Yuri Blanarovich wrote:
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.

Hi Yuri, try this out for your argument in the other group. Using EZNEC:

Example 1: 102' CF dipole with loading coils in the center of each arm
to cause the antenna to resonate on 3.76 MHz. I get XL=j335 ohms.

Example 2: Replace the above loading coils with series inductive stubs
hanging down. Ten foot stubs with six inch spacing between the wires is
what I used. What happens to the current across that six inch gap is obvious
from the current plot using EZNEC. Hint: There is a step function across
that six inch gap just as there will be with a six inch coil.

Then ask: Why doesn't EZNEC treat these two cases the same way?
--
73, Cecil http://www.qsl.net/w5dxp



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Yuri Blanarovich wrote:
There is too much reliance now going on modeling program results, ignoring some
realities.

Yuri, here is a modeling result that you might like. :-) I took a 102' dipole
and loaded it in the center of each leg with an inductive stub that made the
dipole resonant on 3.76 MHz. I added a one ohm series 'load' to each side of
the stub. Drawing one leg of the dipole, it looks like this:

----------R2-+ +-R1----------FP--- ... other half
| |
| | inductive
| | stub
+-+

EZNEC reports 0.85 amps through R1 and 0.57 amps through R2, a difference
of 33%. If one could model the inductive loading reactance as an actual
physical coil instead of a lumped single point impedance, results would
be similar to the above.

Now here is something that might blow some minds. The inductive stub
above is ten feet long. That's about 1/8WL on 20m. A 1/8WL shorted stub
equals +jZ0. The results of running the above antenna on 20m is that the
current through R1 is 185 degrees out of phase with the current through R2.
At the time when the current through R2 is flowing toward the end of the
antenna, the current through R1 is flowing toward the feedpoint. Wonder
what Kirchhoff would say about that. If you replace the stub with a coil
of the same reactance, not much changes.

Tell W8JI to stop using lumped circuit analysis when he should be using
distributed circuit analysis. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Thank you Cecil and Fred!
(Where is Roy? We could use expert guidance in modeling the case.)

I will post your comments on eHam.net.

The analogy using stubs is excellent. That brings the question of using nice
coils, vs. stubs, vs. toroids in shortened antennas. But we will save that for
another thread with proper name.

Now watch for W8JI twisting into: "I said that all along" see him changing his
web page and become a guru who "discovered" that current accross the loading
coil in the antenna is significantly different and Yuri (et al) will remain the
idiot who can't get the things right :-)....
Happened many times before :-(

Thanks again!

Yuri, K3BU/m

As Ken, K7GCO keeps saying: "Don't they get tired of being wrong?"

Yuri Blanarovich wrote:
Thank you Cecil and Fred!
(Where is Roy? We could use expert guidance in modeling the case.)

I sent Roy a copy of the EZNEC file that I sent to you. Anyone else
who wants a copy of those files, send me an email.

I will post your comments on eHam.net.

Which forum/topic?
--
73, Cecil, W5DXP

Cecil wrote,

Tell W8JI to stop using lumped circuit analysis when he should be using
distributed circuit analysis. :-)
--
73, Cecil http://www.qsl.net/w5dxp


You know, it's against the law to kill people, Cecil. I
almost choked to death on my morning cup of
Bo Lee when I read that. :-)
73,
Tom Donaly, KA6RUH

oSaddam (Yuri Blanarovich) wrote in message ...
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.

This would apply to a non top loaded antenna, but I wonder about a top
loaded "large hat" antenna. The current distribution is fairly steady
up the whip, even without using a coil.

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).

But the coil placement on mine was mechanically limited. Yes, 2/3 up
is the best appx place, but I can't do that, except when parked. With
the 10 ft "stock" version, the upper 5 ft is stinger whip. I don't
have a sturdy support for the coil any higher than 5 ft up. Besides,
I'd hang the coil up on too many tree branches. The base of my antenna
is about 4.5 ft off the ground. My coil is nearly 10 ft in the air as
it is. Thats why I only use the lower extender mast only when parked.
My coil is then nearly 13 ft in the air. And the stinger still 5 ft
higher than that.
This is why I do add a lower 3 ft mast when I'm parked. I then do
approach the 2/3 level. There was no error involved. I just can't do
everything I'd like when it comes to that antenna. If I'm working 75m,
and I'm doing say 10 over 9 to someone with the 10 ft version,
switching to the 13 ft version will usually bump me up to 15 over 9.
Thats the usual difference I see in the real world.


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.



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.

What does this refer to? The comparison of current at each end of the
coil, or the overall antenna performance changes due to varying the
height of the coil?
If this is total performance increase from raising the coil, I don't
doubt it.
But thats not what this is really about. We are talking about the
differences in current at each end of the coil, and the ramifications
IF it's proven that the current drops off a good bit at the upper end
of the coil, compared to the lower end. I contend that even if you
prove that there is a major current difference at each end of the
coil, it will have no real impact on the design of short verticals. We
are already using the optimum coil heights, whether the current
difference is large, or not.
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.

Yes, maybe modeling, but not the design of the antennas as I use them.
I don't use modeling to design short verticals in general. I never do
for mobile antennas. I prefer to calculate using a program like
vertload. I don't consider vertload as "modeling". I consider it a
calculator program. The only time I model short verticals is if I'm
adding a top hat, and this would usually be something like a large
160m antenna for home use. I like to model it to get an idea of the
current distribution. But I don't rely on that model to calculate
efficiency.
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.

