Gee, Reg. Until now I've always gotten the impression that you already
learned everything there was to know about this stuff. :-)

73, AC6XG

=============================

Jim, if you promise, cross-your-heart, not to tell anybody I'll let you into
a long-hidden secret.

When it comes to the the distribution of current along a loading coil on a
vehicle-mounted whip I am severely handicapped in that I have never been
mobile in a motor car with a radio transmitter, never owned a motor car,
never even held a driver's licence.

Otherwise I am quite a normal person who takes an interest in electric
currents flowing along wires, rods, through coils and around the surfaces of
such things as vehicle bodies. Normallity extends to world-wide objections
against being choked by petrol fumes and the now common practice of
financing pirate expeditions to obtain the diminishing raw material from
which the poisonous liquid is refined.

It seems Yuri is interested in modelling short, coil loaded antennas. He
refers to controversy. I can set his mind at rest and assure him there is
none.

When the length of a loading coil is short in comparison with the overall
height of the antenna, certainly in comparison with a wavelength, the
current into one end can be assumed, with negligible error, to be equal to
that which comes out of the other end as with any other coil in an L,C,R
network analysis. Its stray capacitance can be ignored except for
investigating its self-resonant frequency.

It is a lump of inductance effectively concentrated at its midpoint. For
estimating antenna behaviour and performance it is necessary only to add
half the length of the coil to the length of the lower portion of the
antenna, and to do likewise to the upper length. The antenna's distributed
radiation and wire loss resistance can be sufficiently accurately estimated
from these dimensions, all being transformed to the feedpoint according to
the normal transforming action of the lengths of transmission line (the
antenna parts) involved.

================================

When coil length is nearly as long as the antenna, ie., a close-wound
helical for the lower frequencies, in which coil loss for a high inductance
is minimised by using thick wire rather than an inconvenient, very large
diameter coil, the antenna is best considered as a continuously loaded
1/4-wavelength transmission line in which its uniformly-distributed
capacitance, loss resistance and radiation resistance per unit length is
taken into account.

================================

For practical purpose, these different-proportioned sorts of short vertical
antennas all have the same, simple, well known radiation pattern. Any slight
differences are overwhelmed by variations, entirely out-of-human-control, in
the local environment and along the propagation path. What matters is
radiating efficiency. The standard of radiating efficiency is that of a very
high 1/2-wave dipole of any orientation and there's no need to be concerned
here where the radiation disappears to.

There is only one question of consequence. At what height up a short
vertical is a coil of given intrinsic Q to be located to maximise radiating
efficiency? It is never at or very near the top! As coil height increases
the required inductance and number of turns increases rapidly. Coil loss
resistance always overtakes the improvement in radiation resistance due to
the change in distribution of current along the antenna. A high resistance,
self resonant coil of many turns of fine wire right at the top of the
antenna eventually fails.

================================

There are various special cases which are dealt with by simple programs
available from the website below.

There is one program which covers from helicals, via screwdrivers, to lumped
coils. The coil can slide up and down the antenna to find the location of
maximum efficiency for given coil length and diameter. The number of coil
turns and wire gauge are automatically recalculated to maintain the same
required antenna resonant frequency. Program name is LOADCOIL.exe
Download and run in a few seconds.
----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================

On 30 Oct 2003 01:16:24 GMT, oSaddam (Yuri Blanarovich)
wrote:
but article
has details, measurements, pictures of meters and facts of life.

Hi Yuri,

Then what's left to be said here?

73's
Richard Clark, KB7QHC

Reg writes:

It seems Yuri is interested in modelling short, coil loaded antennas. He
refers to controversy. I can set his mind at rest and assure him there is
none.

When the length of a loading coil is short in comparison with the overall
height of the antenna, certainly in comparison with a wavelength, the
current into one end can be assumed, with negligible error, to be equal to
that which comes out of the other end as with any other coil in an L,C,R
network analysis. Its stray capacitance can be ignored except for
investigating its self-resonant frequency.


There is none? You are confirming there is one by your above statements.

The point is that W9UCW measured, that difference in "normal" loading coil
(not long coils or helicals) is in order of 40 to 60% less at the top of the
coil. That is significant in calculating or optimizing the efficiency of loaded
antenna. MEASURE it and don't rely on myth perpetuated since 1955 by Belrose
till today's ARRL Antenna Book. ON4UN has it right in his book.

Resonance is no big deal, efficiency is greatly affected, modeling programs are
way off especially if you include more loaded parasitic elements.

Did you read my article, facts and measurements? You can try to repeat the
measurements to validate the effect. That's what I am looking for, if we are in
error, would like to have it pointed out. Not speculations that it "should be"
like that.

Yuri

Then what's left to be said here?

73's
Richard Clark, KB7QHC


See the comments from the flat earth society, even Reg has and "hasn't" have a
problem.

