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Strange that I should writing about this - I've probably made my
last transmission on HF APRS. The good old ute has been sold,
something new and complicated is coming in its place (Pajero), with
no clear options for mounting antennas of any sort (That's still to
be confirmed, but the outlook is not good). <br>
<br>
So what did I use? A couple of different antennas - both home brew
- as much as I could. I have more or less standardised on the
Terlin base / mounting spring combination using 1/2" UNC threaded
antennas.<br>
<br>
In the early days I used a system of home brew top loaded antennas
with resonators / loading coils based on the Scalar system. <a
href="http://www.users.on.net/%7Etfneumann/Amateur/Magna/Magna.html">Here</a>
is an example from the past concentrating on the antenna base
rather than the antenna itself, but perhaps you can get the idea. I
have a full set of loading coils from 160 through to 10 metres
including some of the RFDS frequencies. Top load is good from the
RF side, but there can be mechanical complications.<br>
<br>
More recently I had used a DSE / Mobile One no compromise, high end
helical antenna originally intended for 27Mhz. I scored two of
these at a ridiculous price at a time when DSE was quitting stock.
Happily they both used 1/2" UNC bases and indeed came with the bases
as well. A further enhancement was that they had adjustable tuning
tips at the pointy end - top quality antennas at a bargain price.
I stripped the heatshrink and the windings, and rewound one for 40
metres and another for 20. They have both performed in a very
satisfactory manner on the bullbar of the ute, and generally I've
enjoyed excellent results on APRS using a Tiny Track with an IC-706
running around 50 watts.<br>
<br>
I've also made up a HF helical using a commercial fibreglass
electric fence space about 1.5 metres long - in this instance for
40 metres. I incorporated an adjustable tuning tip in this one as
well. Originally made for tractor usage, it's also been tested on
the ute with good results both radio wise and in the mechanical
integrity aspect.<br>
<br>
All of this assumes that one has access to a lathe and supplies of
hexagonal brass - and you like doing this sort of thing (I do).
Otherwise it's something commercial.<br>
<br>
Apart from the RF considerations it's extremely important that the
mechanical integrity of the entire antenna installation is 100% at
likely highway speeds. The wind forces on the antenna are
impressive, and keep in mind that if you are driving into the wind,
the force becomes the sum of ground and wind speeds (110 kph vehicle
speed + 40 kph head wind = 150 kph force on the antenna). There is
an obligation not to start shedding bits of antenna at following
traffic, or at yourself if it's mounted at the front of the vehicle.<br>
<br>
HF APRS is good fun. Too bad I don't see myself doing much of it
in the future <span class="moz-smiley-s2"><span> :-( </span></span>
<br>
<br>
Mind you it wouldn't work at all if it wasn't for dedicated amateurs
who offer their stations as Igates for the rest of us to enjoy.
Thanks everyone whose facilities I enjoyed over the years; it was
greatly appreciated. <br>
<br>
73<br>
Terry<br>
VK5ATN<br>
<br>
<br>
<br>
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