

But in this case, such an alternate parallel network should not be on the very unique 144.39 +600 alternate input frequency but could use any other of the dozens of available 2 meter digital channels.  Further, most weather stations are fixed locations, and do not really listen and usually are not used for receiving much traffic.  Again, this is probably not the ideal use of the national alternate-input channel (144.99).

In Summary:  The advantages of an order of magnitude improvement in network reliability for local mobiles and HT's by using the alternate input (144.39 +600) channel are lost if there are digipeater outputs on that channel as well.

Just some thoughts.

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