A Cheapskate's Approach to Packet Radio (Part 1 receiving)
Packet radio Terminal Node Controllers (TNCs) are packet radio modems capable of communicating with each other over the air using the AX.25 protocols running at 300, 1200 or 9600 baud.
The radio side has connections for transmit and receive audio as well as the PTT (press to talk). The TNC also has a 12 volt power input and an async serial data connection allowing it to communicate with a computer (and human).
When operating packet with VHF radios, most use 1200 baud because it works well with voice radios and is a relatively forgiving data mode.
Since packet radio is an old digital mode, it's hardware is often found at hamfest flea markets. Some common brands were Kantronics, MFJ, Pacomm and AEA.
Because the TNC's are often 30 years old, they sometimes need some TLC to use them again.
My point here is your mileage may vary.
Another option that can work well is a virtual software TNC running on your computer.
This method uses the computer's sound card to listen for and emit the 1200 baud modem audio (packet racket the old timers called it).
While there are likely options for every computer type, I'll discuss a popular Windows program suite.
A Ukrainian Ham named Andre UZ7HO has created and maintained a popular packet radio program who's latest version is called soundmodem114.
He also has a terminal program called easyterm49 that operates seamlessly with soundmodem running.
The terminal program provides the user a TNC like interface that is functional and uncluttered while soundmodem is like the packet "engine under the hood".
Installing:
Andre's packet radio TNC webpage:
The 3 files needed:
soundmodem114.zip
easyterm49.zip
user_guide114_EN.zip
I created a sub directory named /packet which holds the unzipped programs.
Unzip the downloaded files to your new directory.
Make desktop shortcuts for the programs.
The user_guide114_EN.zip is a very handy .pdf document.
Receiving packets:
The following assumes your computer soundcard's MIC input is connected to a 2 meter FM radio's audio out. This usually takes a 1/8" stereo connector patch or jumper cord.
Set the frequency to a packet channel like 145.670 (local node WILMAR) or 144.390 APRS. You'll want some activity to see things work.
First start the soundmodem114 program. Then start the easyterm49 program.
The unplugged volume should be adjusted low audible with squelch open. When the program is running and it's hearing the radio's open squelch, the effects should be visible on the waterfall display.
An outdoor antenna will likely yield much more packet activity than one indoors, which will likely only hear the local node WILMAR.
When a packet is decoded you will see it a a lot of info on the soundmodem screen, while the easyterm screen is much more like what you would see with a real TNC.
Once you get it working, one of the fun things to do is listen in on the International Space Station (ISS) when they pass overhead several times a day on 145.825 MHz.
You might even hear me use them as a APRS digipeater, sending the world my location.
73
Mike N0QBH
N0QBH Mike Berg