Significant impact on modeling software. If the stuff is not accomodated
properly, then results (mainly efficiency) are way off.

Mobile antennas, shortened antennas can be made better if we have handle on the
thing.

Yuri, K3BU/m

Yuri Blanarovich wrote:
Any experiences out there, rather than more "reasons" why it ain't so?
Proper accommodation in modeling programs can give substantial improvement in
loaded elements modeling.

Assume a transmission line with an SWR of 10:1. Put a series inductor
in series with the transmission line. Assuming negligible losses, the
forward current is the same at each end of the coil and the reflected
current is the same at each end of the coil. The question is: Do the
superposed currents, Ifwd+Iref, remain constant? Of course not, because
of phase shifts. With a large enough coil, one could cause a current
maximum point on one side of the coil and a current minimum point on
the other side.

That same principle holds true for standing wave antennas which are
antennas with (surprise!) standing waves. The current is NOT the same
at each end of the coil (unless a current maximum or current minimum
occurs in the middle of the coil). However, for traveling wave antennas,
the current at each end of a loading coil would be close to equal.
--
73, Cecil http://www.qsl.net/w5dxp



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Yuri Blanarovich wrote:
In a nutshell, I (and W9UCW, etc.) found that current diminishes accross the
coil. W8JI using Kirchoff and Ohm says it can't.

If you put a loading coil 1/3 of the way up on an end-fed 1/2WL
vertical, the net current will increase across the coil. The
net current can decrease, or increase, or be the same magnitude
for special cases. Think Ifwd+Iref with the coil causing major
phase shifts.

Open-ended antennas like dipoles are standing wave antennas. The
forward current is relatively constant through the coil and the
reflected current is relatively constant through the coil. But
the phasor sum of those two currents can vary wildly from end
to end in the coil because of phase shifts.
--
73, Cecil http://www.qsl.net/w5dxp



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Thanks Cecil,
the standing waves do it again!

So far the best argument against W8JI's Kirchoffs and Ohms!

With your permission I will post this public posting back at the eHam.net.

Now see what Reg says, hopefuly after reading the article.

Yuri

Open-ended antennas like dipoles are standing wave antennas. The
forward current is relatively constant through the coil and the
reflected current is relatively constant through the coil. But
the phasor sum of those two currents can vary wildly from end
to end in the coil because of phase shifts.
--
73, Cecil http://www.qsl.net/w5dxp


Don't we have a case of coil being RF choke to certain extent?
Also I think that behaviour of radiator before and after the coil defines the
magnitude of the current, no?

Yuri, K3BU

On 29 Oct 2003 18:58:40 GMT, oSaddam (Yuri Blanarovich)
wrote:

It looks like this factor is not properly accommodated in modeling programs
using loading inductors in antenna elements. In view of the above, time
permitting, I will try to do some work and try to shed some more light on the
subject.

Hi Yuri,

With respect to the data:
Here are some actual measurements of current below and above loading coils.
92" mast, using a HI-Q coil (openwound airdux, 2 1/2"d) with small thermocouple
type meters mounted on the insulated coil support. First for 40m, moving the coil
in the mast from base to center to top (with hat) and reresonating.
Base --100ma below & 66ma above
Center --100ma below & 45ma above
Top --100ma below & 37ma above

This speaks more of simple Resistive heat loss supported by your own
direct observation of:
I fried the loading coil with 600W into Hustler resonator,
melting heat-shrink tubing and wire at the bottom of the coil.

As you, through personal testimony, offer that the bottom of the coil
was warmer (hotter even) than the top; and further, given that the
measuring device is caloric based (thermocouple) it stands to reason
that the close association to heat inflates the base reading not
through the evidence of higher current, but higher (and demonstrable)
heat. The pictures show the close proximity of the thermocouples to
the heat source/coil and also reveal no readings at greater distances
from the coil.

You go to great lengths to portray the current distribution along the
entire length of the radiator from authoritative sources, and yet you
do nothing to confirm them; except over the small portion that
fascinates you and is prone to just such systematic error as I
describe.

73's
Richard Clark, KB7QHC

Yuri Blanarovich wrote:
I post fact of life, Tom pompously chimes in that it can't be so with some
added ridicule, and I won't budge to a bully that is barking up the wrong tree.
This is about 5th time. Watch him to become expert in few moths "he said it all
along" :-) and will become a guru.

I don't understand what the fuss is all about. The forward current hits
the end of the dipole and is reflected. There are standing current waves
on a standing wave antenna. A loading coil shifts the phase between Ifwd
and Iref so anything is possible across a coil, increasing currents,
decreasing currents, or equal magnitudes of currents. It all depends on
the phases involved. For dipoles shorter than 1/2WL, the current decreases
across the loading coils. For a 1WL dipole with the loading coils located
1/3 of the way from the feedpoint, the net current through the coils will
actually increase.
--
73, Cecil http://www.qsl.net/w5dxp



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