Steve N. wrote:
. . .
While what Roy says is correct (there should rarely be any doubt) , I
believe Jim's interpretation is not in-valid and can be a very good "mental
model" of antennas.. The monopole needs something to be its "other half" as
Jim says. In a ground plane antenna, the "plane" is the other half. In a
dipole (vertical or otherwise) the bottom half is the "other half". Any and
every circuit needs two connections to have a complete circuit. Sometimes,
with antennas, the "other half" is not obvious to everyone, but it will
always be there and Roy's words; "...the same amount of current which flows
into a monopole must flow somewhere else .." addresses this "other half"
issue. I believe Jims words are sufficently close to Roy's words to be
equivalent. So, I would say, Yes. There ain't no such thing as a monopole,
you just don't realize what is acting as the other half (If you think you
have one).
I believe this basic concept can be applied to almost every antenna
configuration to help understand, to a great extent, what is "going on"

I agree with that.


Jim:
the element is
reflected by ground or a ground plane or a counterpoise and is
effectively a two element set up anyway.
Roy:
No, an element isn't "reflected". The field from any antenna, monopole
or dipole, is reflected by the ground, creating interference with the
unreflected field.

Actyually I'd say Yes. I believe the following words describe precisely the
same thing Roy said.
In electromagnetic field theory studies, there is something called the
"Method of Mirrors". This is a way to calculate a field when there is a
conducting plane nearby. To model the conducting plane is mathematically
very complex. You actually can model the conducting plane as a mirror which
has produced a "reflection" of the element in it. This reflection, because
of the properties of the field, has the opposite sign (polarity) and is
mathematically handled as though it was another element in the place
conducting plane. . It turns out that with this new model, it is much
easier to do the math to get the total field. (NO I can't do it any
more...don't actually think I was doing it the first time! bad instructor).
So, I would say that it is a pretty good analogy to say that the ground
plane "has the effect of" reflecting the monopole and therefore providing
the "other half".

The reasons I objected to the concept are twofold. First, there's some
ambiguity in what constitutes the "ground plane" which does the
"reflecting". The "ground plane" of a ground plane antenna reflects
nothing except very high angle radiation. So it doesn't act as a mirror
in any sense. Secondly, the Earth does reflect the field, but the result
isn't anything like a mirror. Only a perfect ground ends up creating the
same shaped field from a grounded vertical as a free space dipole. Real
ground creates a field shape which is very significantly different. So
the model of a mirror-like ground isn't at all a good one; its use leads
to incorrect conclusions so I don't feel it should be used unless the
user clearly understands that the conditions under which it's valid
aren't ones you can achieve very well at all -- except with salt water
or with a very large (several wavelength radius) conducting plane
perhaps at UHF.

so, somebody take me to
school on this; what about a ground plane whip such as a cb antenna with
another identical whip mounted upside down below it?
This constitutes a vertical dipole, a common antenna type.

You have, ideed described a dipole turned vertical...and I feel you have
identified a very important concept in antennas. Just about every antenna
can be understood better if you start out by finding the "dipole" hidden
within. beams, G5RVs end-fire arrays, broasdide arras, J-Antennas,
colinears...


would the bottom
whip appear equal to a ground plane and allow the antenna to radiate as
it was intended to?
No, it wouldn't "appear equal to a ground plane". As for the antenna
radiating as it's intended to, how do you intend for it to radiate?

Per my above, I'd say that Jim's description is a relatively good model
to have in your head. If the groundplane "acts" similar to a mirror giving
rise to the equivalent of the "other half" of the dipole, and that result is
equivalent to another "half" mounted below it, I feel this only leaves us to
debate the real meaning of Jim's words "...appear equal to ...". I think
that is a pretty good way to think about it.

Well, no, a typical "ground plane" doesn't act like a mirror, as I
explained above. And even the Earth does a very poor job of acting like
one. At best, it's a dirty mirror which isn't uniformly dirty. The only
common exception is salt water, which does a fair job.

would this work in an application where a decent rf
ground cannot be achieved?
Sure. A vertical dipole is a very common type of antenna. A google
search should bring you a wealth of information.

Ditto on what Roy said, plus: The antenna commonly called a "J-Pole" is
"the same thing" as a half wave dipole, but stood on end,
vertically...EXCEPT it is fed power at its bottom end not in the middle.

As an asside...I'll also add that this concept of a "decent RF ground"
is something that many seem to believe is some sort of an absolute thing.
Comments labout wanting to get a wire to a ground rod as being the solution
to all things...the hunt for the "good RF ground". I have a tongue-in-cheek
twist on an old saying: "A ground's a ground, the world around." An RF
ground is somwhat of a myth, even on a solid sheet of copper, but some think
there is this absolute GROUND that all things must be connected to in order
for things to work correctly, or solve problems. Of course I know that, as
Jim states, that the bottom half of this vertical dipole is the "other half"
of the antenna and might be considered a "ground" but is is nothing more
than the other half of the antenna and hads, and needs, absolutely no
relation a zerp potential reference, whatever that is.

Yes, "ground" is a concept which is almost universally misunderstood and
misused. Calling something "ground" seemingly imparts magical properties
to it. Very often, it gets in the way of understanding how antennas
work, and people would be better off forgetting about it. It's not
uncommon for an EZNEC user to ask how to model a "ground" or to specify
that something is a "ground", meaning some sort of elevated
"counterpoise" or "ground plane". I explain that the program doesn't
know or care what the user might consider to be "ground" -- all it knows
and cares about are conductors and currents. People hoping to understand
antennas would be well served by doing the same.

. . .

Roy Lewallen, W7EL