[aprssig] OT:: Jeep and Radio

[Stephen H. Smith]

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]
Home Page:          http://wa8lmf.net

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