Thursday, September 21, 2017


APRS can be a valuable tool to establish what resources are around you. But what if you aren't hearing very many packets or you're not getting digipeated on VHF? APRS via HF may be a good work around.

I've spent a lot of time working with APRS on 144.390 MHz VHF. It's a good tool for figuring out what kind of ham radio resources are around you. The whole purpose of the system is so you can sit in an area for about 15 minutes decoding packets and get an idea of what the local repeaters are, where other hams are around you, get weather data, along with a bunch of other useful information.

Even though the APRS network is vast, occasionally you may find yourself in an area where you don't hear much activity and/or cannot get digipeated. Don't fret, if you have access to an HF transceiver, you can jump on APRS at 10.151 MHz.

If you're paying attention, you might have noticed that 10.151 is not a ham frequency. That's because, according to official documentation, you are supposed to use LSB which will put the mark/space falling inside the 30 meter band. I never liked that idea. In fact most ham transceivers will not allow you transmit at that frequency even though technically you would be inside the ham band. So, I deiced to try it a different way.

Using the good old UZ7HO SoundModem and my EasyDigi sound card interface, you can easily setup APRS on HF without using the lower side band trick.
Select AX.25 300db
You will notice at the top of the SoundModem window, there is a drop down menu with various emission types and speeds. The default is AFSK AX.25 1200bd at 1700, which is the correct setting for VHF. All you need to do is select AFSK AX.25 300bd at 1700 instead. The tricky part is tuning in your radio to just the right spot. I've found that 10.147.55 MHz USB is the perfect spot to place the AFSK signal at the 1700 center mark inside the software. (This may vary slightly by radio.)

You don't necessarily have to do it that way, I like keeping the center mark location at 1700 in the software since I switch back and forth between VHF and HF APRS. However, you can tune to 10.148 and wait for a packet to show up on the waterfall, then click that location on the waterfall. This will cause the software to listen and transmit at that location.
SoundModem Decoding Properly

Watching the software for a while, you should begin seeing decoded packets in the main window. If not, the center mark location is probably not set correctly and should be reset.
APRSIS/32 Connected to SoundModem

Once the SoundModem software is setup and decoding packets, you can setup your favorite APRS software to connect to the UZ7HO software as a TCP KISS TNC. You will want to make sure that you don't set a path and you're beacon is set for a longer duration. (Don't try to use HF APRS for turn by turn tracking. You won't make many friends.)

Experimenting on HF, I've found that it's a little different. Since you are on HF a single hop for your packet could be thousands of miles, so you don't need a digipeater at all. Chances are an iGate will hear your packet and forward it on to the internet side of APRS with little issue. Since the 300bd on HF is a lot slower than the 1200bd of VHF, you will only want to beacon every 30 minutes or so unless you are trying to convey more information. Even still, you will want to limit how frequently you send packets so you don't "hog" the frequency.

HF is a completely different system from the usual VHF APRS network. If you are listening to HF traffic, you will only see the stations that are also on HF. In order for your packets to appear on the VHF network, an iGate that has the ability to transmit must be setup to forward nearby HF stations that show up on the internet side and beacon them as objects to local VHF stations. Unfortunately there isn't an iGate setup with this capability near me. (Perhaps this could be a good future project.)

Even though APRS on HF is much slower, I've found it to be very reliable. You can use the same features on HF as you do on VHF. A good example is the ability to send email messages. The only downside is you shouldn't expect a response as traffic originated from the internet is not supposed to traverse the HF APRS network. (If you need two way messaging, you should probably use Winlink.)

APRS on HF is another tool to put in your ham toolbox. I think it's something that could be used more and it would be nice to see more of an interconnect between the HF and VHF networks.

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