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Old November 9th 03, 08:46 AM
Roy Lewallen
 
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My challenge to you was rhetorical. Based on past experience, I had no
real expectation that you'd be able to actually calculate a current.

Our educations differ a great deal. Mine enabled me to give a numerical
prediction, which as anyone who has read my earlier postings, is 1.
Yours has evidently not prepared you to meet this onerous challenge.

Does anyone else feel up to the task of calculating the currents in a
simple circuit? It used to be that you'd have to be able to do this to
get a first phone license, or probably an amateur extra. Now, it appears
that even American engineering education isn't always up to the task.

Roy Lewallen, W7EL

Cecil Moore wrote:
. . .
Balanis would be surprised to know that you consider the material that
he teaches in his classes at ASU to be pseudo-analysis. Some of the
stuff I have posted is in Balanis' book, _Antenna_Theory_ which you
haven't read. In particular, he says: "Standing wave antennas, such
as the dipole, can be analyzed as traveling wave antennas with waves
propagating in opposite directions (forwards and backwards) as represented
by traveling wave currents If and Ib in Figure 10.1(a)."
. . .


  #262   Report Post  
Old November 9th 03, 02:46 PM
Cecil Moore
 
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Roy Lewallen wrote:
My challenge to you was rhetorical. Based on past experience, I had no
real expectation that you'd be able to actually calculate a current.


Here's how to mask the effects that we have been discussing:

1. Choose a small inductance that replaces a very small number of
degrees of the antenna.

2. Use a ferrite coil designed to minimize distributed effects.

3. Mount the coil at a place in the antenna where the slope of the
current is virtually zero.

That's what you have done.

Here's how to showcase the effects that we have been discussing:

1. Chose a large inductance that replaces an appreciable number of
degrees of the antenna.

2. Use a typical air-core loading coil like a bugcatcher that has
appreciable distributed effects.

3. Mount the coil at the center of the antenna where the slope of
the current curve is near maximum.

When you perform your experiment with an 8 foot center-loaded
bugcatcher on 75m, then you will be taken seriously.
--
73, Cecil http://www.qsl.net/w5dxp



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  #263   Report Post  
Old November 9th 03, 05:50 PM
Yuri Blanarovich
 
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W5DXP wrote to W7EL:

Question: Why didn't you use a 75m bugcatcher coil for the experiment?



And why did you put toroid at the base? We are talking about loading coils that
are installed at half to 2/3 up the radiator. Try 12 foot radiator with loading
inductor at ~65% up from the feedpoint. The results should be "magnified". Our
theory is that the current drop across the inductor should be roughly
proportional to
the current in the radiator (in degrees) that it replaces (Cosine law).

Judging by description, I would guess that there wasn't much difference. Put
that coil up but don't use scope probes, they will detune the antenna, no
wires, use thermocouple meters. Probes at the base they probably do not distort
the measurements much.

Nice disertation on engineers and modeling. The only small problem is what and
how you model. If your modeling uses 0 size inductance and real measurement
shows something else, maybe there is a reason to question modeling how well it
reflects reality. I went to university first, I designed parts that human life
depended on, and I would think twice about relying on some numbers without
testing and verifying it. Space shuttle tiles modeled OK?

Yuri

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Old November 9th 03, 06:31 PM
Cecil Moore
 
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Yuri Blanarovich wrote:
Judging by description, I would guess that there wasn't much difference.


The feedpoint of the radiator alone is 35-j185. The impedance of the loading
toroid is 0.6+j193. Assuming perfect predictability, that gives the antenna
system a feedpoint impedance of 35.6+j8, i.e. it is *longer* than resonant.
That moves the current maximum point inside the toroid making the current
in and out even closer to equal. If a coil is installed at a current maximum
point or a current minimum point, the current in and out will be the same.
If a coil is installed at a place where the slope of the current envelope
is positive, the current will actually increase through the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Old November 9th 03, 06:48 PM
Cecil Moore
 
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Roy Lewallen wrote:
Our educations differ a great deal. Mine enabled me to give a numerical
prediction, which as anyone who has read my earlier postings, is 1.
Yours has evidently not prepared you to meet this onerous challenge.


Roy, I have repeated a statement three or four times earlier on this newsgroup.
My statement predicts a result of 1. Here is that statement again:

"If a loading coil is placed at a current maximum point, the current in and
out of the coil will be equal." I have been assuming that is why your coil
was placed at the current maximum point, to ensure that the currents would
be equal. Depending upon where the coil is placed, the currents in and out
of the coil can be equal, greater than, or less than.
--
73, Cecil http://www.qsl.net/w5dxp



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Old November 9th 03, 07:16 PM
Richard Harrison
 
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Roy, W7EL wrote:
"And, Richard (Harrison), who said something like "an inductor without
phase shift is like"...I don`t recall ; hot dog without ketchup or
something."

My analogy may not have been apt, but fact is that you don`t have an
inductor without phase shift. The current lags the voltage in an
inductor.

My dictionary says that phase is a particular stage or point of
adbvancement in a cycle; the fractional part of the period through which
the time has advanced, measured from some arbitrary origin.

Apply a voltage or the voltage across the inductance. Current does not
change instantaneously in an inductance, but it lags the imposed voltage
change.

