On Nov 3, 2:25 pm, Roy Lewallen wrote:


My comments were made when envisioning an HF antenna. I believe that if
you move the same HF antenna from one car to another, or change the
mounting position, the resonance will change. Haven't you found this to
be true?

Actually, on mine, I don't see much difference in the resonant point
of the whip from car to car. Just the degrees of loss, depending
on the mount location.
IE: I ran my usual HF antenna on a chevy monte carlo for
quite a while. It was mounted on the trunk, and the car
body forward of the whip was a good bit longer than to the
rear.
I stuck the same antenna on one of my trucks, with the whip
mounted on a piece of angle iron bolted to each side of
the toolbox/bed right behind the back window.
The tuning was basically unchanged.
But the efficiency was poor..
I came to the conclusion there was not enough metal
under the antenna base, so I moved it to the side toolbox
where there is a much wider strip of metal under the
whip vs the narrow angle iron.
That pretty much cured the problem, but didn't effect
tuning much at all
Then I mounted it on another truck, but this time on a
ball mount bolted to the side of the cab.
Again, the tuning changed little. The antenna worked
great, and that truck has the highest mount, and
good metal at the base of the whip. I consider it the
best of my vehicles for mounting an HF antenna.
But then I later tried mounting it on the trunk of my
honda accord. Again, little change in tuning.
It worked fairly well. About like the monte carlo,
except maybe a tad less efficient being the car
is smaller. On those 4 vehicles, I never had to do
any drastic changes as far as tuning the whip with
the number of coil turns used.
One thing that bothers me about the mobile antenna
= a perverted dipole theory..
You would think that the amount of metal under the
whip would not matter too much in that case as long
as the connections are real good.
But.. This was not the case. The amount of metal
under the whip seems to be quite critical.
So... This makes me believe the antenna acted more
like a typical short vertical, than a offset dipole
even if trunk mounted.
As with a typical ground mount vertical, the amount
of metal under the whip seemed to be critical.
IE: most suggest more short radials, vs long ones,
as it gives more metal under the whip.
The typical HF mobile seems to act about the same
way.
I don't doubt that the pattern with a rear mount is
a good bit different than a roof center mount though
due to the "dipole" effects of the offset mount.
But I've never seen the tuning of the whip change
much between those four vehicles I mentioned..
I use the same "fixed" coil taps on all of them.
I still haven't tried mounting it on my newest car
"toyota corolla", as I'm still chicken to bugger it up
and drill holes in the car. :/
MK