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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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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Yuri Blanarovich wrote:

Thanks Cecil,
the standing waves do it again!

See if you can get Tom to assert that the current into and
out of a coil in series with a transmission line with reflections
is also constant. :-) Same principles apply.

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

You can get a ballpark estimate of those currents by comparing
a 1/2WL dipole to a loading coil dipole. Assuming the following
two dipoles are resonant on the same frequency:

-----y----------x-----FP-----x----------y-----

-----coil-----FP-----coil-----

Assume the feedpoint impedances are the same and the losses in
the coils are negligible. The net current into the coil is close
to the current at 'x'. The net current out of the coil is close
to the current at 'y'.
--
73, Cecil http://www.qsl.net/w5dxp



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Yuri Blanarovich wrote:

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.

Don't we have a case of coil being RF choke to certain extent?

RF chokes are usually high enough impedance to drop virtually all
the RF voltage across the choke.

Also I think that behaviour of radiator before and after the coil defines the
magnitude of the current, no?

It can be thought of as a very lossy transmission line where
the loss is radiation. Please see my other posting comparing
a 1/2WL dipole to a loaded dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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oSaddam (Yuri Blanarovich) wrote in message ...


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.

Let the games begin :-)

Yuri, K3BU

Dunno, I think it varies. But I sort of agree with Tom, I think it's
fairly constant across the coil. The reason I think so, is because the
electric field across the coil is also constant, and I can easily see
that using a fluorescent bulb across the antenna and coil. Sure, there
may be some decrease, but that could be due to wire resistance loss.
Also, the construction of the antenna itself could vary the current
across the coil. If the whip is top loaded, and the current would be
linear up the whip anyway, I would think it would also be linear
across the coil, no matter where it was. If the coil were base loaded,
with the sharply tapering current distribution up the whip, the taper
across the coil *might* be more. Another reason I think it's fairly
constant is because if I elevate the coil up the whip, I also elevate
and improve the current distribution. The current will be fairly
constant all the way up to the top of the coil, and then start to
taper off as you go up the "stinger" part of the whip. If the
distribution were not fairly linear across the coil, I don't think you
would see this. I'm sure it's not perfectly linear, but I think it is
for all *practical* purposes. And like I said, probably can vary from
antenna to antenna. I haven't read the eham thing yet, but I guess
this is my wishy washy vote... MK

Can any of you guys tell me which of the waves on the antenna does the
radiating - is it the forward or is it the backward wave ?
---
Reg

Richard KB7QHC wrote:

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.

No, it confirms that there is a significant (not negligible) difference in the
current at the bottom vs. top of the coil. Yes, Hustler has small (almost
resistive) wire on 80m resonator. If you trasmit for short period of time (not
enough for heat to equalize) and feel it, or use thermal strips to check
temperature, you would see the taper in the current from bottom to top. It is
in order of 50%, not negligible. Coils in tests are good quality, not
"resistive" wire, current relatively low (100mA) as shown in W9UCW measurements
and pictures.

The point is, if the current was constant or close to it, you would not see the
difference as we see it. Heat rises to the top, if anything the top would be
warmer if the current was constant. If the coil is uniform colenoid, same wire,
diameter (resistance), spacing and it shows difference in heat produced accross
the coil, then we can, using I2R formula, deduct that that current at the
bottom is greater than on the top. W9UCW measurements confirm that, Cecil
explains. Speculations that Earth must be flat might satisfy those reading the
(wrong) books, but will not jive with reality.

Simple way to test it, transmit 100W to 80m Hustler resonator, and feel the
coil. Even insensitive people can feel the significant difference in
temperatures. Put 500W to it for longer period and watch the heatshrink tubing
shrivel from the bottom up. This eliminates all the "errors" with meters to
prove the point.

Yuri, K3BU/m

NM5K

Dunno, I think it varies. But I sort of agree with Tom, I think it's
fairly constant across the coil.

MEASURE or FEEL it! Or disprove what W5DXP is saying.

It appears that current drop is proportional to the current drop in the section
of the antenna that is "missing" - replaced by the coil. You can express it in
electrical degrees and it appears to correspond to cosine distribution.

Yuri, K3BU

Reg Edwards wrote:
Can any of you guys tell me which of the waves on the antenna does the
radiating - is it the forward or is it the backward wave ?

An electron experiences the sum of those two waves and emits a
photon when it is energized enough. So the answer is both.
--
73, Cecil http://www.qsl.net/w5dxp



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Yuri Blanarovich wrote:
MEASURE or FEEL it! Or disprove what W5DXP is saying.

This is easy to see using EZNEC. Model a 102' G5RV on 20m
and look at the current distribution. There are three
current maximums and four current minimums. If you install
a loading coil at a current maximum or current minimum, the
current magnitude will be the same on both sides of the coil.

If you install a loading coil at a point where the slope of
the current is negative (decreasing), the current at the bottom
of the coil will be greater than the current at the top of the
coil. This is the usual case for mobile antennas.

If you install a loading coil at a point where the slope of
the current is positive (increasing), the current at the bottom
of the coil will be less than the current at the top of the
coil.

Note: 'Top' of coil is the end closest to the the ends of the
antenna. 'Bottom' of coil is the end closest to the feedpoint.
--
73, Cecil http://www.qsl.net/w5dxp



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