[OZAPRS] UHF APRS Channel - 434 or 439

[Chris Hill]

Hello All,

I have recently purchased a 70cm LIPD transceiver, which has prompted me
to
look at the frequency assignments (LIPDs vs Amateur).

This is summarised on the WA VHF Group's site at
http://vhf.worldsbest.com.au/LIPD.htm

In the above summary, I have shown 434.175MHz as "Packet - APRS".

Personally, I think that 434.175MHz should be denoted as the Australian
UHF
APRS frequency, on a national basis...  either for simplex use, or as the
uplink to a bit-regenerative repeater.

Any comments?

(Also, can anyone provide any additional info on frequency usage in
Australia on the 433.075MHz to 434.775MHz sub-band?)


Regards,



Chris vk6kch





-----Original Message-----
From: ozaprs-admin at marconi.ics.mq.edu.au
[mailto:ozaprs-admin at marconi.ics.mq.edu.au]On Behalf Of Grant Willis
(VK5ZWI)
Sent: Tuesday, 23 July 2002 9:23 PM
To: ozaprs at marconi.ics.mq.edu.au
Subject: [OZAPRS] UHF APRS Channel - 434 or 439


Hi Folks,

Only recently subscribed but have read the archived digests.

There is merit in 439 and 434 approaches. However I favour putting
it somewhere on 434. Here is my reasoning:

(a) Packet transmitters make the channel unatractive to LIPDs - they will
go away :-)
(b) Packet repeaters with DCD-State machine Squelches (ie bit stream
detecting rather
than noise or audio detecting) will be immune to lockout by LIPD
transmitters - ie they will
transmit over the top of them if they hear them

The only remaining argument with (b) is that then stations accessing
repeaters etc need to be able
to get enough signal into the repeater to overcome the LIPD. From what I
have seen of high-site
voice repeater LIPD interference, the EIRP reaching the repeater is not
very high - but it is enough
to open squelches and keep them open. Most users when they transmitted
over
the top could
override the interference. This does limit the range a little that a
single
high site repeater could
work over, but with other home stations on channel able to act as relays -
the signal level problem
will go away to some extent in the uplink to a WIDE repeater, and if the
WIDE high-site APRS
repeater runs reasonable power (most single regional WIDEs should run
probably 25-50W at least
to give APRS-Messaging a chance) then interference into the roaming
mobiles
will be limited also,
particularly as they are mobile, and in a mobile environment LIPDs do only
reach 200-500m due
to ground clutter. (This clutter is lost if you go ground to high site -
as
there are reduced quantities
of local obstructions).

I can also demonstrate that LIPDs dont seem to be bothering other 434
packet networks.
Here in Adelaide, we have a 4800 baud noise-gate squelch based inter BBS
network sitting
on 434.050 - and we know that locally we can hear LIPDs on 434-434.1 MHZ.
The BBS
network doesnt seem to have blinked and it is still keeping on forwarding.

So - I would still recommend 434.175 be chosen as the national APRS 70cm
channel.

Incidently, there is a case for APRS using 70cm in a mobile environment
more-so than 2m!

2m is very noise prone - from ignition noise, and other man made noise
sources - far more
so than 70cm. Packet is particularly susseptible to noise - as noise
generates bit-errors on
receive. So you might actually find that 70cm in urban environments (ie
cities) is a better
choice for running APRS mobile and in particular APRS messaging than 2m.
Of
course,
the argument is reversed when you get out into regional areas with
reeaters
many tens or
even hundreds of km away - where the lower path loss on 2m wins the day -
and the environments
are generally quieter. I have moved most of my mobile voice activity to
70cm in part due to
this reason (and because I got sick of the endless chatter on the main 2m
repeater here :?)

Something for you all to think about.

Regards,
Grant VK5ZWI



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