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..

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

On Jun 4, 4:40*pm, Roy Lewallen wrote:
Richard Fry wrote:
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.

Could you please explain why, if the same fundamental equations given
in antenna engineering textbooks and I.R.E. papers are used by
modeling programs, the results of their use do not always support each
other very well?

If it is accepted that the radiation resistance of a short monopole is
independent of the loss resistance in the loading coil and r-f ground
either alone or together, then what is the basis for the variation in
radiation resistance that you report?

BTW, the equations in the Carl Smith paper I referred to earlier in
this thread produce a radiation resistance of 0.113 ohms for a 1.65
MHz, 9.84' (3-m) x 0.25" OD, base driven monopole -- which is not
_hugely_ different than the values calculated by EZNEC.

RF

Richard Fry wrote:

Could you please explain why, if the same fundamental equations given
in antenna engineering textbooks and I.R.E. papers are used by
modeling programs, the results of their use do not always support each
other very well?

I'm not aware of any cases where engineering textbooks and papers
disagree with modeling programs. NEC, for example, has been very
extensively tested against both theory and measurement. If there are
cases where the programs seem to disagree with theory, it's very likely
due to careless modeling resulting in a model which isn't the same as
the textbook model. Can you cite an example of disagreement between
computer model and textbook theory?

If it is accepted that the radiation resistance of a short monopole is
independent of the loss resistance in the loading coil and r-f ground
either alone or together, then what is the basis for the variation in
radiation resistance that you report?

It is indeed accepted that the radiation resistance of a monopole over a
perfect ground of infinite extent has the characteristics you ascribe,
and computer models show this independence as they should. (I haven't
yet received your model which you feel seems to show differently.) But
it's neither true nor "accepted" when the ground system is much less
than perfect. The variation is due to interaction between the vertical
and ground system, just as the radiation resistance of a VHF ground
plane antenna changes as you bend the radials downward. Altering the
number, length, depth, and orientation of radials has more of an effect
than simply adding loss.

BTW, the equations in the Carl Smith paper I referred to earlier in
this thread produce a radiation resistance of 0.113 ohms for a 1.65
MHz, 9.84' (3-m) x 0.25" OD, base driven monopole -- which is not
_hugely_ different than the values calculated by EZNEC.

EZNEC gives a result of 0.1095 ohm with 20 segments, converging to
around 0.103 ohms with many more segments. Keep in mind that the model
source position moves closer to the base as the number of segments
increases.

The author's result is good. If you examine the paper carefully, I'm
sure you'll find that the author had to make some assumptions and
approximations to arrive at his equations -- the most fundamental
equations can't be solved in closed form, and many, many papers and
several books were written describing various approximations to
calculate something as basic as the input impedance of an arbitrary
length dipole. If you do some research, you'll find that the many
different approximating methods all give slightly different results. The
small disagreement in the cited paper is really a measure of how good
his approximations were. Modeling programs have to use numerical methods
which are limited by quantization, but they have the advantage of not
needing the various approximation methods required for calculation by
other means.

Roy Lewallen, W7EL

wrote:
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.

Can you give measured data for the losses? (or reasonably high fidelity
model data). I don't think the losses are *big* in either case, so you
might be looking at the difference between 5% loss and 7% loss, which is
negligible in the overall scheme of things.

I'd be willing to bet a six pack of frosty cold beverages (not the
antenna) that it's not a 75% loss vs 10% loss situation..




Maximum current will be at the tuner which is not desirable.

yes and no. I don't think, on a short antenna (2 meters long, here, but
let's say up to 3 meters) the difference will be significant;especially
when viewed in the context that the whip is next to a big giant metallic
object. They're ALL short compared to a wavelength, so the difference
in ideal gain is going to be somewhere between 1.64dBi (infinitely short
dipole) and 2.15dBi (half wavelength dipole)... And, given it's a
(mostly) vertical antenna, for which you have no real control over the
propagation path, who's to say that the fatter lobe on the infinitely
short dipole might not be better than the slightly skinnier one on the
half wavelength one.

