Current through coils
 

Cecil,

I don't remember why you chose a frequency of 5.89 MHz for all of your
analysis, but in any case I believe that frequency is slightly out of
the 75 meter band. The FCC will be calling. 8-)

I tried to adjust the frequency downward, but the segment length limit
is reached for the coil model. I redrew the coil with only six segments
per turn, rather than eight. Now the frequency can be lowered to about
3.94 MHz without EZNEC guideline check warnings.

In order to get the 90 degree phase curve I extended the horizontal wire
to about 55 feet. (not sure why this matters if the antenna is purely a
terminated traveling wave antenna, but, hey, no loose ends.)

The result from EZNEC is that the phase shift in the coil is about 9
degrees.

You might observe that this shift is a bit smaller than the "tens of
degrees" noted below, and it is also smaller than the guru-inspired
transition point of 15 degrees.

I would like to model a coil more typical of common use than the strange
beast you designed, but the segment length limits in the NEC engine seem
to preclude such models. (I have never seen a mobile coil that is 12
inches high, 6 inches in diameter, with 2 turns per inch.)

If I was the cynical sort I might think that your choice of coil
dimensions and frequency were picked to get a phase shift of 16 degrees,
which is just over the guru limit. But since I am a straightforward kind
of guy I won't think such thoughts.

73,
Gene
W4SZ

Cecil Moore wrote:

Technical facts *are* a deeply personal issue with me. So I ask
you the same question he When are you going to correct the
technical errors on your web page? The delay through a 75m
mobile bugcatcher coil is tens of degrees, not faster than the
speed of light.

Current through coils
 

Gene Fuller wrote:
. . .
I tried to adjust the frequency downward, but the segment length limit
is reached for the coil model. I redrew the coil with only six segments
per turn, rather than eight. Now the frequency can be lowered to about
3.94 MHz without EZNEC guideline check warnings.
. . .

Of the Guideline Check warnings, the one about short segment length can
most often be ignored without a problem. If in doubt and where
practical, do a test case. For example, model the inductor with a wire
through the middle directly connecting top and bottom and with a source
in the middle. Note how the source reactance changes with frequency to
see that it follows what it should theoretically do. Another test you
should run when in doubt is the Average Gain test, described in the
manual. Either is probably adequate to have confidence in the results.

The Guideline Check warnings are based on NEC-2 recommendations. I've
found that you can often make segments much shorter than the
recommendations without causing a problem.

Roy Lewallen, W7EL

Current through coils
 

Reg Edwards wrote:
How does EZNEC make its calculations? If you don't know you are
placing your faith in a mirage.

It uses the Moment Method (MM) sometimes called the Method
Of Moments (MOM). It's described in Kraus and Balanis
and credited to Roger Harrington in the 1960's.

Each segment in EZNEC is assumed to have constant current.
In the aforementioned 8-sided coil, there are 200 segments,
each with an assumed constant current. The standing wave
current in each segment depends upon where it is inserted
in the standing wave environment as shown at:

http://www.qsl.net/w5dxp/1WLDIP.EZ
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Gene Fuller wrote:

Gene, I want to commend you on a rational, mostly technical,
posting. Hopefully, others will follow your lead.

I don't remember why you chose a frequency of 5.89 MHz for all of your
analysis, but in any case I believe that frequency is slightly out of
the 75 meter band. The FCC will be calling. 8-)

:-) Here's the history. I tried to model my 75m bugcatcher coil in
EZNEC. It appears to be impossible without violating EZNEC's guidelines.
So I changed the coil from 4 TPI to 2 TPI. That moved the resonant
frequency from 3.8 MHz to 5.9 MHz, close enough to the 60m band
that I thought no one would object. Do you think the FCC calls everyone
who uses EZNEC to model an antenna out of the amateur bands? :-)

I tried to adjust the frequency downward, but the segment length limit
is reached for the coil model. I redrew the coil with only six segments
per turn, rather than eight. Now the frequency can be lowered to about
3.94 MHz without EZNEC guideline check warnings.

Hey, great.

In order to get the 90 degree phase curve I extended the horizontal wire
to about 55 feet. (not sure why this matters if the antenna is purely a
terminated traveling wave antenna, but, hey, no loose ends.)

I found the same thing. Seems no matter how one changes things, it
appears to be a diverging series. The main goal is to get the reflections
reduced to a low level, maybe not to eliminate them entirely.

The result from EZNEC is that the phase shift in the coil is about 9
degrees.

Yes, but that's for a coil designed for a 6 MHz antenna. You need to
add a lot of turns to make it typical of a 4 MHz coil. Doubling the
turns would make for an 18 degree phase shift - a detail you seem to
have missed. Please expand the coil until it resonates an 8 foot
antenna on 4 MHz and repeat your findings.

You might observe that this shift is a bit smaller than the "tens of
degrees" noted below, and it is also smaller than the guru-inspired
transition point of 15 degrees.

Of course, a 60m mobile coil used on 75m is going to have a smaller
phase shift. But you dropped a bit in your logic, Gene. What you need
to do is go back and create a coil that resonates an 8 foot antenna
on 4 MHz. Then do your phase calculations. When you do that, the delay
in the larger coil will turn out to be tens of degrees. Could you email
me your EZNEC file? (Hopefully, without the worm/virus I received from
someone else recently.)

