[aprssig] HF options - Further Thoughts

[Stephen H. Smith]

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.
>


Just had some further thoughts.  Do these stations need to be linked in 
a linear fashion (A---B---C) talking to each other, or is it just that 
each station needs to get what it hears to some common distant point; 
i.e. command post, net control or igate?


If the latter, then an alternate approach would be to use the 10 Mhz "30 
meter" band.  30M is open for hops of 100-500 miles nearly 24 hours a 
day, and  has very little interference on it since no voice ops are 
allowed on the band.  

The antennas become more reasonably-sized. (A full-sized 1/2-wave dipole 
is only 46 feet end-to-end, and a mini-dipole made from two 30M mobile 
whips end-to-end is efficient enough to actually be usable.)   One would 
probably use the same NVIS approach of a horizontal dipole low to the 
ground for high take-off angle, as on the lower bands. 

If the "command post" or "igate" station was placed at least100-150 
miles away from the area of operations, this should work reasonably 
well.    

[Note that the existing APRS HF system operates at the top of this band 
at mark/space freqs of 10.149.2 MHz / 10.149.4 MHz. There is an 
extensive infrastructure of igates and HF<>VHF gateways already in place 
on this frequency that ensures that you can transmit virtually anywhere 
in North America and be seen on the Internet. ]   

However, given the very low data throughput at 300 baud compared to 
1200, and considering that you are occupying the channel for a radius 
that may cover 1/3 of North America at times (depending on propagation), 
you may be better off operating elsewhere in the band with your own 
network. especially if you are going to generate any significant volume 
of traffic.



--

Stephen H. Smith    wa8lmf (at) aol.com
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