"Nat Gurumoorthy" wrote Reg,
I recommend the following pair of wonderful articles.
Part one is in HAM RADIO, April, 1977, pp. 52-58. The second in May,
1977, pp. 29-39.
The full reference is: Boyer, Joseph M. (W6UYH): "The
Antenna-Transmission
Line Analog".

Mr Boyer simplifies the antenna analysis by equating parts of the
antenna to transmission lines along the same lines that you have.
Regards
Nat

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

Nat, Antennas and transmission lines are not just analogues - Antennas
ARE transmission lines with controlled 'leakage'. In fact, as people
forever complain on these walls, it is impossible to prevent a transmission
line FROM leaking.

Genuine guru's lump the names 'lines' and 'antennas' together in one volume.
They may mention in passing that the mathematics are identical to both in
case a casual reader doesn't realise it. But if Terman disdainfully omits
mention of the obvious there's always the danger that his disciples may
think it doesn't exist.

I produced the brief semi-serious description just for the purpose described
in the subject line ;o)
----
Reg, G4FGQ

Reg Edwards wrote:
Antennas and transmission lines are not just analogues - Antennas
ARE transmission lines with controlled 'leakage'. In fact, as people
forever complain on these walls, it is impossible to prevent a transmission
line FROM leaking.

The reason they are considered separately is that their functions are
different. Steps are taken to minimize transmission line 'leakage'.
Steps are taken to maximize antenna 'leakage'.

I was surprised that, for the purpose of a ballpark conceptual analysis,
Kraus considers the reflected current on a dipole to be equal to the
forward current. But then I remembered 50% of the power can be radiated
while the current drops by only 29.3%.

Here's what Kraus says: "It is generally assumed that the current
distribution of an infinitesimally thin antenna is sinusoidal and
that the phase is constant over a 1/2WL interval ... A sinusoidal
current distribution may be regarded as the standing wave produced
by two uniform (unattenuated) traveling waves of equal amplitude
moving in opposite directions along the antenna."

Exactly the same thing can be said about a lossless unterminated
transmission line. If lumped circuit analysis doesn't work on
transmission lines with reflections, why should it be expected
to work on antennas with reflections?
--
73, Cecil http://www.qsl.net/w5dxp



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Exactly the same thing can be said about a lossless unterminated
transmission line. If lumped circuit analysis doesn't work on
transmission lines with reflections, why should it be expected
to work on antennas with reflections?
--
73, Cecil http://www.qsl.net/w5dxp



While I agree with this statement, it still doesn't prove anything
about the value of the characteristic impedance and electrical
length of a loading coil and the relationship between these
parameters and the inductive reactance of the loading coil.
This is what bothers me about the claims that the "cosine law"
can be used to predict the current taper in the loading coil.
Seems that there is a big leap of faith taking place when going
from the observation (which I believe is correct) that current
taper is caused by standing waves on the resonant antenna,
and a "law" that says what we can predict that taper with a
simple formula without saying anything about the shunt
capacitance per unit length and series inductance per
unit length of the loading coil, quantities which normally
bear directly on the characteristic impedance and velocity
of propagation of the EM structure (at least in an
analogous transmission line case).

73 de Mike, W4EF.............................................



Exactly the same thing can be said about a lossless unterminated
transmission line. If lumped circuit analysis doesn't work on
transmission lines with reflections, why should it be expected
to work on antennas with reflections?
--
73, Cecil http://www.qsl.net/w5dxp



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Michael Tope wrote:
This is what bothers me about the claims that the "cosine law"
can be used to predict the current taper in the loading coil.

It is certainly NOT a "cosine law". It is at best an approximation.
From _Antennas_For_All_Applications_ by Kraus & Marhefka, third
edition, page 464: "The difference between these (dashed) curves
and the solid curves is not large but is appreciable." The solid
curves are cosine curves. The dashed curves, indicating the actual
current, are not cosine curves but are relatively close approximations.

The only time pure cosine curves will result for net current is in
a lossless situation which is certainly not entirely valid or accurate
for a radiating antenna.

If the magnitudes of the forward current and reflected currents are
not equal, there will be a drift away from a pure cosine shape.
--
73, Cecil http://www.qsl.net/w5dxp



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Mike, W4EF wrote:
"---standing waves on the resonant antenna, and a "law" that says what
we can predict that taper with a simple formula---."

It`s as simple as ON4UN`s Fig 9-22 center loading diagram. The resonant
element is 90-degrees long. Any missimg wire length must be provided by
the loading.

Best regards, Richard Harrison, KB5WZI

On Sat, 6 Dec 2003 03:54:24 +0000 (UTC), "Reg Edwards"
wrote:

I produced the brief semi-serious description just for the purpose described
in the subject line

So, are you feeling better? ;o)

You have missed the point again. It is I who should be asking YOU if you are
feeling better ;o)

On Sat, 6 Dec 2003 20:23:12 +0000 (UTC), "Reg Edwards"
wrote:

You have missed the point again. It is I who should be asking YOU if you are
feeling better ;o)

Never been better OM. I'm off to Buenos Aires for a couple of weeks
to escape the rain. Hope you are feeling better soon.

73's
Richard Clark, KB7QHC