[aprssig] APRS on HF

Stephen H. Smith wa8lmf2 at aol.com
Sun Feb 20 06:30:35 EST 2011

On 2/20/2011 12:50 AM, Julian, G4ILO wrote:
> Having run a 30m IGate (G4ILO-1) for several months now I have often
> been the station that gates European HF mobiles to APRS-IS. I
> understand that in areas where there is little VHF coverage (which
> includes much of the UK) a mobile has a better chance of being gated
> if they use HF, even though they will be gated by someone 1,000 miles
> away. So is that the only real purpose of APRS on HF, to gate HF
> mobiles? And if so, why do HF IGates need to transmit?
> Julian, G4ILO
> G4ILO's Shack: www.g4ilo.com

Support of mobiles in remote locations has been the main use of HF APRS in 
North America.

In the American west (and all of Canada more than 100 miles/160KM north of the 
US border), the population density is EXTREMELY LOW and VHF infrastructure very 
sparse.    In the US alone, this is an area approximately ONE THOUSAND MILES 
SQUARE (1600 KM square) in the western one-third of the country.    (Once you 
go more than about 100 miles east in from the Pacific coast, coverage shrinks 
to almost nothing, except along the major Interstate highways and islands of 
coverage in cities that are hundreds of miles apart.)

Turn off the Interstates onto the secondary roads and scenic byways in the 
west, especially in mountainous terrain, and you drive off the edge of the 
world APRS-wise on VHF.     HF coverage covers this mountainous Great Basin 
area (the interior west between the Sierra Nevadas in California and the Rocky 
Mountains of Colorado) very well.    Depending on the time of day, you are 
either propagating eastward to igates in the MidWest and/or the East Coast, or 
westwards to igates on the populous Pacific Coast, or both.

HF APRS is also widely used by pleasure boats in the Caribbean, and off the  
west coast of the US and Mexico, once you get more than about 30 miles from 
shore.   Given the very limited capacity of a 300 baud (or slower) channel, and 
the fact that every transmission occupies the channel for a radius of 500-2000 
miles, depending on propagation,    absolutely the last thing thing you want to 
do is gate VHF traffic onto HF.

About twice a year I drive from Los Angeles, CA to East Lansing, MIchigan (my 
home town where my sister and mother still live) --- a trip of about 2200 
miles/3500 KM each way. For about 600 miles of this trip there is just NO VHF 
coverage at all.

One-way igates break a major feature of  APRS operation:  two-way messaging.

I have carried on extensive two-way messaging from my mobile with the HF APRS 
Messenger application on the last three or four cross-country trips.   Most 
contacts have been RF<-->RF, but I have had exchanges with a station in Los 
Angeles coming out of an igate on the East Coast while I was mobile in the 
southern Utah desert -- an RF haul of over 2000 miles.        The effectiveness 
of PSK63 over weak signal paths is dramatic when working from a mobile in the 
boondocks of the Great Basin.



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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