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The way to get better performance out
of your monopole-over-conductor (ground-plane) antenna is to understand how
it operates. Here's my attempt, and I hope it's not too feeble. Picture a wooden
pencil, the kind with an eraser, standing on-end, eraser up, in the center
of a round mirror which has a diameter of two pencil lengths. Now let's look at
this from a distance, looking down at a 45 deg angle to the
pencil. The distance must be enough so that the image we see is coming at us as
close to a plane-wave as possible, where all parts of the image are at the same
angle as received by the eye. At least ten mirror-diameters will be fairly
good. What we will see is a pencil with an eraser on each end that has a
total length of 1.4 pencil-lengths. Note that the eraser on the lower half is
right at the forward edge of the mirror. The image of the pencil has become
inverted and added-on to the bottom of the real pencil. Now
let's drop our line-of-sight to where we are looking down at 30 deg. Now
we see a pencil whose upper-half is 87% of a pencil-length long, but the
lower image is only a partial pencil; the upper portion has dropped off the edge
of the mirror. If we made the mirror 73% larger, we would just be able to see
the eraser on the bottom.
Basically this is the same as the
monopole, the pencil, over a ground-plane, the mirror. The analysis of the
monopole speaks about the "image" antenna, the one that forms the bottom portion
to a plane wave arriving from a distant source. We can say that the incident RF
wave bounces or reflects off the ground plane up into the monopole to give us
twice as much energy. Just as in our light reflection, the RF wave, in
reflecting off the ground plane, has its RF voltage inverted so that it adds to
the direct monopole reception. And just as when the mirror wasn't long enough
initially to show us the whole lower-half of the image at lower angles, so, too,
does our reception drop off at lower angles when the ground-plane is short.
There is less energy being reflected up into the antenna element. You
can see that to get full rececption at very shallow angles requires a very long
ground plane. And where does most of our reception come from? Very shallow
angles! That is why the dipole is so much better than a monopole with an
abbreviated-length conductor below it. The only thing that prevents the monopole
from performing too badly is that most of the antenna's current, which is what
causes the radiation, is in the feed-end; maximum at the feed, zero at the
tip. So even the abbreviated image is of the stronger radiating
portion.
Now I spoke of the signal bouncing or
reflecting off the ground plane. It doesn't! What actually takes place is that
the incident electromagnetic wave induces a current in the ground plane which
then re-radiates the signal. If the ground plane is not highly conductive, then
the re-radiated signal will not be as strong. This current that is induced is
only in a very, very thin layer of the ground plane in what is called
skin-effect. For copper at 20C it is 2.61 inches / F^1/2 ,
or 20.6 nano-inches, 2.06E-08" at 127 MHz! So even 0.001 copper will give an
excellent ground-plane at our Localizer, VOR, and Comm frequencies; it will
be almost 50,000 times as thick as the necessary thickness. But I can't imagine
that a paint highly-loaded with conductive material would be able to have
the required conductivity in such a thin layer. It may look highly conductive in
thicker layers, but that's not what counts.
BTW; the antenna transmits
just as well as it receives with the wave going in the opposite direction;
an antenna is considered a reciprocal radiator. That is, as long as the
transmitted power doesn't get the ground-plane too hot where its conductivity
decreases or it melts!
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