Yuri Blanarovich wrote:
My "case" is to bring this to attention of those who are still "knowing" that
the current in loading coils is the same at both ends.

The current can be the same at both ends if the coil is positioned at a
current minimum or current maximum point which is NOT the case with
mobile antennas.

The key to understanding is to recognize that the coil causes the
opposite phase change in the forward current as it does in the reflected
current so they *cannot* track each other through the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Tdonaly wrote:
What is "the radiating part of the antenna," Yuri?

The part of the antenna under the ideal top hat.
--
73, Cecil http://www.qsl.net/w5dxp



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What happens when the coil is a flat pancake ?
---
Reg.

What happens when the coil is a flat pancake ?
---
Reg.




It tastes good :-)

Wouldn't make a difference. W9UCW used toroid and got the same results. Have
you read the article?

Yuri

oSaddam (Yuri Blanarovich) wrote in message ...

I could buy that to an extent I guess. But say if you had a top loaded
vertical, with linear current distribution, the current across the
coil should be appx equal no matter where the coil is placed. But if
no top loading, maybe so.. MK


Top loaded vertical does not have LINEAR current distribution, that is another
simplification, fallacy. Current in the radiator has cosine distribution.

It's still fairly constant. But the degree would depend on the height
of the vertical vs the length of the top hat wires. If the hat is too
small, I doubt you would see as steady a distribution up the radiator.

At
the base, there is not much difference, just like in cosine of the angle
corresponding to the electrical length of radiator at that point.

I'm not sure what you mean by this. Do you mean the current across the
coil would be more steady than if the coil were higher? Does this
include using the top loading wires?

Again, this subject of current distribution is important in optimizing the
antenna design by fine tuning the position of the loading coil in the antenna,
combination with top loading etc. Morew current flowing in the radiating part
of the antenna - the stronger the field and louder signal.

Sure. But this is old news. I beat Reg's vertload program to death
finding the best overall coil height for my mobile antenna. Basically
I ended up putting it as high as I could. Which ended up a center load
at 5 ft up, with a 10 ft whip. Or 8 ft up on a 13 ft whip when parked,
and using the "Super" mode...

The "linear" current distribution mentioned in ARRL Compendium and Antenna Book
is the simplification propagated from Belrose's 1955 QST article. It is close,
but not exact and introduces confusion as it is demonstrated by the flat earth
society.

Dunno, I've never read it. Maybe calling the distribution as "linear"
is the wrong term, but the current is still fairly steady along the
radiator in the case of the vertical with the large top hat. I'm
looking at a model of one now. The current distribution is almost like
a twin tower standing next to the vertical. There is a slight decrease
from bottom to top, but it's very small. It's still my view that the
difference in current at each end of the coil used in such a case is
fairly small. No matter where the coil was mounted.
I've never said they would be exactly the same. Seems to me I started
off by saying they could vary a bit depending on the antenna. So if
you are saying they would be close, but not exact, you seem to be
saying the same thing I said to begin with, which seems to be the same
thing Cecil is saying. ??? Crap, I'm becoming confused.... I'm not
good at playing these type games. That's why I didn't even bother
reading the "current war" over on e-ham. *Sounds* like a 598 thread
nit pick contest just judging from what I've heard... It's not "that"
important to me, being I don't see what it would do for me, even if I
found there to be a fairly large difference from each end. If you have
pertinent info which shows the current is not even close to being
constant across the coil, please enlighten us. "I guess you are
attempting to" But at this point, I think everyone is starting to
chase their tales and bark at the moon.
I'm not really seeing the point, being the art of improving current
distribution in short verticals using coil placement is old news. MK

Reg Edwards wrote:
What happens when the coil is a flat pancake ?

Don't you have a program for predicting the electrical
characteristics of a flat pancake coil? :-) Seriously,
what are those characteristics? Is there a formula?
--
73, Cecil http://www.qsl.net/w5dxp



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NM5K:

At
the base, there is not much difference, just like in cosine of the angle
corresponding to the electrical length of radiator at that point.

I'm not sure what you mean by this. Do you mean the current across the
coil would be more steady than if the coil were higher? Does this
include using the top loading wires?


It is roughly like this: Consider quarter wave vertical (90 degree radiator),
no coils or loading, you will get current max at the base and then diminishing
towards the end to zero. Distribution is in the form of cosine function, nice
cosine curve. Now if you would insert the coil anywhere in that radiator and
shorten it and tune it back to resonance, the current distribution accross the
coil would correspond to the "missing" portion of the radiator that coil
replaces. Efficiency of the antenna is roughly proportional to the area under
the curve. ON4UN pictures show that. Appears that the best compromise position
for the loading coil is about 2/3 from the base. Having top loading, it
"stretches" the high current carrying portion of the radiator. The lower the
loading coil is, the less difference in current drop accross the coil (cosine
function) but shortening of the more current carrying radiator - less
efficiency.

Again, this subject of current distribution is important in optimizing the
antenna design by fine tuning the position of the loading coil in the
antenna,
combination with top loading etc. Morew current flowing in the radiating
part
of the antenna - the stronger the field and louder signal.

Sure. But this is old news. I beat Reg's vertload program to death
finding the best overall coil height for my mobile antenna. Basically
I ended up putting it as high as I could. Which ended up a center load
at 5 ft up, with a 10 ft whip. Or 8 ft up on a 13 ft whip when parked,
and using the "Super" mode...


