On Jun 3, 3:41*am, "-.-. --.-" wrote:
"-.-. --.-" ha scritto nel ...

Hello,

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

Missed that the expected frequency of the system is between 14 and 30 MHz,
but just curious if i had any chance to work 40 meters

-.-. --.-

It's possible.. But feeding a whip with a tuner usually does not
make
for an efficient mobile antenna. Not only are many/most tuners more
lossy than say using a loading coil on the whip, but current
distribution
suffers. Maximum current will be at the tuner which is not desirable.
The location of the loading coil has a large effect on the current
distribution
and efficiency of the antenna. Where you have it is about the worst
possible
place.
I have lots of people ask me about running whips matched with tuners..
I pretty much have a standard reply.. No! Not on my watch!
Chortle..
My mobile antenna is center loaded in the driving config.. Even higher
if
I add the 3 foot lower mast, but that's only when parked. In the
parked config,
my loading coil is 8 ft above the base of the whip. "14 ft tall whip"
And yes, you can tell a pretty good difference from the normal driving
config,
with the coil at 5 ft above the base. "11 foot tall whip"

"-.-. --.-" 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 3, 11:03*pm, wrote:
But feeding a whip with a tuner usually does not
make for an efficient mobile antenna.

A 11.5 foot (~3.5m) whip driven by an SG-230 autotuner was measured to
be 12 dB down from the top-rated bugcatchers and screwdrivers at one
of the CA 75m mobile shootouts back in the 1980's.
--
73, Cecil, w5dxp.com

*To determine efficiency you'd have to make some field strength measurements
(usually performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space.

The radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks

So if the base current entering the whip itself can be measured, and
whether or not a loading coil was used at the base to resonate* the
antenna system, and regardless of the loss in the r-f ground used by
the whip -- then for practical purposes the power radiated by the
antenna system will be the product of the square of that base current,
and the radiation resistance of the whip.

* However "resonated" antenna systems using a short whip do not have
the same radiation performance as those using a naturally resonant 1/4-
wave monopole. Resonance just means that the capacitive reactance at
the feedpoint of the short whip has been exactly offset by the
inductive reactance of a loading coil, which can allow for the most
efficient power transfer into the radiator.

However the radiation resistance of the whip is not changed by this
process -- it is still very low compared to a naturally resonant 1/4-
wave monopole. Therefore the radiation efficiency of such a short,
loaded, whip antenna system cannot approach that of a naturally
resonant 1/4-wave monopole unless the losses in the loading coil and r-
f ground are nearly zero.

Loading coils do not replace the "missing degrees" of an electrically
short radiator, as far as its radiation resistance is concerned. The
radiation resistance of a whip depends only on the electrical length/
OD of the whip, itself.

RF

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

Richard Fry wrote:
To determine efficiency you'd have to make some field strength measurements
(usually performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space.

The radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks . . .

This is true only if you don't confuse the idealized textbook models
with real antennas. But most of us are unfortunately stuck with using
the latter. In general, the impedance you calculate with the idealized
models doesn't match that of real world antennas. It works pretty well
for AM broadcast installations, where the length and large number of
radials make the impedance relatively independent of ground
characteristics. But this doesn't describe the typical amateur monopole
antenna, either ground or mobile mounted.

An approximation to input resistance can be made by adjusting for an
abbreviated radial system, but this gets increasingly unreliable as the
number of radials decreases. The best readily available modeling program
allowing the inclusion of a buried ground system, which uses the same
well-established equations as textbooks, is NEC-4. It, however, suffers
from a serious shortcoming in doing this calculation -- it assumes that
the ground is homogeneous to an infinite depth. Real ground is typically
stratified, and skin depth at HF is as much as several tens of feet, so
the representation of real ground is very poor. There are many cases
where a single "equivalent" value of homogeneous ground doesn't exist
which gives the same results as actual measurement. I've made very
careful measurements of a simple vertical monopole with various numbers
of buried radials whose impedance couldn't be matched with NEC-4 using
any ground parameters, and I believe this to be a common occurrence. In
no case would I depend on a computer model, let alone an even more
simplified textbook model, to predict the resistance of a real monopole
having an abbreviated ground system with enough accuracy to reasonably
estimate the efficiency.

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.

Roy Lewallen, W7EL

"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, 9:53*am, Roy Lewallen wrote:
Richard Fry wrote:
*To determine efficiency you'd have to make some field strength measurements
(usually performed with a calibrated field strength meter) in order to determine
how much of the power going into the antenna terminals is being radiated
into free space.

The radiation resistance present at the base of an electrically short,
linear, monopole (whip) antenna of various ODs can be calculated
rather accurately using equations found in various antenna engineering
textbooks . . .

This is true only if you don't confuse the idealized textbook models
with real antennas. ...

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.

CASE 1 = Zero loss resistance and reactance in the r-f ground, and
zero loss resistance in the loading coil:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:50:57 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.08578 V at 35.09 deg.
Current = 0.4986 A at 0.0 deg.
Impedance = 0.1408 + J 0.09888 ohms
Power = 0.035 watts
SWR (50 ohm system) 100 (25.17 ohm system) 100


CASE 2 = Same model as above, except with a total of 25 ohms loss in a
loading coil and r-f ground, and no reactance in the r-f ground:

EZNEC Demo ver. 5.0

1650 kHz 3 meter monopole 6/4/2010 10:49:40 AM

--------------- SOURCE DATA ---------------

Frequency = 1.65 MHz

Source 1 Voltage = 0.9386 V at 0.22 deg.
Current = 0.03729 A at 0.0 deg.
Impedance = 25.17 + J 0.09579 ohms
Power = 0.035 watts
SWR (50 ohm system) = 1.987 (25.17 ohm system) =
1.004

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.

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.

RF

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

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 -