On Thu, 2 Mar 2006 21:35:30 -0600, (Richard
Harrison) wrote:

Danny did not specify where he thought I erred in my previous posting. I
said that a whip mounted on a vehicle is not exactly like a dipole. I
meant that the whip did most of the radiating because it carried a
concentration of current in the same direction while in the car body the
current is dispersed in various directions, some of which canncel in
their effects.

That may have some validity in the VHF and higher ranges, but on HF -
particularly on 80 meters - a car body's size is a small fraction of a
wavelength (as is the whip portion). Consequently the vehicle body
acts like the one half of a dipole antenna. Slightly lop-sided yes,
but a dipole just the same - just as Roy stated.

Regards,
Danny

ORIGINAL MESSAGE:

Dan Richardson wrote:

That may have some validity in the VHF and higher ranges, but on HF -
particularly on 80 meters - a car body's size is a small fraction of a
wavelength (as is the whip portion). Consequently the vehicle body
acts like the one half of a dipole antenna.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q. How can a car body which is a "small fraction" of a wavelength act
like one half of a dipole?

A. It can't.

Q. Well, what does it do then?

A. It acts like a short piece of wire leading from the bottom of the
whip to the actual ground plane, namely the earth itself.

Q. Does that help any?

A. Probably a little, but remember the piece of wire (the car body) is
only a few feet long. Not very much on 80 meters.

Q. Thanks, I get it now.

A. You're welcome.

73, Bill W6WRT

Bill Turner wrote:
ORIGINAL MESSAGE:

Dan Richardson wrote:


That may have some validity in the VHF and higher ranges, but on HF -
particularly on 80 meters - a car body's size is a small fraction of a
wavelength (as is the whip portion). Consequently the vehicle body
acts like the one half of a dipole antenna.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q. How can a car body which is a "small fraction" of a wavelength act
like one half of a dipole?

A. It can't.

Q. Well, what does it do then?

A. It acts like a short piece of wire leading from the bottom of the
whip to the actual ground plane, namely the earth itself.

Q. Does that help any?

A. Probably a little, but remember the piece of wire (the car body) is
only a few feet long. Not very much on 80 meters.

Q. Thanks, I get it now.

A. You're welcome.

73, Bill W6WRT
Actually it is acting as one half of a dipole. It is just a non-resonant
half of a dipole. Remember "di" means two.

Dave WD9BDZ

David G. Nagel wrote:

Actually it is acting as one half of a dipole. It is just a
non-resonant half of a dipole. Remember "di" means two.

Dave WD9BDZ

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~

In a strict sense you are correct, but in the context here where one
half of the dipole is an eight-foot whip and the other half is four
feet of car body, we don't have much of an 80 meter antenna without the
coupling from car body to earth ground.

Bill, W6WRT

Bill Turner wrote:

David G. Nagel wrote:


Actually it is acting as one half of a dipole. It is just a
non-resonant half of a dipole. Remember "di" means two.

Dave WD9BDZ


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~

In a strict sense you are correct, but in the context here where one
half of the dipole is an eight-foot whip and the other half is four
feet of car body, we don't have much of an 80 meter antenna without the
coupling from car body to earth ground.

Bill, W6WRT


No argument here Bill. The point I guess I was trying to make is that a
dipole antenna system is two elements no matter what you make them of.
I use a Hi Sierra screwdriver antenna on my Honda Element. Even though
is is an impressive construct I don't harbor any illusions that it is an
efficient radiator. The body of the car is longer than the length of the
screwdriver, coil and whip. I have also used an Outbacker. Some say that
is a good antenna for it type, I have not had that good of a result with
it. Of course I am using my mobile for Civil Air Patrol and the
Outbacker does not fit that frequency very well on the precut tuning jacks.
I have found this thread to be interesting but I think that is had
passed that point.
I do not consider myself to be anything other than an interested amateur
and always consider your comments with great interest. Thank you for
your personal comments.

