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Old October 31st 03, 03:54 PM
Mark Keith
 
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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