(yes, it's a monopole..same idea though)

The location of the loading coil has a large effect on the current
distribution
and efficiency of the antenna.

Quantify "large"...

Is it bigger than 3 dB? (100%)
Bigger than 1 dB? (25%)

You're already taking a 5-6 dB hit just by having it bolted to a car
driving down the road. While I wouldn't say you should capriciously
throw away performance, you're already in a compromise situation.
Having less wind drag or a broader operating band might be a bigger
advantage than a dB or two.


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.

And what's your operating bandwidth? Can you tune anywhere in the 40m
or 80m band? What's the efficiency of your system when you're not right
at the "sweet spot"...

The efficiency of the autotuner system is pretty constant across the band.

Now, it's possible all one needs to do is check into the nets at fixed
frequencies within a few kHz.. In which case the fixed tune system works
fine (assuming you've tuned it while actually driving... )

If you operate "mobile" (as opposed to portable, parked by the side of
the road), the autotuner takes care of the substantial change in antenna
impedance as the wind pushes it back. Or, one could mount it in the
center of the roof and guy it.. (been there, done that)





.. 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"

measured difference?

Cecil Moore wrote:
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.


that's a pretty big difference.. (12 dB implies a factor of 16.. that's
like most of the Tx power being dissipated somewhere, and that sounds
like "component melting" levels)

Have you a link to the data and test methodology?

On Jun 6, 4:58*pm, Roy Lewallen wrote:
This is why, for
example, a center loading coil must have more inductance than a base
loading coil to effect the same change in reactance at the base.

The following is based on a fixed length antenna.

The phase shift at the top of each coil is associated with the abrupt
shift in characteristic impedances at the coil-stinger junction
(according to W8JI). When a base section is added to a base-loaded
antenna, there is an opposite abrupt shift in characteristic impedance
at the base-coil junction. That bottom (negative) phase shift
subtracts from the (positive) phase shift at the coil-stinger junction
so more phase shift must be added through the coil to compensate for
the phase shift lost at the base-coil junction. Increasing the coil
length provides the necessary additional phase shift.

Assume a loading coil has a characteristic impedance of 4000 ohms and
the stinger has a characteristic impedance of 600 ohms at the coil-
stinger junction. Given the impedance looking into the stinger, it is
easy to calculate the phase shift at the coil-stinger junction. Let's
(for instance) say the stinger's input impedance is 0.25 - j2500 ohms.
If we normalize that impedance to the assumed Z0=600 ohms of the
stinger, we get very close to -j4.167. The impedance at the very top
of the coil is the same and if we normalize to the assumed Z0=4000
ohms of the coil, we get -j0.625 ohms. If we subtract the arctangent
of those two values, we get the phase shift: 76.5 - 32 degrees = 44.5
degrees at the top of the loading coil. We can also read that same
value from a Smith Chart.

When we go to a center-loaded coil, the calculations are complicated
by the resistive portion of the impedance, but we will find a negative
phase shift at the bottom of the coil that subtracts from the positive
phase shift at the top of the coil. Since we have reduced the total
system phase shift by moving the coil to the center of the antenna, we
need to add more length to the coil to increase the phase shift
through the coil in order to compensate for the negative phase shift
lost at the bottom of the coil.

One can emulate the loading coil problem using pieces of transmission
line with different Z0s. The basics of shortened dual-Z0 stubs are
covered he

http://www.w5dxp.com/shrtstub.htm

For instance, the following shortened stub has a resonant frequency at
which it is electrically 1/4WL long even though it is only 1/8WL long
physically because of the 45 degree phase shift between the two
sections.

-----22.5 deg 300 ohm-----+-----22.5 deg 50 ohms-----

What happens to the resonant frequency if we move half of the 50 ohm
line to the bottom?

----11.25 deg 50 ohm---+---22.5 deg 300 ohm---+---11.25 deg 50 ohm

How many degrees do we need to add to the 300 ohm line to achieve the
same resonant frequency as before?
Can anyone out there solve this problem?
--
73, Cecil, w5dxp.com