Yuri Blanarovich wrote:
"Roy Lewallen" wrote in message

The currents flowing different directions don't cancel. If they flow in
equal amounts, in phase, in opposite directions, then the fields they
create nearly cancel. And that's the case along the roof of a car if the
car and roof are symmetrical and the antenna is at the center. But it's
not the case where it flows vertically along the sides of the car. There,
the currents are in the same direction.


How do YOU make it flow vertically along the sides of the car and not
distributed along the whole conductive surface of the car's body? How do you
make the current flowing, say, towards the front of the car (or one side)
not to (partially) cancel by current flowing in opposite deirection, towards
the back (or other side)?

I don't make it flow anywhere. The field from the current flowing toward
the front, along the roof, cancels the field from the current flowing
toward the rear along the roof. Once the current reaches the edge of the
roof, all components flow vertically. If the car is symmetrical and the
antenna mounted in the center, all components are in phase. There is no
field or potential gradient to induce current flow horizontally along
the sides, front, and back of the car.

The currents flow from the feed point, base of the vertical in all
directions where supported by conductive sheet metal, or wire or radial.
They might not be exactly symmetrical, but they have opposite direction
portion of the current and corresponding cancellation.

Along the roof, yes, as I explained in my original posting. But they
don't flow in opposite directions along the sides, front, and back of
the car.

The radiation pattern is formed by the sum of all the fields which are
created by currents flowing on conductors. The antenna is one such
conductor. The body of the car is another.


How simple! :-)

Yes, it is indeed.

Please model vertical resonant dipole, then monopole with one radial, then
with two radials, then with 8 radials, then with 64, then with solid sheet,
then with car body. Then move the antenna around, away from the center and
see what happens to the pattern. See how this "conductor" is "another".
Then the case of say 2m quarter wave vertical mounted in the middle of the
roof, sure doesn't look like car is the other wire.

If you're saying that a car doesn't look like a 19 inch wire, you're
right. It doesn't look like the Earth, either.

That's true. And the coupling of the car body to ground alters the amount
of current flowing along the body of the car. This current equals the
current flowing into the antenna.


That is true, but portions of the current flowing in different directions
from the feedpoint are in opposite direction and based on their magnitude
partially cancel. The result is the interaction with monopole, radiator and
forming the radiation pattern, which in practical HF mobile installation is
distorted, lopsided. If it was the"other wire" then it would be symmetrical
like in true vertical symmetrical di-pole.

Sigh. You're right, it's not like a symmetrical dipole. It's like a Yuri
Special asymmetrical dipole.

Suit yourself. I called the system an "asymmetrical dipole". But like
"ground", putting a name on it doesn't change its properties.


I am not suiting myself but describing difference between vertical monopole
working against conductive surface (sheet, car body, radials) with
corresponding current flow and dipole, di-pole that customarily refers to
symmetrical antenna with two (similar) legs.

Ok.

Certainly a whip mounted on a vehicle can be expected to have a different
pattern than a symmetrical dipole, and nothing I've written has attempted
to make a claim that it does. But it sounds like you've grabbed onto the
"asymmetrical dipole" label as a basis for argument. So please go back
over my postings but substitute "Yuri special" for "asymmetrical dipole"
and see if then you find anything I've written which isn't correct.


Sounds more like "Roy special" - everything is assymetrical or symmetrical
dipole. How "dipoly" is the 2 m whip in the middle of the car roof? How can
you tune to resonance 80 m car whip by adjusting the "other wire" - car
body? Implying the other half of "Roy dipole" is a "wire"?

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? At VHF the asymmetry is of course much greater, and the short,
thin part (the whip) of a structure like the VHF whip-car combination
has much more effect on the impedance than the long, fat part (the car).

That's the way I understand it, without involving photons, Gaussss and
other farticles :-)
If you say so.


Other smarter people in their books and modeling in your EZNEC say so.

Roy Lewallen, W7EL

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