Sunday, September 21, 2008

APRS station: shopping list

So the more I think about it the more I'm thinking that an APRS station is going to be this winter's project.

Initially I'm just going to listen. The next step is going to be transmitting WX info from our Oregon Scientific PWS. My location won't allow me a very high antenna, but we're a mile and a half from the Tri-State (at closest approach) and so we ought to be able to hear at least some data from mobiles.

APRS is a variation on AX.25 packet, and historically APRS stations have used modified TNCs to do their work. However, I'm not really interested in buying a TNC, and thankfully I really don't need to. It's perfectly feasible, from what I'm reading, to use a computer for this role, using the computer's soundcard to process the signal from the radio, and software to demodulate the "modem noise" into digital signals. Computers are more expensive than TNCs, but I have several spare computers and no spare TNCs, so computer is the direction we'll be going. "Soundmodem" software already exists for Linux, so that's an easy step.

I already have a radio, an ADI-146 that I picked up cheap off eBay a while back. This is a 50 watt 2m mobile rig, and was sold to me without a microphone. No big deal, don't need a microphone for this anyway as the rig controller will replace the microphone anyway. As a mobile rig, the transmitter has no internal power supply, so I'll also need a 12V (or 13.8V) supply. To round things out, I'll also need an antenna and feedline to connect the antenna to the radio. For good measure, it would probably be a good idea to get a VHF SWR meter before transmitting with this rig at 50W; don't want to blow out the finals, after all. Also, most commercial 2m antennas are tuned to peak around 146 MHz; APRS uses 144.390, which is pretty far downband from the 146 center point, and with most commercial antennas likely to require retuning to get a good match. For that I will need either an analyzer or a SWR meter.

So, this means I need to buy the following:
  • Antenna (I'm thinking something like the Diamond F22A, priced at $89.95)
  • Feedline (about 60 feet required with prices that range between 30 cents and $1.20 a foot, plus connectors and short cables, for a total of between $50 and $125)
  • Rig controller (the Rigblaster Plus at $149.95 should suffice)
  • Power supply (probably somewhere around $100)
  • KVM switch (around $30)
  • SWR meter (MFJ has a cheap one for $15.95)
That puts the total budget in the range of $440 to $515, with probably another $25 for incidentals.  Maybe I can find some cheaper alternatives or get some stuff used or something; I'm not sure I'm ready to spend that much right now.