Simple APRS / KISS TNC

My first dabble with APRS was around 2002 and it seemed like a bit of a gimmick: AX25 Packet frames containing Lat/Lon positions, watching mobile Hams move along a map in real-time.  It was a nice mode, but not particularly useful on its own.  I ran a digipeater/iGate, MB7UCF, for some time in an attempt to see if there was potential for the mode to evolve.  Seems that, as it was, it was quite boring – Better use of the positioning features have been made by RAYNET for large events such as fun-runs and Salt Marsh 75.  The bulk of the traffic back in the day was mostly RF but the UK APRS frequency of 144.800 is still active – but has much changed?

There are a number of APRS weather-stations – Quite a useful real-time indication of conditions around the country.  Oddly, various (useless?) telemetry messages still seem to be sent and I didn’t see very much casual “chatter”.

After the passing of G4IDE in 2004, the UI-View program ceased to be updated.  One of the program’s drawbacks was that, without a 3rd-pary mapping tool, you had to rely on self-made JPG maps.  Then, along came GoogleMaps and a few new APRS clients: AGW Tracker being my favourite – and the ability to connect it with the infamous AGW Packet Engine and its associated apps make it an obvious choice for the tweaker.  AGW is quite useful in that it offers TCP access to any application (LAN or WAN) that you allow access to.  This means that AGW and the radios/hardware could run in the Shack and you could sit indoors with a laptop and connect to AGW via IP.  Same goes with allowing a friend to use your equipment: Just set-up a Port Forwarding Rule (default is TCP 8000) to your AGW computer and set them up with a Callsign/Password: They’ll be able to use your RF ports as if they were local to them.

Modem solutions these days are various: MFJ, KAM, PacComm, Soundcard and homebrew ‘duino – the soundcard option allows for any digimode-equipped station to get onto APRS with ease.  Thanks to USB (no serial-port faffing) it’s possible to run several TNCs.  For the hardware, I took advantage of some quiet time after Boxing Day to build+box a couple of new Shack boxes…  An Arduino-based KISS TNC being one of them.  This is a simple project and uses just a few discrete components – It requires a fair bit of RX audio drive so not ideal for a “data” jack output but is OK if you tap the speaker output and use the AF volume control.

Arduino KISS TNC

Arduino KISS TNC

The radio in use is a Baofeng UV-5RTP which pushes around 7.5w and has a reasonable front-end so doesn’t mind sitting on the outside aerial within spitting-distance of a nearby 153MHz pager site.  For cabling, my OCD always gets the better of me so all home-brew interfaces feature a 5pin DIN connector wired to what I call “TNC standard” – this means that any radio cable I make can be used with the infamous Tiny-2 TNC or any of my own boxes.  Usually it’s a 6pin mini-DIN data plug or a Yaesu 4-pole 3.5mm one.

As I didn’t have a Baofeng/Kenwood cable to hand, I chose the next best solution: Re-purpose a naff speaker-mic!  The cable was snipped just as it entered the microphone casing, stripped and a 5pin DIN soldered on.

The beauty of the Baofeng wiring is that you don’t need to mix the Mic+PTT like a Yaesu, so no 1k resistor+1uF cap to worry about.  Just Mic, RX, PTT and Gnd.  Simples!  If you’re using a smartphone app like APRSdroid which, being sound only, doesn’t have PTT – The VOX on the Baofeng hangs for around 500mS after the audio finishes – Perfectly acceptable in a speech situation but not for the fast nature of AX25 where you may miss your own frame being digi’d (or even clogging-up the frequency with a dead-carrier) so YMMV.

Getting the config right for an internet-RF station can be fun: There used to be a clause in the licence about connecting to non-Amateur networks (ie: the interwebs) so anybody wishes to digipeat frames from an APRS server (and out onto RF) needed an NoV to do so.  This would grant you an MB7Uxx callsign for your system.  These days, the powers-that-be are a little more progressive in their attitudes to technology and the hobby – so gating RX’d traffic from radio to the WWW is something any of us can do.

I opted to set a “loopback” port in AGW so that I could run AGW Tracker with AGW Digi.  The Digi app runs the connection to an APRS server and pipes all traffic onto the internal “loopback” port.  With this configuration, I could RX local traffic on 144.800 on Port 1, and pipe everything from the Internet onto the Loopback port so that AGW Tracker could see everything coming in.  Although AGW Tracker can connect to a server, it seems a bit silly to have two connections running.  If only AGW Digi had its own internal server!  As I’m only sporadic in terms of RF operation, I didn’t want to digipeat lots of stuff – So, for the RF port, I’ve set only CHELMS as a beacon path.  This covers me if I go out with a /M APRS setup and wish to be gated (and re-transmitted) via my own system.

AGW APRS Digipeater - RF Settings

AGW APRS Digipeater – RF Settings

A quick ‘n’ dirty test with the KISS TNC running off the main station (FT-857, V-2000) and me firing AFSK from APRSdroid into a Baofeng using VOX produced satisfactory results (and digipeats).  So, with all this fancy goings-on, just what use is it?  Handy to know where I am (or how fast I’m cycling).  If I was part of a group, and we had APRS-enabled radios, then we’d have distance+bearing available on our radio displays.  Coupled with a smarphone app like APRSdroid, I could run the UV-5RTP in VOX mode on the bike, connect it to the speaker/mic jack on the phone and set it to AFSK mode rather than UDP (iGate) mode.  The phone would handle the GPS, mapping and AX25 encode/decode – the radio would simply transmit+receive via whatever aerial I was using.

Thanks to the interwebs, anyone can see where you’ve been – Is this a good thing?  Depends where you’ve been, I suppose!  Here’s a screengrab from APRS.fi back in February 2015 when I enabled APRSdroid during my trip to the Canvey Island Rally.

APRS track from  Canvey Rally 2015

APRS track from Canvey Rally 2015

So, APRS, a useful system for safety and keeping tabs on people, walkers, runners etc as part of an event but a bit boring for just watching callsigns on a map!