Richard Fry wrote:

For the sake of discussion, below are two pastes from the same NEC
model using the demo version of EZNEC v. 5.0 -- which rather well
support my earlier post that the radiation resistance (NOT the
impedance) of an electrically short monopole is a function of its
electrical length, and not the loss resistance of the r-f ground and/
or the loading coil.

. . .
EZNEC calculated the radiation resistances of these two cases to be
0.14 ohms and 0.17 ohms, respectively -- fairly close, but not exact.
Perhaps Roy could comment on the reason why their agreement using NEC/
EZNEC is not better.

Sorry, I can't tell without seeing the EZNEC description file. If you'll
attach the .EZ file to an email message to me, I'll be glad to answer
your question. I wasn't able to get a radiation resistance that high at
that frequency for a 3 meter vertical of any diameter, so there's
something in the model which isn't immediately apparent.

Those wanting a good resource for the measured results for monopoles
of less than 1/8 electrical wavelength might try to locate the paper
by Carl E. Smith and Earl M. Johnson titled PERFORMANCE OF SHORT
ANTENNAS, published in the October, 1947 edition of the Proceedings of
the I.R.E.

The equation for the radiation resistance of short antennas given in
that paper is independent of the resistive losses in any loading coil
or r-f ground system.

And the same fundamental equations are used by modeling programs. The
problem is that interaction between the antenna, an abbreviated ground
system, and the Earth can modify the radiation resistance as well as
adding loss resistance. You might try modeling a few short verticals
with a few radials just above ground, and looking at the gain with
various radial systems. You'll find that the gain change doesn't exactly
correlate with the feedpoint resistance change when you assume a
constant radiation resistance. This isn't a shortcoming of the modeling
program, but a real effect. I doubt you'll find much about it in
pre-computer age texts, though, because it's probably a very tough, or
maybe impossible, manual calculation.

Roy Lewallen, W7EL

Roy Lewallen wrote:

As a side note, Brown, Lewis, and Epstein's sparse radial results can be
matched reasonably well with NEC-4, but it does require a fair amount of
ground constant adjustment for various numbers and lengths of radials.

Mobile mounted whip antennas fare even worse relative to simple textbook
models. I don't have any experience with comparison of computer models
with actual measurement. Those results should depend on the care with
which the model is constructed and the amount of influence the ground
has on the impedance.

I would figure that getting an accurate ground influence in an antenna
design program would have to be a daunting project, indeed.

While installing my bugcatcher, I did a lot of it in stages, noting the
positive influence on the results. My thinking at this point is that the
make and model, and the size of the vehicle would be critical for the
model, and small changes go a long way.

What's more, the more efficient and narrow the antenna, the more effect
the changes have. Given that a good setup always involves a lot of
custom work like bonding and turning potential radiators like the
exhaust system into more bonded area, it is a really tough exercise.

- 73 de Mike N3LI -

On Jun 3, 3:08*am, "-.-. --.-" wrote:
My poor knowledge of antenna systems think that the ATU, choosing the
appropriate LC value, bring the entire system in a resonant condition: true
or false ?

If you perform a frequency sweep with an antenna analyzer connected to
the tuner input and the impedance goes from R-jX ohms to 50+j0 ohms to
R+jX ohms, the purely resistive frequency meets the definition of the
*system* resonant frequency. Unfortunately, under those extremely
lossy conditions, "system resonance" means almost nothing.
--
73, Cecil, w5dxp.com

"-.-. --.-" ha scritto nel messaggio
...
Hello,

my mobile setup is composed by a 2 meter vertical whip feeded immediately
close to it by an automatic antenna tuner.

Thanks to all. Now is a clear statement that my problem is measuring the
"inefficiency" of the entire system

Only to mention that due to a sort of "quiet life way" with the neighbors,
my home setup is a ATU CG-3000 feeding 9,5 meters height monopole with about
40 random lenght radials (from 5 to 20 meters of lenght) and electrically
connectet to 200 square meters of chicken fence, with real chickens inside
perform quite well on 80, from 40 to 12 meters is probably the best
antenna i ever had compared to old dipoles, in 10 maybe it is too close to 1
WL... but work almost all , even in furiouses pile ups.

Now that I annoyed with this info, the conclusion of the OP or the third
question if you want: 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 ??
Or i miss something important about the 2 feeding methods ??

TIA,
-.-. --.-

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.
--
73, Cecil, w5dxp.com

"Cecil Moore" ha scritto nel messaggio
...
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.

Yes, understand *perfectly*. My energy go somewhere, and this "somewhere" is
heating some toroid into a shield case or radiate on the air. Conservation
law.

Center load i think mean also lower ground loss, IIRC.

And as i understand, a RX shootout of the mobile whip compared with my full
size 1/4 wl vertical antenna with the same transceiver, to have a suitable
relative dB comparison, can give the best test measure i can do with my
actual equipement.

Thanks Cecil an other posters, other suggestions or links to learn more are
very appreciated.

-.-. --.-

On Jun 4, 12:21*pm, "-.-. --.-" wrote:
"Cecil Moore" ha scritto nel ...

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.

Yes, understand *perfectly*. My energy go somewhere, and this "somewhere" is
heating some toroid into a shield case or radiate on the air. Conservation
law.

Center load i think mean also lower ground loss, IIRC.

And as i understand, a RX shootout of the mobile whip compared with my full
size 1/4 wl vertical antenna with the same transceiver, to have a suitable
relative dB comparison, can give the best test measure i can do with my
actual equipement.

Thanks Cecil an other posters, other suggestions or links to learn more are
very appreciated.

-.-. --.-

While that is true your exiting installation may be better than you
think. Best Antenna can be subject to to any number of parameters
from greatest gain to practical operations and installation to
significant others opinion of aesthetic appeal.


Jimmie

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.