I would like to model a coil more typical of common use than the strange
beast you designed, but the segment length limits in the NEC engine seem
to preclude such models. (I have never seen a mobile coil that is 12
inches high, 6 inches in diameter, with 2 turns per inch.)

We do the best we can do with the tools we have. Actually, I
have seen such a coil at one of the CA 75m shootouts. It was
made out of half-inch copper tubing.

If I was the cynical sort I might think that your choice of coil
dimensions and frequency were picked to get a phase shift of 16 degrees,
which is just over the guru limit. But since I am a straightforward kind
of guy I won't think such thoughts.

I tried to model my 4 TPI 75m bugcatcher coil. EZNEC would have
none of that. So I modeled what I could. The phase shift of
16 degrees was a complete coincidence, but interesting, no?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Roy Lewallen wrote:
Of the Guideline Check warnings, the one about short segment length can
most often be ignored without a problem.

The problem that Gene and I are having is the spacing between
adjacent segments. EZNEC won't allow me to do a 4 TPI coil.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Hi Roy,

Thanks. I went back and bravely ran the original coil model, ignoring
the guideline checks. The difference in results from the two coil
models, eight-sided and six-sided, was in the noise. (Or at least well
below the new RRAA error guideline of 59%.)

Emboldened by this apparent success I tried to substitute the now-famous
W8JI coil; 100 turns, 2 inch diameter, 10 inch length. This time the
overall 500 segment count was the limiter (I am cheap), so I had to make
the coil four-sided. The delay through this coil was about 9 degrees at
5.89 MHz and about 6 degrees at 3.9 MHz.

Sooooo, the bottom line for the 17,000 posts in this thread is that the
pure lumped-circuit toroidal coil analog might be just a slight bit
stingy, but it comes pretty close to reality. The full-bore transmission
line model for the coil works as well, but it does not appear to add
much useful information. The real phase shifts are well below the 15
degree transition point claimed by the Tesla coil crowd. I am sure none
of this is news for you or for most others.

Cecil has thanked me on several occasions for bringing up the standing
wave equations. I now must return the favor by thanking him for setting
up the traveling wave model. It did not prove his point, but it helped
bring clarity to the issue.

I am confident that this will not be the end of this immortal thread,
but it is now quite clear through simulation that the entire issue is
much ado about very little.


73,
Gene
W4SZ


Roy Lewallen wrote:
Gene Fuller wrote:

. . .
I tried to adjust the frequency downward, but the segment length limit
is reached for the coil model. I redrew the coil with only six
segments per turn, rather than eight. Now the frequency can be lowered
to about 3.94 MHz without EZNEC guideline check warnings.
. . .


Of the Guideline Check warnings, the one about short segment length can
most often be ignored without a problem. If in doubt and where
practical, do a test case. For example, model the inductor with a wire
through the middle directly connecting top and bottom and with a source
in the middle. Note how the source reactance changes with frequency to
see that it follows what it should theoretically do. Another test you
should run when in doubt is the Average Gain test, described in the
manual. Either is probably adequate to have confidence in the results.

The Guideline Check warnings are based on NEC-2 recommendations. I've
found that you can often make segments much shorter than the
recommendations without causing a problem.

Roy Lewallen, W7EL

Current through coils
 

Cecil Moore wrote:
Roy Lewallen wrote:

Of the Guideline Check warnings, the one about short segment length
can most often be ignored without a problem.


The problem that Gene and I are having is the spacing between
adjacent segments. EZNEC won't allow me to do a 4 TPI coil.

Gee, I wonder why that is?
73,
Tom Donaly, KA6RUH

Current through coils
 

Gene Fuller wrote:
Cecil has thanked me on several occasions for bringing up the standing
wave equations. I now must return the favor by thanking him for setting
up the traveling wave model. It did not prove his point, but it helped
bring clarity to the issue.

Uhhh Gene, you still need to add turns to the 6 MHz coil to bring
it down to 4 MHz with an 8 foot antenna. No fair reporting the
half of the results that agree with your preconceptions while
ignoring the half of the results that disagree.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Tom Donaly wrote:

Cecil Moore wrote:
The problem that Gene and I are having is the spacing between
adjacent segments. EZNEC won't allow me to do a 4 TPI coil.

Gee, I wonder why that is?

My real-world 75m bugcatcher coil is 4 TPI. That EZNEC won't
model it is discouraging.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Gene Fuller wrote:
Emboldened by this apparent success I tried to substitute the now-famous
W8JI coil; 100 turns, 2 inch diameter, 10 inch length. This time the
overall 500 segment count was the limiter (I am cheap), so I had to make
the coil four-sided. The delay through this coil was about 9 degrees at
5.89 MHz and about 6 degrees at 3.9 MHz.
Sooooo, the bottom line for the 17,000 posts in this thread is ...

You wish that was the bottom line. Here's some mud in your eye.

1. I have always been talking about my 75m bugcatcher coil which
is about 6"x6" and designed for actual mobile use. W8JI's coil
is nowhere near what the average ham uses for a 75m bugcatcher
coil. It is much too fragile for long-term mobile use. It can't
even be considered to be a "bugcatcher" because one Texas-
sized bug and it is destroyed.

2. You haven't installed that coil in an 8 foot 75m mobile
antenna so you don't know what the delay is in an 8 foot
4 MHz system. Please feel free to try again - no cigar at
the present time.
--
73, Cecil http://www.qsl.net/w5dxp