[ARC5] Re: [Milsurplus] Re: Antenna tricks

[David Stinson]

Bob Wilder wrote:
 > At 09:16 AM 6/28/2005 -0400, Ray Fantini wrote:
 >
 >> While I was out in Dayton I stopped at the AF museum on Sunday and one
 >> of the things I saw was the HF antenna setup for the B-36, its a long
 >> wire that runs horizontal  from the front of the ship to the tail of the
 >> ship. its feed by about a eight inch vertical stub from the side of the
 >> ship to the horizontal element. dose this mean that the little stub is
 >> the radiating element and the long horizontal element is just a
 >> capacitor?

It depends on many factors- mostly the frequency you're feeding to it
and it's over-all geometry in relation to the body of the aircraft.
If the wire is not parallel along it's length, it's
a radiating element.  If it is, it acts like a section of balanced
feedline and becomes a capacitive loading element.

 > There are some on here who insist that the 8 inch stub is the actual
 > radiating stub.  I say they are dead wrong. I have duplicated this with
 > a 8-10 inch stub to a 100 ft long wire and taking an 11 watt fluorescent
 > light walking the length of the wire seeing the voltage peaks and nulls,
 > proving RF is present on the horizontal portion of the wire, thus it
 > also is a radiator.

Well, hang on a minute--
I assume you were somewhere on the ham bands.
Unless your wire was only a very few feet long, it
is not an "electrially short" antenna and is not,
therefore, addressed by the "capacitive loading" situation.
It will, indeed, act as an antenna with horizontal radiation.

Assuming your "wire" was used at longwave, then
you are looking at one side of what is, in effect,
an open-wire feedline.  Those peaks and nulls
are "mirrored" in the other side of the "feedline-"
which is the ground system- and are phase-canceled
for radiation, just exactly as in any feedline.
If you want to prove it, set up an electrically-small
test antenna, fed at the appropriate frequency,
with a counterpoise wire as your ground simulation
that meets the conditions above:
parallel sides with low frequency feed.
Put an RF ammeter in the horizontal part and
one in the same location under it in the "ground" part.
You will read the same current in both sides.

Taken as a whole, an electrically short antenna (by definition
that is one less than 10% of a wavelength long) is a big,
unsheilded tank circuit.
If you draw this out, it's easy to usnderstand.
It's "primary" operation mode is
where current leaves the top of the coil,
flows up the vertical element, dropping some voltage
and thus, expending power in the "radiation resistance,"
into the horizontal element, which is on side of a big
capacitor plate, down to the ground system, which is the
other side, and back up to the bottom of the coil.
There are other, shorter paths, which are usually leakage
from the coil and other elements close to ground.
They represent losses.
Only that current flowing in the vertical element contributes
to radiation, since currents in the horizontal element
are reflected very nearly 180 degrees out of phase
by the "mirrored" element in the ground system.
Charge a capacitor to +1 volt on one side and you will
see -1 volt on the other.  The horizontal element
and ground work the same way.
The function of the horizontal element is to supply
a large capacitance to
ground relative to those leakage paths near the ground end
and thus, pulling current higher up the antenna
and increasing the "current at X feet height" part
of the equation.  Look at it as a bunch of capacitors
in parallel.  Put a bunch of little ones to the left
and one big one to the right.  Apply AC and the current
in the circuit will flow down all the branches,
but the biggest current will be in the large
cap on the left.  This is your "top hat."
The rest of the caps are "leakage."

The horizontal part is all about getting
RF currents to flow up the vertical element, instead of
through leakage paths, such as from the coil turns to
ground.  You can't avoid leakage, but you can put a bigger
capacitor- and thus, a lower resistance path for current,
up in the air and connected to the vertical, so that current
will flow "up" and contribute to the voltage drop in
the radiation resistance, which is calculated by the
amount of current flowing in an antenna and the
height of that point: more current, higher up means more
radiation.  "Leakage" current, such as from the loading
coil turns to ground or from a cap loading at the base
of the antenna, happens near the ground, so the
"height" part of the equation is near zero and the
resulting radiation is near zero.

All that being said, many aircraft antennas are much longer
than the definition of "electrically short," so the
"horizontal part is a capacitor" rule doesn't apply.
Their actions are much more complex.
Also- most of them are not parallel with the aircraft
body, but "V" away from it.  That makes their horizontal
elements part of the radiating element, rather
than a loading device.

73 Dave S.