[OZAPRS] HF APRS

[Warren VK3BYD]

Hi All,

Given the resent discussion on this list regarding HF APRS I have put
together some notes based on my experiences during the setting up a mobile
HF APRS station.
I don't claim this to be the be all and end all but hope that it may help
others who wish to try HF APRS.


Gatways; VK3MY-3 & VK4DMI are on 10Mhz, VK3MY-4 is on 7MHz. Any refrences
to
VK3MY-4 apply equally to VK3MY-3.

Frequency; the frequency displayed by your radio when successfully
decoding
data is dependent on the tone frequencies used by the tnc. For example my
MFJ1278 uses 2100 & 2300 Hz for hf packet, while my MFJ 1270 tnc uses 1600
&
1800 Hz. So when I swap between these tnc's I need to retune my radio by
500Hz to successfully decode and connect to VK3MY-4. I use VK3MY-4 as the
reference frequency for all HF APRS tuning and would recommend all other
HF
APRS stations to do the same. This will guarantee your station's
transmissions will have the best chance of being received by VK3MY-4  (and
VK4DMI) and sent to VHF and the Internet servers.

Any offset between Tx and Rx tones of the tnc or Tx & Rx frequency in the
radio will also affect the performance. My tnc's have a window of about
+/-
70Hz in which they will decode packets.

I use an xtal locked rig; the xtal will still drift a little when heated
and
cool within the car, however by using a tnc with a tuning indicator I can
fine tune the radio with its clarifier as required, it also helps to have
the ability to display the data being received by the hf radio.

The length of any packet sent will affect its chance of getting through.
This is often the case with the three beacons that VK3MY-4 transmits every
10 minutes. One of the three beacons is much shorter than the others and I
decoded this much more often than the longer beacons. So I keep my
transmitted posits as short as possible to increase the reliability of
being
heard by the Gateway.

Channel load; like with VHF other signals will affect the ability to get
the
data through. Just below the 40mx gateway on 7039 KHz are some VK2 bbs
stations, although these station are 500Hz below the gateway their signal
can often be strong enough to "squash" my mobile station.

My current mobile station consists of an ex-commercial HF rig with xtals
for
7039 & 10149, Kantronics KAM Plus tnc & homebuilt tracker / gps interface
which sends the posits via the serial port of the kam plus tnc . This
arrangement gives me simultaneous hf and vhf mobile tx and rx.

Cheers
Warren

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