On Wed, 09 Jun 2004 00:00:34 -0000, Larry W4CSC
wrote:

Gary Schafer wrote in
:


The diameter of the antenna wire is not too important. Actually the
larger it is the less resistive loss it has and less power will be
wasted in heat. But unless the antenna is significantly shorter than a
quarter wavelength that loss is negligible in the antenna as the
radiation resistance (radiation resistance is where the power goes to
be radiated) is usually much higher than the resistive loss of the
wire.

The diameter of the antenna wire is very important in the antenna's
BANDWIDTH. Go by the CG shore station and look at how WIDE the conical
monopole antenna is:
http://www.tpub.com/content/et/14092/css/14092_35.htm
The whole reason for the wide cone of these broadband HF antennas is to
make it look as if the conductor were several FEET across to the RF from
the feedpoint.

Multiple, parallel conductors are also used to increase antenna wire
apparent diameter in broadband rhombic antennas such as:
http://www.smc-comms.com/rhombic_antenna.htm

To quote the text:
"The simple one wire system has a bandwidth of approximately 2: 1, however
SMC have wide experience in the design of this type of antenna and are
able to offer arrays with 1, 2 or 3 wires per leg to give a bandwidth of up
to 4: 1 and, by careful design, gains of 22 dBi are possible."


However in a very short antenna the radiation resistance can be only
an ohm or a few ohms. Then the resistance of the wire would be a
larger percentage and the heat loss would be greater thus warranting a
larger diameter wire.

Huh?? ANY antenna under 1/4 wavelength long exhibits HIGHER and HIGHER
impedance the SHORTER it gets. The first low impedance of a wire antenna
occurs when its radiator (against a ground, artificial or real) is 1/4
wavelength long. A very short antenna, i.e. a 6' whip on the handrail, has
a very HIGH impedance as frequency decreases on the HF band. That's why we
use an L network to match it to 50 ohms....coil in series, cap to ground to
lower its impedance.


Otherwise a larger diameter wire has the advantage of greater
bandwidth for given tuner settings. But the difference between #10 and
# 16 would probably not be noticeable.

True, that's why we use multiple parallel conductors above.

As you well know, in the case of the ground system as we have said
many times before, it needs to be as short as possible or it becomes
part of the antenna and radiates. "The antenna starts at ground".
Anything above ground is antenna.


Actually, in a plastic boat, the radiation from the ground strap is useful
radiation. You've just moved the FEEDPOINT up the radiating element above
the sea. My feedpoint is about 4.8' above ground on Lionheart. It's
signal strength 5, readability 8 in Moscow, Belarus, UAE, Japan, Brazil,
most of Western Europe on 40 meters and 20 meters. Works pretty good!

73, Larry W4CSC


Oh oh, here we go again. :)

Remember I said that the radiation resistance drops as the antenna
gets shorter. That is the reason the losses go up with a shorter
antenna. Higher current in the antenna and loading coil means more I
squared R loss.

(radiation resistance is equal to the equivalent resistor that would
dissipate the same amount of power that is being radiated) Lower
radiation resistance requires more current for the same amount of
power verses a higher radiation resistance and less current.

The reactance does indeed get higher the shorter the antenna is. With
an antenna shorter than a quarter wave length as you know it looks
like a capacitor. (capacitive reactance) The less capacitance (shorter
antenna) the higher the reactance. The coil in series provides an
equal but opposite inductive reactance to cancel the capacitive
reactance in the antenna.

That leaves only the radiation resistance to feed power to. The coil
AC resistance (not reactance now)is then effectively in series with
the radiation resistance of the antenna. The same current must flow in
both the antenna and coil losses. While the antenna radiates most of
the power it gets, the coil dissipates power in heat equal to the I
squared R loss in the coil.

The capacitor to ground on the other side of the coil and part of the
coil form an L network to match the impedance to the feed line.

Actually we could say that the L network portion really matches the
radiation resistance plus the coil resistance to the feed line.
Because when the coil reactance and antenna reactance are equal we
have resonance and the only component left is purely resistive.

The high reactance in the antenna causes the voltage to go high. But
there is also a phase shift due to the reactance. So the current is
not in phase with the voltage developed across the reactance. That is
why the voltage is high.

Regards
Gary