[aprssig] HF options
jimlux at earthlink.net
Wed Dec 19 14:57:19 EST 2007
ate: Wed, 19 Dec 2007 09:34:07 -0800
From: Scott Miller <scott at opentrac.org>
Subject: [aprssig] HF options
To: TAPR APRS Mailing List <aprssig at lists.tapr.org>
Message-ID: <4769560F.4000203 at opentrac.org>
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I've had an inquiry about a project that would involve linking fixed
APRS stations at intervals of maybe 50 km, with not much chance of
digipeaters. Without favorable terrain, that seems a little doubtful
for direct VHF.
I've done very little HF myself in the past decade, and I've never
worked HF for local (relatively speaking) communications. Can anyone
suggest bands and equipment that would work well for this, assuming 300
baud AFSK is used? Power consumption is a major concern.
What about 6 meters? You can have pretty rough terrain and still get through (The Forest Service uses the 30-88 low VHG band up in the Sierras, for instance)
here are some problems with HF (aside from propagation, about which more later)..
Classic HF packet using AFSK requires very good frequency control on both ends of the link (the assumption was that someone would be sitting in front of the rig to spin the knob, since when HF packet originated, the radios weren't all that precise). 1ppm (which is quite tight) at 15 MHz is 15 Hz, which is enough to screw up your 200 Hz shift AFSK. (typical TCXOs might be 0.2ppm to 2ppm, with 2ppm/yr aging... the 0.2ppm units will be fairly pricey). There are some schemes using a poorman's oven (an appropriate thermistor soldered to the top of the oscillator can, fed from a regulated DC supply).
There's also the ionosphere to contend with (typically, 1-2 Hz doppler shift, and fairly pronounced frequency selective fading that wanders around)
So... how much processor power can you use? One of the multitone signalling schemes where you grab some samples, FFT it, find the current offset, and decode it might be a good way to go.
Is your power limit a peak power or an average power limit. Have you considered a low duty cycle burst scheme? You almost certainly want to use some sort of Class D or Class F PA, which pushes you towards some form of PSK or FSK, or a gang of parallel Tx's each running saturated (like OFDM). Pretty easy to get a peak power of tens of watts with modern IGBTs or FETs.
Now to propagation...
You could go low, and use NVIS. Say, 40m or lower? They're fixed stations, so having a fairly large antenna might not be a problem. 50km could be even "ground wave" on 160m. Can you change frequencies and use some sort of sounding/ALE scheme, or even a pre-planned band change according to the time of day?
There's some issues you'll need to address with respect to automated stations on HF, too.
I guess it comes down to how often do you need to transmit, how much data to send, and whether you've got enough storage to ride through an outage.
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