Lag is to move slowly or fall behind. In a circuit containing resistance
and inductance, almost all real world circuits, current lags the
voltage. This is phase shift by definition. We correct power factor to
overcome phase lag and to eliminate the excess current and loss from the
inductive charging and discharging current of an inductive circuit.
Reactance only stores energy and does no useful work.

I reiterate the accuracy of my postings in this thread, and indeed,
inductance and phase shift are inseparable. Please note that inductance
can be neutralized with capacitance.

Best regards, Richard Harrison, KB5WZI

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Old November 9th 03, 07:16 PM
Tdonaly
 
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Yuri wrote,
Our
theory is that the current drop across the inductor should be roughly
proportional to
the current in the radiator (in degrees) that it replaces (Cosine law).


That's a pretty good theory, Yuri. I'd like to know where you got
this "Cosine law" you keep talking about. I can't seem to find
mention of any such _law_ anywhere but on this newsgroup. Does
that mean I should throw away my method of moments software
because I don't need it any more? And what is a current
drop? I've heard of voltage drops and cough drops but never
current drops. Finally, how do you measure the "current in
the radiator (in degrees)?" Why not use amperes like everyone
else?
I won't believe your theory, Yuri, until you and Cecil take the
time to present it in terms of field theory. Since you guys have taken
EM classes in college you should have no trouble doing this, right?
73,
Tom Donaly, KA6RUH


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Old November 9th 03, 07:51 PM
Cecil Moore
 
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Tdonaly wrote:
And what is a current
drop? I've heard of voltage drops and cough drops but never
current drops.


It's is the decrease in current due to the attenuation
(alpha) factor in equation 1.22 (2) in Ramo, Whinnery, & Van Duzer.
It's all covered in any distributed networks course. According to
Balanis, antennas have an attenuation factor due to radiation and
is similar to (slightly more complicated than) this familiar
transmission line equation for lossy lines.

I = Im(e^-az)d^j(wt-bz)

where a is alpha, w is omega (2*pi*f), and b is beta (phase factor).

I won't believe your theory, Yuri, until you and Cecil take the
time to present it in terms of field theory. Since you guys have taken
EM classes in college you should have no trouble doing this, right?


Please reference Chapter 1 of _Fields_and_Waves_... by Ramo,
Whinnery, and Van Duzer. Start with equations 1.18 (4)&(5)
and 1.22 (1) & (2). Also _Antenna_Theory_ by Balanis, equations
4-81 and 10-1 and one other that I cannot locate right now. :-)
The one I cannot locate is the simplified one for a 1/2WL dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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Old November 9th 03, 08:59 PM
Tdonaly
 
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I won't believe your theory, Yuri, until you and Cecil take the
time to present it in terms of field theory. Since you guys have taken
EM classes in college you should have no trouble doing this, right?


Please reference Chapter 1 of _Fields_and_Waves_... by Ramo,
Whinnery, and Van Duzer. Start with equations 1.18 (4)&(5)
and 1.22 (1) & (2). Also _Antenna_Theory_ by Balanis, equations
4-81 and 10-1 and one other that I cannot locate right now. :-)
The one I cannot locate is the simplified one for a 1/2WL dipole.
--
73, Cecil http://www.qsl.net/w5dxp


I don't have Ramo et al's book, but I do have Balanis' book. I think
anyone who wants to understand equation 4-81 should read the whole
section: 4.5.6 where he makes it clear these equations are
approximations that are pretty good under some circumstances
and lousy under others. The standards he judges them on
are the techniques of Integral Equations and Moment Method
which he explains in another part of the book. He doesn't say
a single thing about a "cosine law" for a real antenna, as Yuri
does.
I think I'll keep my EZNEC.
73,
Tom Donaly, KA6RUH
(P.S. I looked, in Balanis, for a section on inductively loaded
antennas and couldn't find one. That doesn't mean it doesn't exist.
If anyone knows where to look in that book for information on such
I'd be obliged for the information.)
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Old November 9th 03, 09:40 PM
Cecil Moore
 
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Tdonaly wrote:
He doesn't say
a single thing about a "cosine law" for a real antenna, ...


I didn't say a single thing about a "cosine law" either.

If anyone knows where to look in that book for information on such
I'd be obliged for the information.)


Information on inductively loaded antennas seems to be sadly lacking.
But I have found one in _Antennas_ by Kraus & Marhefka, third edition.
I trust that will be a good enough reference for everyone.

On page 823 under "23-13 TRAPS", what he says about traps is not relevant.
But what he says about traps on half their resonant frequency is absolutely
choice.

"At frequency F1, for which the dipole is 1/2WL long, the traps introduce
some inductance so that the resonant length of the dipole is reduced."

On the next page, the current distribution is shown for the trapped dipole
on 1/2 the trap's resonant frequency. Needless to say, it clearly shows
a current drop through the inductive trap.

And talking about phasing using coils: "A coil can also act as a 180 degree
phase shifter as in the collinear array of 4 in-phase 1/2WL elements in Fig.
23-21b. ... THE COIL MAY ALSO BE THOUGHT OF AS A COILED-UP 1/2WL ELEMENT."

Emphasis mine. Now you guys can stop pulling our legs and confess that it
was all a joke.
--
73, Cecil http://www.qsl.net/w5dxp
"One thing I have learned in a long life: that all our science, measured against
reality, is primitive and childlike ..." Albert Einstein



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