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

Exactly!
Vertical mounted on a conductive vehicle body behaves as a vertical
monopole - radiator working against the ground plane - car body. It is
apparent that the idea that car is the "other half" of "asymmetrical dipole"
is misleading and not applicable to the case. Mobile antenna behaves like
any other vertical monopole over conductive ground plane, radial field,
radials - regardless of their size and definitely not as the other half of
"asymmetrical dipole" and practically not affecting tuning, which is the
case with any real di-pole.

73 Yuri, K3BU

wrote in message
ups.com...
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

On Nov 3, 8:22 pm, "Yuri Blanarovich" wrote:
Exactly!
Vertical mounted on a conductive vehicle body behaves as a vertical
monopole - radiator working against the ground plane - car body. It is
apparent that the idea that car is the "other half" of "asymmetrical dipole"
is misleading and not applicable to the case. Mobile antenna behaves like
any other vertical monopole over conductive ground plane, radial field,
radials - regardless of their size and definitely not as the other half of
"asymmetrical dipole" and practically not affecting tuning, which is the
case with any real di-pole.

73 Yuri, K3BU

Just pondering.. Another thing that bothers me about the mobile dipole
theory is that the body element is not resonant in most cases, and
thus
should not act as a decent working dipole leg in those cases.
Now if you by chance had a vehicle with the whip mounted at the rear,
and the front vehicle length by luck happened to be resonant, then
yes,
I could see much more of a dipole effect.
But say with the typical low band HF mobile, the car is not even close
to be capable of being a resonant element.
So I'm also tending to believe it acts more like a typical short
vertical
that is mounted on a varied size sheet of metal, but also coupling
with
the earth.
The coupling to earth can be verified by driving over certain types
of
ground, highways with rebar, etc.. Some areas are noticeably better
than others.
MK

This was done by Don Johnson W6AAQ & described in his book 40 years of HF
Mobileering. He mounted another whip at the front of the vehicle & grounded
it to the frame & tuned it to resonance.
wrote in message
ups.com...
On Nov 3, 8:22 pm, "Yuri Blanarovich" wrote:
Exactly!
Vertical mounted on a conductive vehicle body behaves as a vertical
monopole - radiator working against the ground plane - car body. It is
apparent that the idea that car is the "other half" of "asymmetrical
dipole"
is misleading and not applicable to the case. Mobile antenna behaves like
any other vertical monopole over conductive ground plane, radial field,
radials - regardless of their size and definitely not as the other half
of
"asymmetrical dipole" and practically not affecting tuning, which is the
case with any real di-pole.

73 Yuri, K3BU

Just pondering.. Another thing that bothers me about the mobile dipole
theory is that the body element is not resonant in most cases, and
thus
should not act as a decent working dipole leg in those cases.
Now if you by chance had a vehicle with the whip mounted at the rear,
and the front vehicle length by luck happened to be resonant, then
yes,
I could see much more of a dipole effect.
But say with the typical low band HF mobile, the car is not even close
to be capable of being a resonant element.
So I'm also tending to believe it acts more like a typical short
vertical
that is mounted on a varied size sheet of metal, but also coupling
with
the earth.
The coupling to earth can be verified by driving over certain types
of
ground, highways with rebar, etc.. Some areas are noticeably better
than others.
MK