[OZAPRS] Higher beacon rate HF versus VHF?

[Richard Hoskin]

Hi All,

Remember on HF you are sharing this band with all other APRS users world
wide and even though you can not hear them they may hear you. (You just have
to look at some of the posts on this list from HF APRS operators in other
countries for evidence of this). Also 300 baud is 1/4 the speed of VHF APRS
which means you can only fit 25% of the stations on the channel if you use
the same beacon rate.

Lets see, HF APRS increases the coverage area from roughly 200,000 Sq Km
using 3 hops to potentially 75 Million SQ KM. An increase in coverage area
of 375 times. At 300 baud you get a reduction in band capacity of 4 times. 

Leaving your beacon rate the same as for VHF or increasing it some what
really doesn't make a hole lot of sense.


That's why the world wide recommended minimum APRS beacon rate for HF is 10
minutes or more.

Or another way to look at it; an average length APRS Posit packet takes 3 to
4 seconds to transmit on HF. Assuming a channel efficiency of 30% for ax25
this results in a maximum of 7 stations being able to transmit per minute.
At a transmission rate of one posit per 10 minutes the maximum number of
stations that can be in a single APRS Gate's coverage area is 70 stations.
This will be less as the Net stations transmit frequently and messages
and/or tuning may be under way on the channel. HF Propagation is also an
important factor.

Cheers
Richard
VK3JFK

> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of Jack Chomley
> Sent: Monday, 30 March 2009 6:45 AM
> To: Australian APRS Users
> Subject: Re: [OZAPRS] Higher beacon rate HF versus VHF?
> 
> At 11:00 PM 3/29/2009, you wrote:
> >Jacks success at being received by my HF iGate with 600mW, complaints
> >about QRM etc got me thinking about beacon rates for HF vs VHF.
> >
> >Given the low number of HF stations, the vastness of Australia, the
> >vagaries of HF and the fact that HF stations do not digipeat to HF,
> >and perhaps if I go on, shouldn't digipeat to VHF, I believe beacon
> >rates on HF should be higher than on VHF. What do others think?
> >
> >David
> >VK4MDX
> 
> David,
> I have been using a beacon rate of 2 minutes on my motorcycle system
> running 6 watts on 30m with an Icom 703, SCS DSP TNC, Garmin
> 16-HVS  GPS.  The performance has been good over the last 12 months,
> many of the beacons were missed, but I put that down to the low power
> and antenna waving around while mobile.
> Back in January, I changed the transmit format of the APRS data
> frames to the compressed format and my "hit" rate immediately went
> up. Probably because of the shorter packets, meant less chance of
> them being corrupted.
> See my Jan 15th '09 track log....
> http://aprs.fi/?call=VK4JRC-15&dt=1231977600&mt=m&z=11&timerange=3600
> Since the hit rate has gone up, I can now lower the beacon rate and
> still get a good track record.
> Bike here:
> http://www.radiotelemetry.net/html/hf_mobile_aprs.html
> Personally, I think that IF there are enough IGATES, there is
> probably no reason to have a high beacon rate, nor need to run
> anymore than about 20 watts, but the secret of success is to use
> compressed format APRS data.
> Yesterdays test also proved that when there was QRM on the frequency,
> I managed to get some hits in between the contest stations, that were
> just off the 40m frequency. Shorter frames, less chance of corruption ;-)
> My antenna for yesterday's test was a Buddipole vertical, setup on an
> 8ft tripod, in the back yard, the counterpoise was 1 electric fence
> wire above ground radial, deployed towards the South. The vertical
> element a Versa-T,  1 arm section, coil, with 5 section shock-corded
> military whip, on top of the coil.
> See Buddipole:
> http://www.buddipole.com/
> 
> 73  Jack VK4JRC
> 
> 
> 
> 
> 
> _______________________________________________
> Ozaprs mailing list
> Ozaprs at aprs.net.au
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