ORIGINAL MESSAGE:

wrote:

What we found at the CA shootouts


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Speaking of shootouts, are any scheduled for this year anywhere?

Bill, W6WRT

Bill Turner wrote:

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No doubt that is correct. So how about this: I have a '95 Thunderbird
which I dearly love and don't want to cut holes in. I've been think of
going to a welding shop and having a metal piece made which I could
bolt to the frame in the back and which would stick out about six
inches or so behind the rear bumper, and installing a ball mount on it.
This will keep the lower part of the antenna about a foot away from the
body and allow a nice, long whip overall. The loading coil would be in
the center, homebrew of course. :-)

And not a hole in sight.

Comments?

Bill, W6WRT

I'm not sure why, but most amateurs don't seem to realize that the whip
isn't an "antenna" and the car "ground", but each is half of a
dipole-like antenna. The car part is often much more important with
regard to radiation characteristics and efficiency than the whip part.
With the arrangement you suggest, the antenna consists of a vertical
wire -- the whip -- and a fat, horizontal "wire" -- the car. Whatever
current flows into the whip, an equal current flows over the outside of
the car, originating at the base of the whip.

Any antenna with a low horizontal wire will be quite lossy, because the
wire's current will induce a heavy current in the lossy ground beneath
the wire, or car.

The best arrangement, as others have pointed out, is to mount the
antenna right at the center of the top of the car. This makes the car
"wire" vertical, a much more efficient arrangement, which the
"shootouts" consistently show. You'll also find that larger trucks,
which effectively form a longer vertical "wire" for the car part, outdo
smaller ones for the same whip.

Of course, sometimes you don't have any choice, and you just have to do
the best you can. I once had a bumper mounted antenna consisting of a CB
whip base loaded with an inductor wound on a powdered iron core to
resonate on 40 meters. The car was a VW Squareback, so the antenna had
the increased disadvantage of proximity between the square back and the
antenna. As others have pointed out, this can reduce efficiency farther.
Yet I had a successful QSO with JA while driving down Highway 101,
running 8 watts, CW. So you can still communicate and have lots of fun
even with a very sub-optimal system. But anyone wanting to improve his
system has a much better chance of doing it if he has a basic
understanding of how the antenna really works.

Roy Lewallen, W7EL

Roy Lewallen wrote:

The best arrangement, as others have pointed out, is to mount the
antenna right at the center of the top of the car. This makes the car
"wire" vertical, a much more efficient arrangement, which the
"shootouts" consistently show. You'll also find that larger trucks,
which effectively form a longer vertical "wire" for the car part,
outdo smaller ones for the same whip.

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Well, maybe. The problem is, the higher the mounting point, the shorter
the whip has to be to be legal to drive down the road. If your car was
13 feet five inches tall, your whip could only be one inch long. How
efficient would that be on 80 meters? The point being, everything is a
tradeoff of one thing for another. If the shootouts say a rooftop is
best, ok, but I have to say I'm surprised. At a relatively low
frequency like 80 meters, the car body is more of a coupler to the
earth rather than a real "ground" of it's own. Given that, then the
longer the whip part, the better.

HF mobile antennas are a fascinating subject and one of these days I
will set up a "shootout" range on my 2.5 acres here in the desert and
do some shooting of my own.

Bill, W6WRT

Bill Turner wrote:
So how about this: I have a '95 Thunderbird
which I dearly love and don't want to cut holes in. I've been think of
going to a welding shop and having a metal piece made which I could
bolt to the frame in the back and which would stick out about six
inches or so behind the rear bumper, and installing a ball mount on it.
This will keep the lower part of the antenna about a foot away from the
body and allow a nice, long whip overall. The loading coil would be in
the center, homebrew of course. :-)

The only way to improve on that on 75m would be to mount
a piece of sheet metal on fiberglass poles connected at
the ends of both bumpers. The piece of horizontal sheet
metal, located 13.5 feet from the ground, would have the
same footprint as the T-bird and would be used as the top
hat. You do want optimum performance don't you? :-)
--
73, Cecil http://www.qsl.net/w5dxp

Roy Lewallen wrote:
I'm not sure why, but most amateurs don't seem to realize that the whip
isn't an "antenna" and the car "ground", but each is half of a
dipole-like antenna.