Dave WD9BDZ

I'm afraid people are getting too hung up by trying to squeeze
everything into various pigeon holes like "dipole" and "ground". You'll
have to think beyond those narrow and poorly defined and understood
categories and look at the basics of antenna operation in order to
understand what's happening.

The field radiated from a conductor is determined by two things: the
amount of current on it, and the length of the path the current takes.
Theorists have known this for well over a century. The most
sophisticated antenna analysis programs break the current paths into
very short pieces ("segments"), calculate the current on each piece, and
then calculate the resulting field from the product of the current and
the segment length. Fields from various parts of the conductors can
cancel or reinforce to any degree. (Mathematically, they add vectorally.)

If you don't or can't believe this, you needn't bother continuing.

For those still reading, let's imagine a 16 foot vertical wire with a
tiny 3.5 MHz signal generator at the center. This is known in textbooks
as a "dipole", but how things behave aren't dictated by what we call
them, so feel free to insist it's a "seagull", "pizza", "xfppftm", or
whatever makes you comfortable. The signal generator has two terminals,
and any generator must have equal currents in and out of its two
terminals. If you don't or can't believe that, brush up on Kirchoff's
current law. If that doesn't do it, there's no need to continue further.

Let's suppose the generator is producing one amp RMS of RF current. If,
say, 0.2 amp is flowing upward out of the top terminal at a given
instant, 0.2 amp is flowing upward into the bottom terminal at the same
instant. By inspection, one amp RMS is flowing upward in the vertical
wire immediately above and below the generator. By a number of
techniques, we can show that the current decreases nearly linearly from
the center to the ends. That is, four feet from the center, either above
or below the source, the current is 1/2 amp. At the antenna tips, the
current is zero, which we should expect: there's nowhere for it to go.

It should be obvious that the wire above the source is radiating the
same field strength as the wire below the source -- for each little
piece of the wire above the source there's a piece below the source
carrying exactly the same current. And as it turns out, the fields from
all parts of both wires add completely in phase directly broadside to
the wire, and only partially in phase in other directions. So at least
directly broadside, we can say that the contribution from each wire is
equal and proportional to its total field strength.

Ok, now let's make one of the wires "ground" and the other a "whip",
because we like to do that, right? Let's call the top wire a "whip", and
bottom load it. We add an inductor (very small, physically, to avoid
adding another dimension to this analysis) between the signal
generator's top terminal and its connection with the upper wire. We can
make the inductor the proper value to make the upper wire/inductor self
resonant if it were grounded, or we can make the inductor about twice as
large to make the whole dipole resonant. It doesn't matter. Now let's
see what happened to the radiation from the "whip" and "ground" wires.

There's no change whatsoever! The currents are exactly the same as they
were before, on both wires. They still taper from the center to the tips
as before. They both radiate equally. All we've done is change the
impedance seen by the generator. If you don't believe this, perhaps you
can explain why they won't.

Next, let's replace the lower wire with a cylinder like a tank, say 10
feet in diameter but still 8 feet high. What happens then? Surely it
must now be "ground", and "ground" doesn't radiate, does it?

Well, it does radiate. The one amp flowing into the bottom generator
terminal spreads out radially over the top of the cylinder. Although the
current density decreases as we move out from the center, the total
current also decreases. If only the cylinder top was present and the
rest of the cylinder missing, the current would drop to nearly zero at
the edge. But because of the presence of the rest of the cylinder, the
current at the edge drops to about half the value at the center. The
half which remains flows down the cylinder sides. This would result in
the field from the cylinder being about half the field from the "whip"
if the current decreased to zero at the bottom of the cylinder as it
does at the top of the whip. But the current along the sides of the
cylinder doesn't drop to zero at the bottom of the walls because it can
flow onto the bottom of the cylinder. The average current on the whip is
0.5 amp, and on the cylinder (from a model) about 0.35 amp, so the
cylinder's field is about 3 dB less than that of the "whip". Not quite
what most of envision when we think of a "ground".

