"Yuri Blanarovich" wrote

I (and W9UCW, etc.) found that current diminishes across the
coil. W8JI using Kirchoff and Ohm says it can't.

I am curious if anyone else made measurements (never mind "theory") and
what
they found.

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

If you mean the current distribution along the length of the coil was not
uniform but tapering then it is not surprising.

The important thing is by how much did it taper and in what direction? Up
or down?

Please describe clearly the type of instruments used and how you made your
measurements.

By how much did the current measuring instrument affect the strength of
current flowing, for example by detuning the antenna?

What was the length and diameter of the coil, and the number of turns?

What were the lengths of the antenna above and below the coil?

At what frequencies were the measurement made?

What was the current in amps in the coil wire at the bottom, at the centre
and at the top of the coil?

What does the current distribution in the coil affect, AND BY HOW MUCH?
----
Reg

Reg Edwards wrote:
If you mean the current distribution along the length of the coil was not
uniform but tapering then it is not surprising.

The important thing is by how much did it taper and in what direction? Up
or down?

Please describe clearly the type of instruments used and how you made your
measurements.

By how much did the current measuring instrument affect the strength of
current flowing, for example by detuning the antenna?

What was the length and diameter of the coil, and the number of turns?

What were the lengths of the antenna above and below the coil?

At what frequencies were the measurement made?

What was the current in amps in the coil wire at the bottom, at the centre
and at the top of the coil?

What does the current distribution in the coil affect, AND BY HOW MUCH?
----
Reg

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

73, AC6XG

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

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

Yuri Blanarovich wrote:
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.

All explained by the different phasing of the forward and reflected
currents at that point. If you want to blow Tom's mind, measure the current
in and out of a coil placed 1/3 of the distance up in a 1/2WL vertical.
The current will *INCREASE* from the bottom of the coil to the top of
the coil. How many times have we been warned not to use lumped circuit
theory on distributed networks?
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
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.

But Reg, why do you think they call it a standing wave antenna? Would
you also assert that the current is equal when a coil is installed in
a transmission line with reflections? If it weren't for reflections
from the open ends of a dipole, the feedpoint impedance would be
hundreds of ohms. It's the reflections that reduces the feedpoint
impedance to ~70 ohms.
--
73, Cecil http://www.qsl.net/w5dxp



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What does the current distribution in the coil affect, AND BY HOW MUCH?
----
Reg



Reg it is all there in my article and references I posted in my post. Can you
look up those links or is there a problem? I hate to go over the stuff again.
If you can't look up the links, perhaps I could post the text here, but article
has details, measurements, pictures of meters and facts of life.

Yuri
www.K3BU.us

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

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