James wrote:
> I plan to install a 2 meter radio in my 1980 CJ5 Jeep (softtop). My
> plan
> is to construct a mount that will attach to my rear tire mount. Because
> this will be a custom mount, I can have the base of the antenna even
with
> the top of the spare tire, or I can elevate it some for greater height.
I
> use this Jeep only in mountain areas. Here are some of my questions:
>
> 1. What is the "best" 2 meter antenna type for me (I don't need a dual
> bander. I will only use 2 meters). I know that a 5/8 whip will
> have the best gain, but I have seen some comments that a 1/4 wave may
work
> better
> in the mountains, because of the angles involved. Yet, I know that a
> 1/4 wave needs a good ground plane, which a softop probably will not do
> very
> well. It would seem that with a 1/4 wave, I would want the base of the
> antenna at or near the top of the Jeep. I am also aware of a colinear
> antenna, but that would be quite tall for me !!
Based on mounting a lot of two-way radios in logging trucks in my younger
years, I suggest mechanical strength is a prime consideration, and that if
you are driving on unpaved roads the radiation pattern will vary anyway
from bouncing around on the roads.
I suggest a Larsen 5/8 wave antenna in either NMO or 3/8 in hole mounts.
Personally I prefer the NMO, but you need a 3/4 in hole saw unless you use
one of the available "L" brackets. (Which come in both variants). I
suggest
the Larsen because it's tough. I know at least one logger who was still
able to call for help from his overturned pickup with a Larsen on the
roof.
As for ground plane, you can probably mount it to the spare wheel bracket
or
someplace similar. And while theory says you need to put it in the centre
of a metal roof, you'll be able to communicate very effectively with all
sorts of non-optimum placements. Make it mechanically secure, and out of
the way of opening doors, hatches, wind****elds or hoods. Keep in mind the
higher up you put it the more trouble you have with the whip hitting
things
overhead, like garage lightbulbs.
The other good thing is that the Larsens are easy to unscrew for going
through the car wash after a muddy expedition! And the metal in the
antenna
whip is flexible enough you don't need an antenna spring that can come
loose on rough roads.
>
> 2. Is it best to have the antenna element fully above the top of the
top
> of the Jeep ?
>
> 3. If I use a spring to offer some protection for the whip, doesn't the
> spring add to the length of the antenna, and I would have to tune the
whip
> for resonance ?
>
> 4. What is the best (maybe strongest?) mount type ? I am accustomed
> to
> the 3/8" - 24 type stud mount. It seems strong to me. What are
the
> advantages of the NMO and other mount types for my purpose ?
There's no need to go to the weight and heft of a 3/8 24 mount - it's
really
designed for heavy (and huge) HF antennas. Either the 3/8 snap in or NMO
mounts work fine for the smaller, lighter VHF antennas. The NMOs also are
very low profile when the antenna itself is unscrewed, and both 1/4 wave
and 5/8 wave antennas can be interchanged with the same style mount.
I have no connection with Larsen other than using them for the past twenty
years and more, including all my vehicles.
Bob, VE7HS
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