If we top load the whip with a 10 foot diameter top hat, its average
current increases to about 0.9 amp. But its presence also reduces the
amount of current drop from the center to the edge of the cylinder top
due to mutual coupling. The end result is larger current along the
cylinder sides and very nearly the same field strength ratio between the
"whip" and cylinder.

So far this analysis has taken place in free space. What happens if we
put the cylinder bottom just above the ground, say six inches? Now,
surely, the cylinder is "ground"! But the current still flows down the
sides and radiates just like the old original vertical lower wire did.
And putting the bottom close to ground increases the current along the
sides! The coupling between the cylinder bottom and ground acts somewhat
like a top hat does to a whip, and increases the average current.
Instead of 0.35 amp, it increases to about 0.42. Now the cylinder's
field is only about 1.5 dB less than that of the "whip".

I hope this has encouraged at least a few people to think a little
before declaring every conductor to be either an "antenna" or a "ground
plane" and assuming that by doing so they'll somehow cause it to behave
in some predetermined and only vaguely understood fashion.

Roy Lewallen, W7EL

Bill Turner wrote:
ORIGINAL MESSAGE:

Dan Richardson wrote:

That may have some validity in the VHF and higher ranges, but on HF -
particularly on 80 meters - a car body's size is a small fraction of a
wavelength (as is the whip portion). Consequently the vehicle body
acts like the one half of a dipole antenna.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q. How can a car body which is a "small fraction" of a wavelength act
like one half of a dipole?

A. It can't.

Q. Well, what does it do then?

A. It acts like a short piece of wire leading from the bottom of the
whip to the actual ground plane, namely the earth itself.

Q. Does that help any?

A. Probably a little, but remember the piece of wire (the car body) is
only a few feet long. Not very much on 80 meters.

Q. Thanks, I get it now.

A. You're welcome.

73, Bill W6WRT

Roy Lewallen wrote:

I hope this has encouraged at least a few people to think a little
before declaring every conductor to be either an "antenna" or a
"ground plane" and assuming that by doing so they'll somehow cause it
to behave in some predetermined and only vaguely understood fashion.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~

A very good explanation, thank you Roy.

However... in your example of the giant tin can in free space, the top
of the tin can is acting like a ground plane, the side is acting like
an antenna and the bottom is again acting like a ground plane, just as
we have been saying. When this model is transferred to a car body, the
bottom of the car, in addition to the above, is also acting like one
plate of a capacitor coupling the signal to the earth below it,
commonly known as "ground". If someone disagrees with this I believe we
have a problem with semantics more than physics.

In other words, we are arguing over nothing.

Bill, W6WRT

Bill Turner wrote:
Roy Lewallen wrote:

I hope this has encouraged at least a few people to think a little
before declaring every conductor to be either an "antenna" or a
"ground plane" and assuming that by doing so they'll somehow cause it
to behave in some predetermined and only vaguely understood fashion.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~

A very good explanation, thank you Roy.

However... in your example of the giant tin can in free space, the top
of the tin can is acting like a ground plane, the side is acting like
an antenna and the bottom is again acting like a ground plane, just as
we have been saying. When this model is transferred to a car body, the
bottom of the car, in addition to the above, is also acting like one
plate of a capacitor coupling the signal to the earth below it,
commonly known as "ground". If someone disagrees with this I believe we
have a problem with semantics more than physics.

In other words, we are arguing over nothing.

Bill, W6WRT

I interpreted your comments and those by some others as claiming that
radiation from the car is insignificant, and that it therefore isn't
effectively part of the antenna. I attempted to show that this isn't
generally true. I also showed that coupling to the ground actually
increases radiation from the car. So either I've convinced you by my
illustration, or I misinterpreted your earlier remarks.

Roy Lewallen, W7EL

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

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