"jim rozen" wrote in message
...
In article wY5Ud.56428$uc.36861@trnddc04, Jerry Martes says...


You probably got a mixture of messages while reading these posts.
But, I
would put myself amoungst thoes who wouldnt find value in using the term
"ground plane" when working with a conductor thats shorter than a few
electrical degrees at the frequency of interest.

It's a semanitic issue, I know. For the sake of argument, one
could say that the body of a car is the 'other part of a non-resonant
dipole antenna.'

Because it's a mostly flat part and mostly planar in shape, the
temptation to call it a ground plane is obvious.

If it were a 2 meter whip antanna stuck on the roof of the car,
it would be a very good description. For 1 MHz radiation the description
does leave a bit to be desired as you say.

Circuit board designers call the continuous conductor on their
board a "ground plane" even though that could be a few inches
square.

It is a ground plane in that case, in terms of DC, and in terms of
preventing certain radiation from penetrating it. What it is NOT is a ground
plane in the sense the term is used in antenna theory: as a plane of ground
potential that serves as a sort of clamp against which the radiative energy
emitted from the radiator reflects.

For me the term "ground plane" does not have to have any
particular wavelength of interest to be applied. I like the
term 'ground plane' even for a car body, for 1 MHz am because
it's descriptive of the *shape* of the conductor more than
anything else. That, and the input coil of the receiver is
stuck across that coax feedline, and the shield of the coax
is bonded in most cases to the chassis of the radio, and to
the car body at the other end.

The idea being that there is some rf voltage developed between
the bottom of the vertical and the body of the car, by virtue
of it (the vertical) being immersed in the local rf field.

This voltage is larger than the voltage that would be there, if the
car body were absent; that is if the coax shield simply stopped and
the whip were tagged on the end, out in the middle of nowhere.

Granted not a lot bigger, about a factor of two or three
probably. Remember that while the car is sitting on top
of rubber tires, there's a large capacitence between the car
body, and the actual ground.

No, no. the interelectrode distance is much too great for there to be much
capacitance. The thickness of a tire reduces the capacitance to a value so
low you'd be hard-put to measure it. Do the calculation; it's 'way down
there, despite the area of the car. If it were 1/4 inch or so, it would be
significant -- if the car was sitting on a sheet of metal that was grounded,
which it is not.

My guess is that the whole system is functioning as an unbalanced dipole.
I'll see if I can find anything on it later tonight. I don't recall any
discussions about it in the various ARRL handbooks.

--
Ed Huntress