Cecil Moore wrote:
On Jun 4, 6:19 am, "-.-. --.-" wrote:
... how it is possible that mobile setups with the
"motorized" antennas can have a minimum of efficiency in 40 meters ?? What
the difference from a variabile motoryzed L and an ATU at the feed point ??

Most screwdrivers and bugcatchers are more center-loaded than base
loaded. The section of the antenna that supplies a good part of the
radiation is the straight section between the feedpoint and the bottom
of the loading coil. An ATU driven whip doesn't possess that high-
efficiency, high-current section. The highest current sections in an
ATU system are inside the ATU - not good for radiation. Everything
else being equal, a center-loaded antenna will beat a base-loaded
antenna by ~3-5 dB according to mobile shootout results. The radiation
resistance for a center-loaded 75m mobile antenna is approximately
double that for a base-loaded 75m mobile antenna, i.e. close to double
the efficiency.

According to 75m mobile shootout results, an ATU driven whip is ~8 dB
down from a base-loaded bugcatcher because the bugcatcher coil
radiates and an ATU is usually shielded and often uses powdered iron
toroids for the coils.

As a point of clarification, Cecil, the bottom loaded bug catcher you
refer to - is it the matching coil or the loading coil? I only knew of
mid-loaded bugcatchers.

- 73 de Mike N3LI -

On Jun 4, 12:55*pm, Michael Coslo wrote:
As a point of clarification, Cecil, the bottom loaded bug catcher you
refer to - is it the matching coil or the loading coil? I only knew of
mid-loaded bugcatchers.

Jim, k7jeb, once used a standard 75m Texas Bugcatcher coil as a base-
loaded whip (no top hat) and entered one of the CA 75m mobile
shootouts. He was "only" 3 dB down from similar center-loaded Texas
Bugcatchers (no top hat). This fits well with the radiation resistance
estimate for the center-loaded bugcatcher being double that of the
base-loaded configuration.
--
73, Cecil, w5dxp.com

On Jun 4, 12:55*pm, Michael Coslo wrote:


As a point of clarification, Cecil, the bottom loaded bug catcher you
refer to - is it the matching coil or the loading coil? I only knew of
mid-loaded bugcatchers.

A short whip can be fed at any point on the radiator.
In Cecil's case, I assume the coil was a true loading coil,
and not the matching coil. As per his numbers, the base
loaded was quite a bit better than the "tuner" loaded whip,
which was 12 db down from the center loaded bugcatcher.
In general, appx 3/4 the length of the whip from the base
will be the appx best location for the coil.
The higher the coil is, the better the current distribution.
But.. The higher the coil is, the more turns of wire you
need to tune. So there is a trade off of current distribution
vs coil loss due to the extra turns.
You could have the coil at 95% high, and have great
current distribution, but the losses of all the turns required
would eat you for lunch.
So... usually around 3/4 of the way up will be about the
optimum location. 1/2 way up is good, and a good compromise
between current distribution and coil losses.
For a given length whip, Reg Edwards "vertload" program
can be used to calculate the best location for the coil,
and having played with it, and using the real antennas to
compare, I think it is very close.
Also, it jives with the info and graphs used in the ARRL
antenna handbook on that subject.

On Jun 4, 3:32*pm, wrote:
On Jun 4, 12:55*pm, Michael Coslo wrote:



As a point of clarification, Cecil, the bottom loaded bug catcher you
refer to - is it the matching coil or the loading coil? I only knew of
mid-loaded bugcatchers.

A short whip can be fed at any point on the radiator.

er.. I meant to say the coil can be placed at any point on
the radiator..