A few themes have shown up on this thread that seem to reinforce some
misconceptions about antennas. They might not have been meant that way,
but I think it's a good idea to clarify them in case a casual reader
might drop by and be misled.

The first one is about where the "transmission line" ends and where the
"antenna" starts. The antenna starts at the point where the two current
carrying conductors move far enough apart for significant radiation to
take place. By way of illustration, let's take the example of a simple
vertical mounted on a perfect ground plane. We'll feed it at the bottom
via a piece of coax. It doesn't matter where the coax is, above or below
the ground plane. Inside the coax, there's current on the outside of the
inner conductor, and an equal amount, flowing the opposite direction, on
the inside of the outer conductor. The fields from these two currents
cancel everywhere outside the coax, so it doesn't radiate.

At the point where the coax connects to the antenna, the current from
the inner conductor flows up the antenna. The (equal) current from the
inside of the shield flows over the ground plane, spreading out from the
point where the shield connects to the ground plane. The currents are
now physically separated, so their fields no longer cancel, and net
radiation can result.

An important feature of this setup is the effect of the current flowing
along the ground plane. For every bit of current flowing outward in one
direction, there's an equal bit flowing outward in exactly the other
direction. The fields from these two bits of current cancel at right
angles to the directions of flow. When you look at all the bits of
current, you find that the net field from the whole amount of current is
zero. So the current on the ground plane doesn't result in any
radiation. (By contrast, the current on the antenna wire doesn't have
any cancelling current, so it radiates.) A conductor which carries
current but doesn't radiate (significantly) is often called a
"counterpoise". Misuse of this term is another problem with some of the
postings, and I'll address it in a minute. The important thing to
remember is that a "counterpoise" doesn't radiate significantly. If it
does, it's not a "counterpoise" but a part of the antenna.

What happens if we put a piece of pipe around the bottom, say, 1/4 of
the antenna, weld it to the ground plane, and connect the coax shield to
its inside? Now the pipe is an extension of the transmission line. The
portion of the antenna inside the pipe is the center conductor, and the
pipe is the shield. So the portion of the antenna inside the pipe can't
really be called an "antenna" any more, since it doesn't radiate.

The current on the inside of the pipe reaching the top of the pipe flows
over the edge and down the outside to the ground plane. Here's a very
important point: *This current radiates just like the current on the
antenna wire*. What we have now is a lopsided dipole. The antenna wire
protruding from the top of the pipe is one side of the dipole, and the
outside of the pipe is the other. The "feedpoint" is where the wire
sticks out of the pipe. Once the current along the outside of the pipe
hits the ground plane, it spreads as before. The current on the outside
of the pipe radiates, the current on the ground plane doesn't.

A statement was made about a situation sort of like this "shielding" the
"high current" part of the antenna. This isn't what happens at all. What
happens is that the portion of the original antenna wire inside the pipe
is no longer an antenna at all, but part of a transmission line. The
current on it isn't the same as before the pipe was put there. The
radiating antenna is now the portion sticking out of the pipe, and the
current at its bottom, all other things being equal, will now be greater
than before the pipe was added. No signal or power is lost due to
"shielding". All you've done is shortened the antenna a bit, just as
though you'd cut a bit off the top.

Now let's remove the pipe and bend the ground plane downward into a
cone, with the antenna sticking out of the cone's apex. Here we'll find
that the current flowing on the ground plane does indeed radiate.
There's no net radiation straight up, but it radiates horizontally and
in all other directions. The ground plane is no longer a "counterpoise",
but simply a conically-shaped dipole half. If the ground plane radius
was about the same as the "antenna" length, and the ground plane is bent
downward at a reasonably sharp angle, the radiation from the conical
ground plane will be about the same as the radiation from the "antenna".
Calling it a "counterpoise" doesn't give it magical properties so we can
ignore it -- it's every bit as much a part of the antenna as the "antenna".

Finally, consider an HT. Here, one half of the antenna is the rubber
ducky or other "antenna". If you put a sleeve over part of the outside
of it, the real radiating top part of the antenna is the part sticking
out of the sleeve. The other half of the antenna -- NOT a
"counterpoise", but a real part of the antenna of at least equal
importance to the intended part -- is the outside of the sleeve, and the
outside of the HT, your hand, and your body. Whatever current flows up
into your "antenna" also flows along the other half -- that is, along
the HT and your body -- and radiates.

Roy Lewallen, W7EL