Think for a moment about a wire carrying RF current. RF current in any
good conductor stays very close to the surface, so it's valid to imagine
it flowing right on the surface. On a wire, the current spreads
uniformly around the wire (unless it's very close to some other
conductor), and each little part radiates. But at any distant point, the
fields from the currents at various places around the wire (at a single
location along the wire) are virtually the same, so it acts like a
single current filament flowing on an infinitesimally thin wire. This
is, in fact, how NEC and similar programs model conductor currents.

The same even spreading happens as the wire gets fatter and fatter, but
only up to a point. The model of a single current filament begins
breaking down when the fields from different places around the wire are
noticeably different at a distant point. This happens when the wire
diameter becomes an appreciable fraction of a wavelength. Other things
happen, then, too -- circumferential currents -- ones flowing around the
wire -- develop, resulting in (or being caused by, depending on your
point of view) non-uniform current distribution around the wire. And the
wire itself affects the field. That is, the current on the side away
from a distant point can't directly radiate to the distant point because
the wire is in the way. Because the various current contributions around
the wire won't all add together at a distant point any longer, the
pattern changes.

I can't give any more specific answer to the question, really, than that
the pattern will become more complex. In the case of the example I gave
earlier with the cylindrical "car", if you raise the frequency, you'll
reach a point where these effects happen. One result will be that the
horizontal pattern will no longer be omnidirectional, but develop lobes.
The height of the cylinder or car might affect the way current is
distributed around it -- I haven't thought about it enough to hazard a
guess.

The good news is that today's modeling programs do a good job of showing
these effects. The general technique is to represent flat surfaces such
as a car top or side as a wire grid, to stay within the program's
requirements that wire diameter must be no more than a very small
fraction of a wavelength. As long as the holes in the grid are kept to a
tenth of a wavelength or less, results are quite good.

Roy Lewallen, W7EL

Bill Turner wrote:
Roy, your analogy of the car body as a tin can really got me to
thinking.

With the whip mounted dead center on the top of the car, I can see how
the roof acts like a ground plane (a very short one) but I'm puzzled
about the radiation from the lower part of the car body. If one
visualizes RF flowing through the sides, hood and trunk of the car, the
currents will all be in phase with each other (roughly, of course) but
the currents are displaced in space by several feet.

How does this affect the net radiation from the car body as a whole? Is
there some addition or subtraction due to having the same current, same
phase but at a different location in space, and arranged in a more or
less 360 degree pattern?

An interesting thought.

73, Bill W6WRT

On Fri, 3 Mar 2006 02:28:54 -0800, "Harbin"
wrote:

Howdy Dan:
Thanks for the info on "Mobile Vhf Ant.pdf", very interesting stuff.
I was checking out the rest of your site, and found an interesting pdf on the "Skeleton Sleeve Antenna",
and it's similarity of the J-pole, and you state that the distance from the 1/4 wave elements, and the 1/2 wave
element is not that critical. I have heard this before about the elements on the J-pole too, but what I don't understand
is why this dimension is not critical, it is an electrical path that should be subtracted from both elements, right?
Is it correct to shorten both elements by 1/2 of the dimension between the elements?

Harbin,

By "critical" I meant that the dimension shown can vary somewhat with
no loss of performance. For example, if you do the math you'll see
that the distance (shown in Figure 5) between the center 3/4-wave
element and the shorter 1/4-wave elements is 3.889 inches. If that
figure varied say between 3.5 and 4-inches there would be no
noticeable effect on operation. The important thing is (as I stated in
the article) is to maintain equal spacing the between the center
element and each of the outer 1/4-wave elements. The equal spacing is
the critical thing. What little change there might result in the
resonate frequency can easily be adjusted as stated in the article.

73,

Danny, K6MHE

Thanks, Roy. Interesting.

73, Bill W6WRT

Cecil Moore wrote in news:eOLNf.62289$PL5.32727
@newssvr11.news.prodigy.com:

Roy Lewallen wrote:
A ground plane is a poor model of how currents flow along a car body.

If the car body was 1/2WL in the air, would the antenna
be more efficient?

Probably not. But it might be a BITCH to match!


--
Dave Oldridge+
ICQ 1800667

On Fri, 03 Mar 2006 00:08:42 GMT, Cecil Moore wrote:

Roy Lewallen wrote:
A ground plane is a poor model of how currents flow along a car body.

If the car body was 1/2WL in the air, would the antenna
be more efficient?


