[aprssig] OT:: Jeep and Radio

[Andrew P.]

Watch out for those PL259 feedthroughs. The threads are very fine and the nuts don't spread out very far. I tried this with a mirror-arm mount bracket on my motor home, and it didn't take very many whacks of the antenna against low-hanging tree branches to totally strip the threads on the nuts and barrel connector. Worse yet, I was using an NMO-to-SO239 barrel, which is more expensive than a plain SO239-to-SO239 barrel (though the NMO base was bigger than the nut on the other side). Wish they made the barrels with bigger threads and wider nuts that wouldn't torque off so easily.
Andrew, KA2DDO also known as "Mooserider"

-------- Original message --------
From: "Stephen H. Smith via aprssig" <aprssig at tapr.org>
Date: 09/12/2015  12:48 PM  (GMT-08:00)
To: Nagi Punyamurthula <n0agi at n0agi.com>, TAPR APRS Mailing List <aprssig at tapr.org>
Subject: Re: [aprssig] OT:: Jeep and Radio

On 9/12/2015 11:32 AM, Nagi Punyamurthula via aprssig wrote:
> Team
> I had recently acquired a Jeep and exploring installing my Kenwood d710g in
> it.  Mstly for APRS and voice repeaters on 2m/70cm
>
> Wanted to see if folks here have any jeep install pictures you might be able to
> share for ideas.
>
> The bracket I'm zoning in on is at extremeterrain website
> http://www.extremeterrain.com/rugged-ridge-cb-antenna-jk-1150389.html
>
> While this is meant for CB, I think it can be made to work for 259s
>

Most CB antennas use HF-antenna-style 3/8"-24 threaded studs for mounting; not
PL-259- or NMO-style mounts like most VHF/UHF antennas.

It would appear (judging from the not-very-clear pictures on the site you
linked) that this bracket is intended for a fitting that has a 3/8-24
inside-threaded "hole" to accept the male-studded whip on one end, and a 3/16"
or 1/" inch hole to accept a matching short screw and ring terminal (from the
coax center conductor) on the other.  The entire fitting is "RF hot". It is
mounted through the hole in the bracket with sandwiched nylon shoulder washers
to keep it from touching the vehicle body/ground.

To use it with VHF/UHF antennas, you would have to acquire a 2M/70CM antenna
with a PC-259 base (not the more common NMO).  Then you would have insert a
PL-259 female-to-female "barrel plug" (i.e. coax-to-coax "splicer") through the
hole in the bracket. You would screw the antenna to the top end of the barrel
plug and a coax line with a PL-259 plug to the bottom end.  Make sure your
barrel plug comes with a pair of hex nuts around it's outer circumference --
not all do.

However, since a barrel plug has a 5/8" outside diameter while the normal hole
for a shoulder-washer insulator mount is only 1/2", you would have to drill or
ream the hole in the bracket to enlarge it.

Even after all this, you will get a lousy lop-sided radiation pattern since
about half of the VHF/UHF whip's height will be blocked by the vehicle body,
and the spare tire.  In the forward and backwards directions, you will have far
less radiation than off the sides or backwards.     (The much taller CB whip
has most of it's length above the roof.) Signals from the sides that were
usable initially will take a 10-15 dB dive when you make a right-angle turn, so
that the front or rear of the vehicle are now facing them.

To get a reasonably circular antenna pattern (i.e. same coverage in all
directions), the vast majority of the antennas's length MUST be ABOVE the roof
line.

The best location will be a permanent or magnetic mount in the center of the
roof.  If the roof is non-metallic (i.e. fiberglas), you will need to use a
no-ground-plane-required  end-fed  half-wave  design like the Diamond 770
series.    (The much more common 1/4-wave or 5/8ths-wave-type antennas MUST
have a substantial horizontal metallic surface underneath them to work properly.)

_________________________________________________

--

Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
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