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

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

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

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 -