Current through coils
 

Gene Fuller wrote:
Again, what extra information would be gained if somehow the traveling
wave components could be measured?

Here's a pretty good animation of forward, reflected,
and standing waves.

http://users.pandora.be/educypedia/e...stwaverefl.htm
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

That's probably what he has in mind: using longer coils to
get the correct phase shift. He'll have to use the formulas
in the reference to make coils that will test his ideas.
I don't understand why he uses a coil at all, in that case,
since he could just as easily use a length of coiled up
transmission line to accomplish the same thing. I think he's
been trying to prove that coils, as people currently use them, are
really transmission lines that automatically shift the current
phase the correct amount to cancel antenna reactance. If
he applies his reference formulae to one of Tom's coils and it
doesn't show the correct phase shift, though, his theory is in trouble.
73,
Tom Donaly KA6RUH




chuck wrote:
Hello Tom,

I understand that on page 6, the reference qualifies the statement in
the abstract by saying that for heights " . . . less than 15 degrees . .
. one passes to the lumped element regime . . ."

I thought Cecil was drawing examples for heights greater than 15
degrees. Have I misunderstood?

73,

Chuck, NT3G


Tom Donaly wrote:

Cecil Moore wrote:

Tom Donaly wrote:

What's the formula, Cecil?




http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (32)

The velocity factor can also be measured from the self-
resonant frequency at 1/4WL. VF = 0.25(1/f)

I suppose you also
have something that will tell us how to find your coil's characteristic
impedance; o.k., out with it.




http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (43)

The characteristic impedance can also be measured at
1/2 the self-resonant frequency at 1/8WL. For a lossless
case, the impedance is j1.0, normalized to the
characteristic impedance so |Z0| = |XL|.
For a Q = 300 coil, that should have some ballpark accuracy.

We don't need extreme accuracy here. We just need enough to
indicate a trend that the velocity factor of a well-designed
coil doesn't increase by a factor of 5 when going from 16
MHz to 4 MHz.

In "Antennas for All Applications", Kraus gives us the phase
of the standing wave current on standing wave antennas like
a 1/2WL dipole and mobile antennas. 3rd edition, Figure 14-2.
It clearly shows that the phase of the standing wave is virtually
constant tip-to-tip for a 1/2WL dipole. It is constant whether
a coil is present or not. There is no reason to keep measuring
that phase shift over and over, ad infinitum. There is virtually
no phase shift unless the dipole is longer than 1/2WL and then
it abruptly shifts phase by 180 degrees.

I agree with Kraus and concede that the current phase shift in
the midst of standing waves is at or near zero. There is no
need to keep providing measurement results and references.



You load your antennas with a Tesla coil? Did you read the part
about a Tesla coil going to a lumped inductor when it was shortened?
73,
Tom Donaly, KA6RUH

Current through coils
 

Cecil Moore wrote:

Tom Donaly wrote:

You load your antennas with a Tesla coil? Did you read the part
about a Tesla coil going to a lumped inductor when it was shortened?


A minimum Tesla coil is 1/4WL. My 75m bugcatcher coil
mounted on my pickup as a base-loaded coil with no
whip is 1/4WL on 6.6 MHz. Going from 6.6 Mhz to 4 MHz
is only 40% shortening. I think the lumped inductor
crossover point is probably pretty far below 4 MHz.

Why don't you crunch the numbers using your reference and find out for
sure? (If your reference is correct, that is. Some of the papers by
academics on the web don't always give information that
corresponds to reality.) You should be able to analyze your
bugcatcher easily and report what you find. It sure beats
sitting around drinking Ripple and feeling persecuted.
73,
Tom Donaly, KA6RUH

Current through coils
 

Tom Donaly wrote:
If
he applies his reference formulae to one of Tom's coils and it
doesn't show the correct phase shift, though, his theory is in trouble.

His reference formulae are for traveling waves, not standing waves.
We already know that the phase of the standing wave current on a
1/2WL thin-wire dipole varies not one degree over that entire
180 degrees. Yet we know the forward wave undergoes a 90 degree
phase shift from feedpoint to tip and the reflected wave undergoes
a 90 degree phase on the trip back to the feedpoint. Standing wave
phase is virtually unchanging and is therefore useless for trying
to determine the electrical length of a wire or a coil.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

John P. wrote, "A network analyzer is way beyond my budget."

Though I'd love for you to buy a nice new Agilent Vector Network
Analyzer, I have to say that for things up through low VHF at least,
the very economical project at
http://users.adelphia.net/~n2pk/index.html is well worth looking at.
The performance, when properly calibrated, needs no apologies, for
sure. Paul is one of the Good Guys in ham radio, and not just for
making this project available.

One of Tom's links probably won't work for you, but I'd highly
recommend some of the ap notes you can find after a somewhat diligent
search. I'm sorry to say that the search engine on the Agilent web
site is a poor relative of Google, but you should be able to find these
two ap notes the

Agilent AN 1287-1: Understanding the Fundamental Principles of Vector
Network Analysis

Agilent 1291-1B: 10 Hints for Making Better Network Analysis
Measurements.

They are in PDF files, and I just saved a copy of each...just in case
you can't find them.

