[aprssig] HF options

Stephen H. Smith wa8lmf2 at aol.com
Wed Dec 19 13:15:53 EST 2007

Scott Miller wrote:
> 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.
> Thanks,

To operate this close-in on HF is going to require radiating nearly 
straight up (i.e. NVIS - Near Vertical Incidence Skywave)   In turn, 
this implies operating on 1.8 MHz (160M) or 3.6 MHz (80M). [The higher 
bands won't reflect back to earth at high angles; they just head off 
into space.] 

Actually probably 5 Mhz ("60M") would probably be the ideal band, except 
that we can only run USB voice on the five fixed channels in this band, 
not data.  :(

NVIS is a relatively "lossy" mode meaning you have to run fair amounts 
of power to achieve a reasonable S/N and data error rate. (The lower HF 
spectrum tends to be noisy.)  This means typical full-power 100W HF/SSB 
transceivers with reasonably efficient antennas. This would typically be 
full-sized dipoles (130 ft end-to-end),  inverted-Vs or large horizontal 
loops close to the ground (ideally 10-20 feet up), ideally over a 
reflecting ground mat.   No 9-foot loaded mobile whips here!   

The 100W out (approximately 200W DC in) transceivers wouldn't 
necessarily  be a deal breaker, watt-hour-wise if you use short MIc-E 
formatted bursts at an infrequent TX rate.

Note that optimal TNCs on HF are not just VHF modems running at 1/4 the 
speed. Normally you want substantial audio conditioning and band-pass 
filtering before the actual modem chip.  For the best results, you will 
probably want to install 500-800 Hz bandwidth "CW" or "RTTY" IF filters 
in the transceivers.  

Further, for this kind of application (data operation at low S/N) the 
digital signal processing IF and audio filtering found in recent HF 
transceivers (that "de-noises" and "de-hisses" SSB voice operation) is 
usually less desirable for data, than the classic passive IF crystal or 
mechanical filtering (and analog audio filtering).    [The DSPs in 
low-end ham gear tend to create phase jitter that mangles data.]


Stephen H. Smith    wa8lmf (at) aol.com
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