That perhaps points to some error in validity of the formula, and confirms our
findings. Experience and W5DXP reported shootout results point to reverse
dimensions, 10 feet mast and 5 ft whip, coil about 2/3 up the antenna. You want
to have as much as possible the mast length and then best compromise between
the coil inductance (properties) and remaining whip (and hat).

The "linear" current distribution mentioned in ARRL Compendium and Antenna
Book
is the simplification propagated from Belrose's 1955 QST article. It is
close,
but not exact and introduces confusion as it is demonstrated by the flat
earth
society.

Dunno, I've never read it. Maybe calling the distribution as "linear"
is the wrong term, but the current is still fairly steady along the
radiator in the case of the vertical with the large top hat. I'm
looking at a model of one now. The current distribution is almost like
a twin tower standing next to the vertical. There is a slight decrease
from bottom to top, but it's very small. It's still my view that the
difference in current at each end of the coil used in such a case is
fairly small. No matter where the coil was mounted.
I've never said they would be exactly the same. Seems to me I started
off by saying they could vary a bit depending on the antenna. So if
you are saying they would be close, but not exact, you seem to be
saying the same thing I said to begin with, which seems to be the same
thing Cecil is saying. ??? Crap, I'm becoming confused.... I'm not
good at playing these type games. That's why I didn't even bother
reading the "current war" over on e-ham. *Sounds* like a 598 thread
nit pick contest just judging from what I've heard... It's not "that"
important to me, being I don't see what it would do for me, even if I
found there to be a fairly large difference from each end. If you have
pertinent info which shows the current is not even close to being
constant across the coil, please enlighten us. "I guess you are
attempting to" But at this point, I think everyone is starting to
chase their tales and bark at the moon.
I'm not really seeing the point, being the art of improving current
distribution in short verticals using coil placement is old news. MK

Well, looking at fairly simple example of typical 40m loaded mobile model
antenna, as W9UCW used, having current vary 40 to 60% is significant, measured
differences in field strength are in order of 10 dB and that is significant. I
guess it must be like religion, you believe what you want and if the reality
doesn't matter, than let everybody be happy. But this has tremendous impact on
modeling especially in loaded parasitic arrays. If W8JI showed that Eznec
calculated current to be different by fractions and the measurements show
around 50% difference, then we have huge discrepancy and warning not to rely on
results like that.

There is too much reliance now going on modeling program results, ignoring some
realities. Some people are becoming "experts" on antennas based on modeling
results, without building one. But, even lightbulb can radiate and make some
people happy, but it is not my intention to argue with those. My goal is to
maximize the performance of the antenna and take advantage of propagation modes
for maximum results in the contests, where every fraction of dB counts.

It just amazes me that some people go to great length to speculate, calculate,
rather than go and verify the measurements and see what it REALY is. You can
see that in the threads after the articles.

What I was looking for is to see 1. if anyone else MEASURED the current in
loading coils, and what results they arrived at (and if we are wrong, then
where did we go wrong). 2. If this is right than to have modeling software
implement it with least error. I would like to use that for optimizing, say,
loaded elements for receiving arrays on low bands, optimizing mobile antennas,
loaded multielement beams, etc.

Yuri, K3BU/m

Mark Keith wrote:
I've never said they would be exactly the same. Seems to me I started
off by saying they could vary a bit depending on the antenna. So if
you are saying they would be close, but not exact, you seem to be
saying the same thing I said to begin with, which seems to be the same
thing Cecil is saying. ???

Nope, not what I am saying at all. I'm saying the magnitude of the forward
current doesn't change much through the coil and the magnitude of the
reflected current doesn't change much through the coil. That satisfies
Kirchhoff. But the net current, which is the superposition of those two
currents, can change drastically because of the relative phase differences
on each side of the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Current through a coil in an antenna.

If we feed an antenna at the current point, the current decreases as the
voltage increases along the antenna element from feed point to end..

That being said, a coil replacing a segment of an antenna (in order to
physically shorten it) will exhibit the same properties (relating to
currents) as the segment it replaced.



"Richard Clark" wrote in message
...
On 30 Oct 2003 22:59:26 GMT, oSaddam (Yuri Blanarovich)
wrote:

If we suppose the loading coil is heating up equally

Hi Yuri,

You have already testified twice that it does not - so why IF it
around?
1.)
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.
2.)
Put 500W to it for longer period and watch the heatshrink tubing
shrivel from the bottom up.

73's
Richard Clark, KB7QHC

w4jle wrote:
Current through a coil in an antenna.

If we feed an antenna at the current point, the current decreases as the
voltage increases along the antenna element from feed point to end..

That being said, a coil replacing a segment of an antenna (in order to
physically shorten it) will exhibit the same properties (relating to
currents) as the segment it replaced.

Yep, if the feedpoint impedances are the same and both are lossless,
that has to be true.

Here's a repeat of a diagram I drew earlier.

-----y----------x-----FP-----x----------y----- 1/2WL dipole

-----coil-----FP-----coil----- loaded dipole

Assume the physical length of the loaded dipole is 1/4WL.

Each coil replaces the section between 'x' and 'y'. The currents
at 'x' and 'y' are quite different, being 1/8WL apart.

Consider an 8 foot center-loaded 75m mobile antenna. 87% of the
electrical length of the antenna is in the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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