A Google search on phrases like "network analysis application note" and
"VNA application note" should yield some interesting things. Here's
one thing I found, which has links to others:
http://na.tm.agilent.com/vnahelp/appnotes.html

Finally, there is an old HP ap note on S-parameters that you should try
to find. It _may_ be on the Agilent web site, but if not, a Google
search will probably turn it up. Though a Vector Network Analyzer does
not necessarily have to do S-parameter measurements, the 8753 is set up
to do them as its fundamental measurement, and they are generally
useful in making higher frequency measurements, since the standard
methodology in the industry is to use S parameters to characterize both
passive and active devices. If you look at network analyzers on eBay,
you may see ones offered without the S-parameter test set, and you can
find the S-parameter test sets offered separately; that's all fine so
long as you understand what you are looking at.

Cheers,
Tom

Current through coils
 

K7ITM wrote:
John P. wrote, "A network analyzer is way beyond my budget."

Though I'd love for you to buy a nice new Agilent Vector Network
Analyzer, I have to say that for things up through low VHF at least,
the very economical project at
http://users.adelphia.net/~n2pk/index.html is well worth looking at.
The performance, when properly calibrated, needs no apologies, for
sure. Paul is one of the Good Guys in ham radio, and not just for
making this project available.

One of Tom's links probably won't work for you, but I'd highly
recommend some of the ap notes you can find after a somewhat diligent
search. I'm sorry to say that the search engine on the Agilent web
site is a poor relative of Google, but you should be able to find these
two ap notes the

Agilent AN 1287-1: Understanding the Fundamental Principles of Vector
Network Analysis

Agilent 1291-1B: 10 Hints for Making Better Network Analysis
Measurements.

They are in PDF files, and I just saved a copy of each...just in case
you can't find them.

A Google search on phrases like "network analysis application note" and
"VNA application note" should yield some interesting things. Here's
one thing I found, which has links to others:
http://na.tm.agilent.com/vnahelp/appnotes.html

Finally, there is an old HP ap note on S-parameters that you should try
to find. It _may_ be on the Agilent web site, but if not, a Google
search will probably turn it up. Though a Vector Network Analyzer does
not necessarily have to do S-parameter measurements, the 8753 is set up
to do them as its fundamental measurement, and they are generally
useful in making higher frequency measurements, since the standard
methodology in the industry is to use S parameters to characterize both
passive and active devices. If you look at network analyzers on eBay,
you may see ones offered without the S-parameter test set, and you can
find the S-parameter test sets offered separately; that's all fine so
long as you understand what you are looking at.

Thank you for all this useful information.

Current through coils
 

FWIW, the "single loop terminated in a diode" that provides both
magnetic and electric coupling at the same time is not the only way to
make a directional coupler. It can be done with a ferrite toroid to
measure the current and a capacitive voltage divider to measure the
voltage; it can be done with a pair of identical RF transformers, one
to monitor the voltage (connected step-down across the line) and one to
monitor the current (connected step-up in series with the line). In
fact, RFSim99 has a window you can access from the
Tools--Component--Coupler pulldown menu, that will help design a
directional coupler in several different ways. (Beware that the
coupling they tell you for coupled lines is only for coupling sections
1/4 wave [or 3/4 or 5/4 or...] long.) Of the ones shown there, only
the transformer one is broadband.

Cheers,
Tom

Current through coils
 

On Wed, 15 Mar 2006 01:39:54 GMT, Cecil Moore wrote:

Tom Donaly wrote:
You load your antennas with a Tesla coil? Did you read the part
about a Tesla coil going to a lumped inductor when it was shortened?

A minimum Tesla coil is 1/4WL. My 75m bugcatcher coil
mounted on my pickup as a base-loaded coil with no
whip is 1/4WL on 6.6 MHz.

An 11.4 meter tall bugcatcher coil - sure.... Such are the rewards of
a Xerox based education.

Current through coils
 

Tom, K7ITM wrote:
"FWIW, the "aingle loop terminated in a diode" that provides both
magnetic and electric coupling at the same time" is not the only way to
make a directional coupler."

Agteed, but the Bird Electronic Corporation has been successful making
the plug-ins for their "Thruline Wattmeter" that way for about 50 years.

Best regards, Richard Harroison, KB5WZI

Current through coils
 

Richard Clark wrote:

Cecil Moore wrote:
A minimum Tesla coil is 1/4WL. My 75m bugcatcher coil
mounted on my pickup as a base-loaded coil with no
whip is 1/4WL on 6.6 MHz.

An 11.4 meter tall bugcatcher coil - sure.... Such are the rewards of
a Xerox based education.

The physical height of my 75m bugcatcher coil is about 0.167 meters.
Dividing 0.167m by 11.4m gives the velocity factor equal to 0.015.

Your 11.4 meter value assumes a VF of zero. Multiply the 11.4 meters
by the VF of the environment and you will obtain the physical length
for something with an electrical length of 90 degrees.

To obtain an electrical 90 degrees using RG-213:

11.4m * 0.66 = 7.5m

To obtain an electrical 90 degrees using my 75m bugcatcher coil:

11.4m * 0.015 = 0.167m
--
73, Cecil http://www.qsl.net/w5dxp