[OZAPRS] Antenna for HF-APRS...

[Gordon Taylor]

Terry,

                There are plenty of ways to mount antennas on Pajeros, I
have 5 on mine and two of them are HF.

 

I have a Terlin base and spring and an old RFDS whip retuned with some
alfoil and heatshrink to get it from 9.995MHz to 10.14670 MHz, this is
mounted on the bulbar.

 

The other HF antenna is a Sierra Sidekick screwdriver mounted on a Terlin
spring on a custom mount on the back door that attaches via the mounts for
the spare tyre cover swing frame.

 

I can send you some pictures if you like.

 

Yes newer cars take some ingenuity but it can be done.

 

Regards

 

Gordon  VK4VP

 

From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Terry, VK5ATN
Sent: Sunday, 1 March 2015 20:18
To: ozaprs at aprs.net.au
Subject: [OZAPRS] Antenna for HF-APRS...

 


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). 

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.

In the early days I used a system of home brew top loaded antennas with
resonators / loading coils based on the Scalar system.  Here
<http://www.users.on.net/%7Etfneumann/Amateur/Magna/Magna.html>  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.

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.

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.

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.

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.

HF APRS is good fun.   Too bad I don't see myself doing much of it in the
future   :-(  

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.      

73
Terry
VK5ATN






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