[aprssig] OT:: Jeep and Radio

Nagi Punyamurthula n0agi at n0agi.com
Mon Sep 14 14:56:13 EDT 2015

Thanks for the notes below.  
I was planning on machining a wider hole for the pl-259 should the stock port is too small.  Looks like I will be exploring the wider hole on this bracket.  On the note of the "blockade" for a circular pattern.  I was thinking thru that hurdle.  I will give the behind the spare wheel route a shot and want to see the performance before I drilled holes on my jeep's hood area or the side metal panel  (the roof is fiber glass - so that is not an option as you note below).


-----Original Message-----
From: Stephen H. Smith [mailto:wa8lmf2 at aol.com] 
Sent: Saturday, September 12, 2015 2:43 PM
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]
Home Page:          http://wa8lmf.net

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