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Ed · March 9th 07, 07:24 AM posted to rec.radio.amateur.antenna

wrote in news:1173412553.256780.306530@
8g2000cwh.googlegroups.com:

On Mar 5, 11:40 pm, Ed wrote:
I am wondering how ground loss/resistance is effected by my
having the bottom/feedpoint of the antenna so high off the ground,
not
to mention, I wonder how to calculate or guess what it is at all.

The bigger the vehicle, and the higher the mount, the lower the ground
losses. But the type mount will effect also. IE: mounting directly on
a
roof will almost always be better than mounting on a ladder, with both
at the same height. I've found that the more metal under the base of
the antenna , the better. Mounting on a ladder, thin metal rail,
angle
iron, etc will usually result in inferior performance. But..Sometimes
there is nothing you can do about it if the mounting options are
limited. I ran into this on one of my trucks. At first I mounted the
antenna on a piece of angle iron that ran across the truck bed,
just behind the back window. Both ends were well grounded.
But, I kept seeing overall lousy performance. So I decided to
move it over on top of one the tool boxes, which has a good deal
more metal area under the antenna base. Made a large difference,
and woke the antenna up. That truck has a utility bed, and the
boxes are pretty much part of the body. I'm not talking about
a toolbox thrown into the back of the truck. Often those are
not the greatest, unless very well grounded. I don't see why
you need the exact ground loss. It's not going to effect your
antenna design. But when doodling with vertical programs, I'll
just stick in about 6-8 ohms as a "very good" mobile ground
example.. A huge truck with the antenna on top could be slightly
lower, but many small cars will be much higher. Knowing the
exact loss is really not critical. Your antenna design will still
come
out the same.
MK

Thank you very much for the helpful info to my situation. The RV roof
is fibreglass. All I have to really work with is the ladder, which in
itself is bonded to the aluminum frame in the back walls, as well as very
well bonded to the chassis underneath by means of large copper straps.
Bout the best I can do. The figures you suggested should help too.
Thanks again.

Ed K7AAT