[OZAPRS] Some HF thoughts

Tue Mar 31 10:22:09 EST 2009

Hi All
	 The net station should set myalias to tune. Then you set your
beacon rate to every 10 seconds and path to tune adjust your TX until
you hear your packet being digied back once you have your self netted
then change the beacon rate and path back to the correct settings. What
I do is find the lowest and highest frequency that the net station
decodes then split the difference and you should be in the middle of the
pass band of the net station this also allows for a bit of TX drift.
Remember that with most TNC's you only have at max 50 Hz to ply with 

Also many rigs the dial will not be correct even my TS430 does not TX
and RX on the same frequency. It's out by 12 hertz you need a really
good signal generator to check the RX even our good HP here at work is
not good enough if its not locked to the 10MHz external reference.
Remember 1ppm at 10MHz is 10Hz 20% of all you have to play with. Then
you also have drift to contend with my Kenwood also drifts about 10Hz as
well with temperature changes these are small but they do matter.

Ideally the net station should have a 0.5ppm TXCO that's why I am not
keen on being a net station as my gear is not really good enough.

Regards Rob Thirkettle  ZL3RX         
Dept  Physics & Astronomy
University of Canterbury     
Private Bag 4800,         
Christchurch 8020,
New Zealand
Tel: +64 3 364 2510
Fax: +64 3 364 2469
mailto:rob.thirkettle at canterbury.ac.nz

-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Geoff
Sent: Tuesday, 31 March 2009 11:47 a.m.
To: 'Australian APRS Users'
Subject: RE: [OZAPRS] Higher beacon rate HF versus VHF?

10 minutes for normal stations, however the PRIMARY NET STATION should
beacon more often, to give everyone else a reference to tune to.
I think we had defined the primary and secondary net stations to beacon
every 5 minutes.

On 30m we already have these established (Richard/Ray please correct me
if
I'm wrong)
Primary = VK3MY-4
Secondary = VK4DMI-4

On 40m and now 20m we need to work out who will be running a 24x7 HF
Gate on
these bands and allocate NET roles accordingly....

Regards,
Geoff

> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au]
On
> Behalf Of Richard Hoskin
> Sent: Monday, 30 March 2009 8:01 PM
> To: 'Australian APRS Users'
> Subject: RE: [OZAPRS] Higher beacon rate HF versus VHF?
> 
> 
> 
> 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
> > http://aprs.net.au/mailman/listinfo/ozaprs
> 
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