Well, that is fine. I hope the "correct" answer is known sooner or
later. But I don't rely on modeling when designing short verticals, so
I really doubt the "correct" answer will effect me much one way or the
other. It's not really an issue to me, being I already know the best
appx coil placement heights to use. And this was determined quite a
few years ago, before modeling was even common.

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.

I've built all mine. "different coil heights" And like I said, I
don't use modeling for mobile antennas. I use "calculator" programs.
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.

I agree, and do the same thing. I wouldn't bother totally rebuilding
my mobile antenna if this were not the case. My old coil was only 2 ft
up..

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.

I'm not really set up to measure this accurately. I'd have to rig up
some gear/couplers, whatever, to do that. Besides, I've never really
worried about it much. I don't think it would have any effect on how I
build my short verticals.

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).

I haven't done that. I'm just going by other related real world
observations I see.

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.

I have no problems with that, if indeed it does cause a modeling
error.

MK

Tdonaly wrote:

Cecil wrote,
Tell W8JI to stop using lumped circuit analysis when he should be using
distributed circuit analysis. :-)

You know, it's against the law to kill people, Cecil. I
almost choked to death on my morning cup of
Bo Lee when I read that. :-)

I apologize for that, Tom. If you had choked to death, would
there have been enough evidence to convict me? :-)
--
73, Cecil, W5DXP

Cecil wrote,

I apologize for that, Tom. If you had choked to death, would
there have been enough evidence to convict me? :-)
--
73, Cecil, W5DXP


Not in California, Cecil. It would have been the
perfect crime.
73,
Tom Donaly, KA6RUH

"Mark Keith" wrote
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.

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

There are 3 losses - coil loss resistance, ground loss resistance and
radiation resistance.

To calculate and maximise efficiency all 3 values must be transformed to a
common point - the base feedpoint.

The length of radiator between coil and base behaves is a transmission line
transformer which transforms the coil loss resistance (XL/Q) to another
value at the base. Would-be modellers should take this into account.

The Cosine current distribution along the radiator is a direct consequence
of its behaviour as a lossy line. (Actually, it is not an exact cosine shape
because of end-effect)

Efficiency = Rrad / ( Rrad+Rcoil+Rground ).

Rground is constant. For short antennas Rrad is the smallest of the 3
resistances.

As the coil is moved further up the antenna both Rrad and Rcoil increase.

But even if the coil is located at the extreme top of the antenna, radiation
resistance cannot increase to more than 4 times the radiation resistance
when the coil is located at the base feedpoint. Usually it is considerably
less than 4.

So the rapidly increasing coil loss resistance very soon overtakes the
increase in radiation resistance. Even if coil Q remains constant, coil
loss resistance increases just by virtue of its necessary increase in
inductive reactance.

To maintain resonance coil inductance increases inversely proportional to
the length of the whip above it. So when the coil is located 95% of the way
to the top of the antenna its loss resistance is TWENTY times greater than
that of a base loading coil even when Q is unchanged. In practice, a coil
having 20 times the inductance but with the same overall dimensions will
very likely have a lower Q and an even higher resistance.

Its easy to see the fixed value of Rground in the above efficiency formula
has the following effects -

When coil loss is less than ground loss, higher radiating efficiency is
achieved by placing the coil nearer to the top of the antenna.

And vice-versa.

When ground loss is very small (zero if antenna is a pair of two
back-to-back radiators to form a dipole) efficiency is relatively high
anyway, maximumum efficiency perhaps occurring with the coil located in the
lower half of the antenna. The slight improvement relative to base loading
(as part of a tuner) may not then be worth the mechanical inconvience of
fitting a coil in the antenna anyway.

An important factor, not considered quantitatively by anybody, is that a
mobile antenna is not just a loaded vertical - the vehicle body, just by
looking at it, obviously forms the major portion of the antenna and is
floating above ground. The vehicle body plus loaded whip should be
considered to be an off-centre-fed, short, 1/2-wave resonant vertical dipole
and modelled as such.

---------------------------------------------------------

Regarding antenna modelling - program LOADCOIL considers all 3 parts parts
of the antenna, the mast, loading coil and whip, as consecutive lengths of
transmission line each with its own Zo and loss resistance. It is obliged to
do this because it covers actual antenna heights approaching 1/4-wavelength
as may be erected in your backyard.

And it continues to do this for very short antennas with very short loading
coils even where there would be negligible error by assuming the current
going into one end of the coil is the as what comes out of the other.

There's a companion program TOPHAT2. There's no coil in it.
----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================

NM5K snip, snip and...

Besides, I've never really
worried about it much. I don't think it would have any effect on how I
build my short verticals.

Good for you :-)
Granted, we are close to optimum with mobile verticals.
If I understand how and why things work, I can do better job on optimizing and
maximizing the performance of the antennas, especially when it comes to more
element loaded arrays. You would see significant difference in
designing/optimizing say 3 element loaded beam. Modeling software uses currents
in elements and calculates mutual interaction between the elements. If the
current distribution and magnitude are off by 50% then we have major problem.

Yuri