Maybe we can get Fear Factor's engineers to calculate the number of
boxes needed to perform the test. One of us will have to drive our
car off a cliff and make signal reports as the car goes down into the
boxes


--
73 for now
Buck
N4PGW

On Thu, 02 Mar 2006 19:35:15 -0500, Amos Keag
wrote:

Cecil Moore wrote:
Dan Richardson wrote:

Roy is quite correct in stating that a vehicle's body behaves as one
side of a dipole. A lopsided dipole to be sure, but one half the
antenna just the same.


Seems the truth might lie somewhere in between. If the ground
plane of a vertical antenna is near the ground, there are
losses. If the ground plane of a vertical antenna is located
1/2WL above ground, the losses are a lot less. I'll bet that
if the vehicle were located 1/2WL in the air, the efficiency
would increase.

Kind of tough though going under power lines, bridges and overpasses :-)

That depends.... If you take cecil up on his idea, you may go over
them.... of course, that you may want to be on the low bands when you
cross some bridges....
--
73 for now
Buck
N4PGW

"Buck" wrote in message
...
On Thu, 02 Mar 2006 19:35:15 -0500, Amos Keag
wrote:

Cecil Moore wrote:
Dan Richardson wrote:

Roy is quite correct in stating that a vehicle's body behaves as one
side of a dipole. A lopsided dipole to be sure, but one half the
antenna just the same.



Not exactly. Body of the vehicle behaves more like a ground plane and a
capacitor coupling to the ground. As soon as "radials" or ground plane has
antenna mounted somewhere else than on the edge, you get cancellation of
current along the body - two sides "working against each other". We are
talking vertical antenna here, not a goofy dipole.


Seems the truth might lie somewhere in between. If the ground
plane of a vertical antenna is near the ground, there are
losses. If the ground plane of a vertical antenna is located
1/2WL above ground, the losses are a lot less. I'll bet that
if the vehicle were located 1/2WL in the air, the efficiency
would increase.


Depends. If vehicle is going over metal grid frame bridge or good ground,
like salty beach, the efficiency goes up. Body of vehicle is like a plate of
capacitor, coupling the ground plane to effcient ground and performance of
the vertical goes up, especially at low angles. Drive over the bridge or
ocean beach and see the S-meter go up.

Kind of tough though going under power lines, bridges and overpasses :-)

That depends.... If you take cecil up on his idea, you may go over
them.... of course, that you may want to be on the low bands when you
cross some bridges....
--
73 for now
Buck
N4PGW

Yuri, K3BU

On Thu, 20 Jul 2006 17:11:10 -0400, "Yuri Blanarovich"
wrote:


"Buck" wrote in message
.. .
On Thu, 02 Mar 2006 19:35:15 -0500, Amos Keag
wrote:

Cecil Moore wrote:
Dan Richardson wrote:

Roy is quite correct in stating that a vehicle's body behaves as one
side of a dipole. A lopsided dipole to be sure, but one half the
antenna just the same.



Not exactly. Body of the vehicle behaves more like a ground plane and a
capacitor coupling to the ground. As soon as "radials" or ground plane has
antenna mounted somewhere else than on the edge, you get cancellation of
current along the body - two sides "working against each other". We are
talking vertical antenna here, not a goofy dipole.


Seems the truth might lie somewhere in between. If the ground
plane of a vertical antenna is near the ground, there are
losses. If the ground plane of a vertical antenna is located
1/2WL above ground, the losses are a lot less. I'll bet that
if the vehicle were located 1/2WL in the air, the efficiency
would increase.


Depends. If vehicle is going over metal grid frame bridge or good ground,
like salty beach, the efficiency goes up. Body of vehicle is like a plate of
capacitor, coupling the ground plane to effcient ground and performance of
the vertical goes up, especially at low angles. Drive over the bridge or
ocean beach and see the S-meter go up.

Kind of tough though going under power lines, bridges and overpasses :-)

That depends.... If you take cecil up on his idea, you may go over
them.... of course, that you may want to be on the low bands when you
cross some bridges....
--
73 for now
Buck
N4PGW

Yuri, K3BU


Thanks for the reply, I suppose you figured out I was playing on
Cecil's reply.

Tonight, I did have a related experience. I have an FM BC band xmtr
in the car attached to my CD player. I use it to monitor the cds on
the car stereo. I noticed tonight when I crossed a very high bridge
that Radio signals drown out my little monitor transmitter, only while
I was on the bridge. The car was specifically over 1/2 wave above the
highway below it.

for what it's worth....


Buck

--
73 for now
Buck
N4PGW