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[aprssig] THOUGHTS ON RELIEVING APRS CONGESTION ON 144.39

David Dobbins ddobbins at gmail.com
Wed Nov 17 03:28:45 UTC 2010


Hello fellow APRS enthusiasts. I’m curious to know what the rest of the
world is doing to relieve some of the congestion of APRS traffic on the
primary freq 144.39? In my wide travels, my experience has been the larger
metro areas are suffering from APRS indigestion, meaning if you turn the
volume up on 144.39 and listen, it’s uncommon to hear any break in the
packets the closer you get to the big cities.



I’d like to share what we’ve been doing about this oversaturation of the
primary freq in the Puget Sound area. For the past couple years we’ve been
experimenting with, and done some implementing of APRS at 9600bd on both UHF
and VHF. The initial alternative frequency/speed experimentation began on
UHF 440.800MHz at 9600bd by Bob King, K7OFT. We have several UHF APRS
digipeaters around the Seattle area. The typical digi setup is an Icom
IC-207 and Kantronics KPC-9612, using the same configuration settings found
on the primary frequency. The biggest group of users are mobiles, typically
those with TM-D700/D710 or the Yaesu FTM-350R and similar handhelds. Those
tracking on the UHF frequency are gated to the APRS-IS, and those with an
internet connection at their home station will see both the VHF primary, and
UHF secondary APRS activity. Our VHF 9600bd effort commenced a little later
than the UHF interest, but has grown significantly over the past year, with
several high level digipeaters serving the alternate VHF frequency
144.35MHz. Scott Cronk, N7FSP has been leading this effort. We eliminated
the possibility of interference between the two freqs (144.39 and 144.35) by
replacing an existing APRS digipeater VHF radio with a Kenwood TM-D700 or
D710, and retaining the existing KPC-3 (or an Argent TNC). The internal
radio TNC operates at 9600bd on 144.35, and the other side of the radio is
set to 144.39 and uses the external 1200bd TNC. No change to the filter
solution has been necessary, or need to add or change antennas, since the
two freqs are close together. We’ve had three arguments presented why this
solution wouldn’t work, and all three have been dismissed. First, folks said
9600bd won’t work on VHF. It does, and quite well. Second, folks said the
two freqs were too close together and would cause interference. Not so, as
the D700 becomes “deaf” on one side of the radio when the other side is
transmitting, thus no issue. Third, while one side was transmitting, there
would be packets not digipeated on the other side. We’ve found this to be
not much of an issue, and the increase in packet loss at the digipeater is
insignificant. We have a growing number of mobile users throughout Puget
Sound using the 144.35 APRS @ 9600bd with very good results. With several
iGates, including direct IGating from the mountaintop digipeaters with
internet access, everyone trying to get their data to the APRS-IS is getting
through. We’re encouraging home and other fixed stations to make the switch
to the alternative frequency to further relieve some of the congestion on
the primary frequency. It’s too early to tell just how much relief we’ve
seen on the primary frequency, and difficult to measure. We've also been
experimenting with cross-banding of data, feeding packets and messages back
and forth between the VHF 1200bd, VHF 9600bd, and UHF 9600bd channels.



We’re also encouraging makers of the VHF trackers to add 9600bd capability
to their products. For those folks in areas oversaturated by APRS packets, I
suggest the next time you make the mountaintop digipeater run to replace the
existing VHF radio with a Kenwood TM-D700/D710 and give dual-APRS ops at
1200bd and 9600bd a try.


Dave K7GPS

NWAPRS Spokane Area Coordinator
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