Antenna Loading Coils
 

GENTLEMEN, behave yourselves.

The ONLY way of modelling the effect of a loading coil in an antenna wire is
to consider it to be a section of a continuous-inductance-loaded
transmission line which also has a radiation resistance.


Which, of course, is what it actually is. It can then be incorporated in
the remainder of the system which consists of other radiating transmission
line sections.


As with all other lines it has length and diameter (that of the coil
former).


It has a uniformly distributed capacitance (to the rest of the world) per
unit length.


It has normal distributed inductance per unit length PLUS the MUTCH extra
inductance due to being wound as a coil.


It has the normal RF wire loss resistance.


It has a uniformly distributed radiation resistance according to the length
of the coil former. (NOT of the length of the wire on the coil.)


R, L and C are all calculable, or at least can be estimated, from
dimensions.


So in an antenna system, in general, we have 3 consectutive transmission
lines sections with the loading coil forming the center section.


Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will have
a much higher value than that of a wire of the same straight length.


If a generator (transmitter) is applied at one end then currents, voltages
and phase relationships at any point along the overall length can be
calculated. Mismatches between Zo's of the various sections are
automatically taken into account.


But we are ultimately interested only in input impedance, efficiency and
power radiated. All the intermediate stuff which may be available in the
process is just so much waffle for the old wives to haggle about.


As is well known, the coil alone, a simple helix, if of appreciable former
length relative to a 1/4-wavelength, will radiate.


Download in a few seconds program HELICAL and run immediately. Enter the
following values -


Height = 2.5m
Coil dia = 50mm
Coil turns = 750
Wire dia = 2.5mm
Rod length = 0mm
Rod dia = 0mm
Ground loss = 8 ohms

Computed results are the performance of a Helical 160 meter band antenna
which was popular a few years back amongst UK mobile amateurs.


Its a case of the height being so low and the loading inductance being so
big that it occupies the whole length of the antenna. Why waste the space?
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg Edwards wrote:
So in an antenna system, in general, we have 3 consectutive transmission
lines sections with the loading coil forming the center section.
Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will have
a much higher value than that of a wire of the same straight length.

(Rhetorical Question)
Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Reg Edwards wrote:
So in an antenna system, in general, we have 3 consectutive transmission
lines sections with the loading coil forming the center section.
Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will have
a much higher value than that of a wire of the same straight length.

(Rhetorical Question)
Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?
--
73, Cecil http://www.qsl.net/w5dxp

Maybe it's a half wavelength long? ;-)

73, Jim AC6XG

Cecil Moore wrote:

Jim Kelley wrote:
Maybe it's a half wavelength long? ;-)

For a real-world air-core coil to exhibit identical currents at the
input and output, it would have to be one wavelength long, lossless,
and non-radiating. That's a pretty tall order just to support an
old wives' tale. :-)

I happily stand corrected.

73 de jk

Reg wrote,

The ONLY way of modelling the effect of a loading coil in an antenna wire is
to consider it to be a section of a continuous-inductance-loaded
transmission line which also has a radiation resistance.


Another absolutist, fairly assinine statement from across the pond. How do
you know it's the ONLY way, Reg, have you tried any others?

Tom Donaly, KA6RUH

Jim Kelley wrote:
Cecil Moore wrote:
(Rhetorical Question)
Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?

Maybe it's a half wavelength long? ;-)

Strangely enough, that won't do it, Jim. A coil equivalent to 1/2WL
reverses the phase of the current such that current is flowing into
both ends at the same time. According to some gurus, that violates
Kirchhoff's laws. But Kraus shows how phase-reversing coils are
used in collinear arrays. Reckon Kraus knows he is violating Kirchhoff's
laws?

For a real-world air-core coil to exhibit identical currents at the
input and output, it would have to be one wavelength long, lossless,
and non-radiating. That's a pretty tall order just to support an
old wives' tale. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil wrote,

Reg Edwards wrote:
So in an antenna system, in general, we have 3 consectutive transmission
lines sections with the loading coil forming the center section.
Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will
have
a much higher value than that of a wire of the same straight length.

(Rhetorical Question)
Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?
--
73, Cecil http://www.qsl.net/w5dxp


An invidious post if ever I saw one. You've got to get out of the
habit, Cecil, of believing the things you think up in your head.
You've got a horrible case of I-think-therefore-it-is syndrome.
Experiment more. Sit at the feet of Richard Clark and learn how
to measure. Learn the truth first and then make up your theories,
knowing beforehand that every theory is an abstraction. Leave the
tunnel vision and its resultant dogmatism to the local baptist
minister. Quit believing that you can win an argument with slippery
evasions and insults, or that there is even any advantage in winning
at all. If they are true, your ideas will fight their own battles, and if
they're false, no amount of bluster and tortured logic will make people
believe them. You've spent countless hours arguing the case for your
interpretation of how waves work, and the only thing you've accomplished
is that you've antagonized a group of people more knowledgeable than
you are.
I'm not asking you to give up, but it would be nice, both for the
benefit of the newsgroup and for your reputation if you would temper
your fanaticism with just a bit of experimentation, dispassionate
reflection and self-doubt.
73,
Tom Donaly, KA6RUH

73,
Tom Donaly, KA6RUH

Tdonaly wrote:
You've spent countless hours arguing the case for your
interpretation of how waves work, and the only thing you've accomplished
is that you've antagonized a group of people more knowledgeable than
you are.

A group more knowledgeable about EM waves than Eugene Hecht? It is
not my interpretation of how waves work, Tom. It is the consensus
of opinions of experts from the field of optics. Non-glare glass
is exactly the same thing as a 1/4WL matching section in a transmission
line and how the interference causes the match is fully understood. If
proven, accepted knowledge from a closely related technical field
antagonizes a bunch of closed-minded ivory tower gurus on this newsgroup,
so be it.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards, G4FGQ wrote:
"It has a uniformly distributed radiation resistance according to the
length of the coil form."

That may be the case of "treating" radiation resistance as if uniformly
distributed. Radiation resistance may be defined as the resistance which
would take the same power as that radiated when placed at the
high-current point of the antenna.

While a transmission line of uniform cross section may have uniform
inductance and capacitance per unit length, it is unlikely that an
antenna has uniform capacitance per unit length. Electric field lines of
force have a varying concentration along equal small segments of wire
length. It`s usually a function of distance between wires and this
varies in an antenna because the antenna is meant to radiate.

Variation of capacitance along an antenna causes a variation in surge
impedance along the antenna, but a useful average can be used for
calculations. Straight wire or coiled as in a rubber ducky, an antenna
is subject to this variation in capacitance and surge impedance.

Best regards, Richard Harrison, KB5WZI

"Tdonaly"
Reg wrote,

The ONLY way of modelling the effect of a loading coil in an antenna wire
is
to consider it to be a section of a continuous-inductance-loaded
transmission line which also has a radiation resistance.


Another absolutist, fairly assinine statement from across the pond. How do
you know it's the ONLY way, Reg, have you tried any others?

Tom Donaly, KA6RUH

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

Tom, beware the Green-eyed Technology Goddess.

WE have modelling programs which actually WORK.

And FREE to US citizens.

Is this 'dumping' of shoddy goods?

After gigabytes upon gigabytes of civil war amongst yourselves - YOU
havn't!
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........