[ML-Hams] APRS / Digital Comm

[Kris Kirby]

On Tue, 29 Mar 2016, Jake Polatty wrote:
> I have a GE Delta that I set aside for something like this.  It would 
> take some modification, but it would work great as a APRS 
> gateway/repeater.

> > Would anyone be interested in an "APRS / Digital Communication via
> > Ham" infrastructure class / presentation?
> 
> Do any of us have equipment capable of getting a two way communication going? :)

I strongly suggest you involve Tim Cunningham, N8DEU. Tim has more or 
less lead the implementation of APRS in the area for the better part of 
a decade. 

At this stage of the game, more power isn't needed. APRS is a 
single-frequency network, and there are multiple nodes with overlapping 
coverage areas.

The I-Gate requires an internet connection, but a digipeater doesn't. 
3.4 GHz is the better route to go for IP connectivity since it's not 
shared with anyone except USAF. YAAC & AGWPE are possible solutions for 
that, but one has to watch the packet traffic on 144.39 MHz to make sure 
that packets aren't being repeated excessively. It is entirely possible 
to configure AGWPE to act as a digipeater while connected to an 
OpenTracker acting as both a KISS TNC and a digipeater, resulting in two 
sets of repeats, one from the OT and one from the AGWPE software. That 
is not a desirable condition on a slow-speed simplex channel. 

I'd suggest finding one of the historical channels (145.01, 145.65, 
144.97, 145.05) and doing some packet from TNC A to TNC B while watching 
from TNC C to get a feel for how things work. There is likely a local 
BBS to connect to, or at least Winlink running somewhere in the area. 

http;//www.harc.net/programs/Winlink2000_RMS%20Express-Intro-.pdf

There are also caveats from radio to radio and TNC to TNC with respect 
to TXDelay values (330 - 500 ms) as well as amplitude values of the 1200 
Hz and 2200 Hz tones. Some TNCs won't reliably decode if one tone is 
louder than the other, but this is a different problem than the TX 
audio.

I'd also suggest reaching out to someone with the test equipment to set 
deviation and check the levels of both tones with a given radio to make 
sure that they are more or less level. The pre-emphasis of PM and FM 
tend to start kicking in around 2KHz and excessive pre-emphasis results 
in no decoded packets on those XR2212-based TNCs. (Ergo, if you have a 
true-FM radio, you shouldn't have issues provided you're using the RX 
audio output and not the discriminator output.)

David Kelly, N4HHE, may also be able to help. He has extensive 
experience in this area as well and can tell you more about op-amps and 
radio impedance matching than you care to know.

--
Kris Kirby, KE4AHR
Disinformation Architect, Systems Mangler, & Network Mismanager