My S10 trailer hitch mounted configuration exhibited
considerable directivity toward the front of the
pickup on 17m.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

The only way to improve on that on 75m would be to mount
a piece of sheet metal on fiberglass poles connected at
the ends of both bumpers. The piece of horizontal sheet
metal, located 13.5 feet from the ground, would have the
same footprint as the T-bird and would be used as the top
hat. You do want optimum performance don't you? :-)

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I think my T-Bird might actually fly. :-)

73, Bill W6WRT

Roy Lewallen, W7EL wrote:
"I`m not sure why, but most amateurs don`t seem to realize that a whip
isn`t an "antenna" and the car "ground", but each is half of a
dipole-like antenna."

Not exactly..

In a common balanced dipole, each half has the same current quantity and
direction, though in one half the current flows toward the feedpoint
while it flows away in the other half.

From such a dipole, both its halves contribute equally to its radiation.

Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.

An antenna is also called an aerial. It is defined as that part of a
radio station which radiates or receives radio waves into or from space.

An antenna ground system is defined as that portion of an antenna system
closely associated with the earth and including an extensive conducting
surface which may be the earth itself.

Most radio amateurs have it right.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"I`m not sure why, but most amateurs don`t seem to realize that a whip
isn`t an "antenna" and the car "ground", but each is half of a
dipole-like antenna."

Not exactly..

In a common balanced dipole, each half has the same current quantity and
direction, though in one half the current flows toward the feedpoint
while it flows away in the other half.

From such a dipole, both its halves contribute equally to its radiation.

Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.

A ground plane is a poor model of how currents flow along a car body.

Consider an antenna mounted on top of a car. From the base of the
antenna, the current flows equally in all directions away from the base
of the antenna, like a ground plane. This current doesn't contribute
much radiation, for the reasons you state. But then it reaches the edge
of the top of the car and flows downward. All the portions of the
current are now flowing the same direction, and their fields don't
cancel but add in phase. The net result is the same as if it were just
flowing down a fat wire the height of the car. If the car is eight feet
high, the field from the car will equal the field from an eight foot
whip. In fact, unless the whip is top loaded to make the current
uniform, the car will radiate more than the whip, because the
capacitance of the car to ground will tend to give the car a uniform
current distribution, like a top hat does to a whip. This will increase
the radiated field strength from the car.

Now consider a bumper mounted antenna. The current will spread from the
base and proceed around the car. More will probably flow on the bottom
than the top and sides due to coupling with the ground, but all portions
will be flowing in the same direction and all will radiate. There is no
place on the car where the current distribution or flow pattern
resembles current on a ground plane.

An antenna is also called an aerial. It is defined as that part of a
radio station which radiates or receives radio waves into or from space.

An antenna ground system is defined as that portion of an antenna system
closely associated with the earth and including an extensive conducting
surface which may be the earth itself.

The problem here is that the currents don't care how you define things.
They flow where the physical laws dictate. Defining "ground" doesn't
make them behave differently.

Most radio amateurs have it right.

If your view represents that of most amateurs, they don't.

Roy Lewallen, W7EL

On Thu, 2 Mar 2006 10:56:15 -0600, (Richard
Harrison) wrote:

Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.

Boy Richard, you sure missed the boat on that one!

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.

A number of years ago I did a study using NEC modeling comparing VHF
mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different
vehicles. I created wire frame vehicles models for a full and mid-size
passenger cars, a small pickup truck and an SUV. The results for the
same antenna mounted top-dead-center on the different vehicles was
quite noticeable sometimes substantial.

An article I wrote on the subject can be viewed at:
http://k6mhe.com/files/mobile_vhf_ant.pdf

73,
Danny, k6MHE







email: k6mheatarrldotnet
http://www.k6mhe.com/

Quite interesting reading. Have you received peer comments?

Dan Richardson wrote:

On Thu, 2 Mar 2006 10:56:15 -0600, (Richard
Harrison) wrote:


Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.


Boy Richard, you sure missed the boat on that one!

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.

A number of years ago I did a study using NEC modeling comparing VHF
mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different
vehicles. I created wire frame vehicles models for a full and mid-size
passenger cars, a small pickup truck and an SUV. The results for the
same antenna mounted top-dead-center on the different vehicles was
quite noticeable sometimes substantial.

An article I wrote on the subject can be viewed at:
http://k6mhe.com/files/mobile_vhf_ant.pdf

73,
Danny, k6MHE







email: k6mheatarrldotnet
http://www.k6mhe.com/