Last February I got my Technician class amateur radio license. Why? Because I became really interested in high altitude ballooning. After looking at those pictures how could anyone NOT become interested? The hobby consists of launched a weather balloon up in excess of 90,000 feet. The balloon carries a payload that contains a GPS unit for tracking, and a radio to transmit the GPS coordinates back down to the ground, among other things. Another device called a Terminal Node Controller (TNC) takes the GPS data and turns it into an audio signal to send over the radio. The TNC also presses the Push to Talk (PTT) switch on the radio before sending the audio. Essentially a TNC is a radio modem. It takes digital data and turns it into audio to transmit through the air. TNC’s can allso to the reverse, and take an audio signal input from a radio to turn it back into digital data. Without this technology, high altitude ballooning wouldn’t be possible. Why send up a balloon to take photos if you can’t recover the payload?
This got me thinking. You can send GPS data over radio, so why can’t you send other data over radio? Well, you can. In fact, people have been doing this for a LONG time. My problem was, TNC’s are expensive. Several hundred dollars expensive. After some Googling, it found out that you can actually use your computer’s sound card as a modem. Makes sense right? There is software called soundmodem that runs on multiple operating systems that does just this. It creates a virtual KISS TNC interface in your system for use with packet radio programs. All you really need is some audio cables, and a PTT interface. Luckily, a serial PTT interface is really easy to build.
I did some testing yesterday morning with packet radio using a manual switch. To make the switch, I just cut the ground wire of a 1/8″ stereo audio cable and soldered a momentary pushbutton switch to it. When I want to send audio out of my radio from the computer, I just hold the switch down manually and set the packet radio software to transmit. It took some fiddling, but I finally got it working. I transmitted from my PC running a virtual Ubuntu install through one radio. The signal went through the air and out the microphone jack of a second radio plugged into my laptop running Windows XP. I sent the message “KE7SAL testing testing” and the laptop recieved it. There were some extra garbage characters before and after the message, but none in between. It was pretty awesome. The software I was running is called fldigi. It runs on both Windows and Linux and worked really well. At first I tried transmitting with fldigi and receiving with digipan, but digipan could not pick up the signal properly. I tried tweaking settings but to no avail. Once I switched to using the same program for send and receive it worked flawlessly.
Now that I can send data to myself, I want to send data to somewhere else! Although it’s cool knowing that I can do it, it’s just not as cool as actually using it for a purpose. Until some friends of mine get set up with packet radio gear I will need to find another way to go about this. That’s where a BBS comes in. A radio BBS is kind of like an old telephone computer BBS. You can “dial” into it with your radio and a computer and send/receive messages. Most BBS’s now adays are also connected to the Internet so you can send and receive emails via radio. There are a few in my city, although I’m not sure I will be able to hit them from my apartment. Maybe if I use an external antenna and place it in a good location outside. Regardless, I’m going to start getting my computer’s set up to deal with BBS’s this week. Here is a breakdown of the steps as far as I can tell:
1) Make sure the AX.25 protocol is enabled in my kernel. This is the network protocol that packet radio uses to communicate with BBS’s among other things.
2) Make sure KISS mode TNC’s are enabled in the kernel.
3) Install soundmodem.
4) Configure soundmodem using the soundmodemconfig utility.
5) Install jnos2. jnos2 is a packet BBS software. I believe it can be used as a BBS client or as a BBS server. This could prove to be fun to experiment with later on.
6) Configure jnos2 to use the soundmodem interface
7) Build the PTT interface.
That should be about it… I think. After all of that is done I believe I should be ready to communicate with BBS’s , or just communicate using the AX.25 protocol in general. If I get a friend to run jnos2 we should be able to send/receive email with each other via radio whenever. I’m getting pretty psyched about this for some reason. There’s just something cool about knowing you can transmit any data you want all over the place wirelessly without requiring any type of phone, cable, internet, etc service. All you need is electricity!