Current through coils
 

Ian White GM3SEK wrote:
There is one typo in that statement: I posted it part-way through
changing from "In other words, you use the inductance in exactly the
same way as you would in any other circuit" to "In other words, it [the
inductance] behaves in exactly the same way as it does in any other
circuit". I stand by both of those statements.

Too bad you are standing by false statements. :-)

Quoting: http://www.ttr.com/corum/index.htm

"... one needs transmission line
analysis (or Maxwell's equations) to model these electrically
distributed structures. Lumped circuit theory fails because it's
a theory whose presuppositions are inadequate. Every EE in the
world was warned of this in their first sophomore circuits course."

Seems you weren't listening that day, Ian.

"Lumped element representations for coils require
that the current is uniformly distributed along the coil - no wave
interference and no standing waves can be present on lumped elements."

It's because you modeled a real-life coil, whose length and diameter are
each a significant fraction of the size of the whole antenna.

A 75m bugcatcher coil is a real-life coil, Ian. Contrary to what
W8JI asserts, it is a significant fraction of the size of the
whole antenna. It uses 42 feet of wire, for goodness sake.

You are hung up on something far more fundamental. You are
misrepresenting the fundamental electrical properties of inductance to
make them fit your theory.

I am using distributed network theory known to work in a standing
wave environment. You are using lumped element theory known to
fail in a standing wave environment. A 75m bugcatcher mobile
antenna is a standing wave environment. So exactly who is
"misrepresenting the fundamental electrical properties of
inductance to make them fit his theory"?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Amos Keag wrote:
I agree with Cecil.

Good to hear.

Conclusion: the coil, at 75 degrees of the circuit, has to be treated
differently from DC or LF models.

Yes, I just ran an experiment that would support that statement.

My 75m bugcatcher coil is mounted on a one foot bottom section
on my pickup. I'm sorry but I can't fasten the coil directly
to the mount for reasons of clearance. But one foot at 75m is
only about 1.5 degrees so let's call it negligible.

I have one of those 12 foot telescoping whips from MFJ. It's
great for portable operation from my pickup. It is adjustable
from 2 feet to 12 feet. The first measurement I made was with
no whip at all, just the bugcatcher sitting on top of a one
foot bottom section. I used an MFJ-259B for the measurements
connected through a two foot W2DU choke. Here are the results.

resonant
Stinger frequency
0' 6.7 MHz
2' 5.1 MHz
4' 4.3 MHz
6' 3.8 MHz
8' 3.5 MHz
10' 3.2 MHz
12' 3.0 MHz

It's more than obvious that with a stinger length of 0', the
coil is very close to 1/4WL and is NOT a lumped inductance.
The current at the top of the coil is obviously zero.

So moving down the frequency in 2' increments, exactly when
does a coil made with 42 feet of wire become a lumped inductance
in the presence of standing waves?
http://www.ttr.com/corum/index.htm says never.

When I get my MFJ current meter, I will actually measure the
current at the top and bottom of the coil.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
There's not one of the clever buggers who can design a coil-loaded
whip for a given frequency using a pencil, paper and a pocket
calculator.

Sure we can, Reg. Just design the coil too big and
jumper the un-needed turns. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Hmm ... ARRL Antenna Book gives method ... No?

Cecil Moore wrote:

Reg Edwards wrote:

There's not one of the clever buggers who can design a coil-loaded
whip for a given frequency using a pencil, paper and a pocket
calculator.


Sure we can, Reg. Just design the coil too big and
jumper the un-needed turns. :-)

Current through coils
 

"Cecil Moore" wrote
Reg Edwards wrote:
There's not one of the clever buggers who can design a coil-loaded
whip for a given frequency using a pencil, paper and a pocket
calculator.

Sure we can, Reg. Just design the coil too big and
jumper the un-needed turns. :-)
==========================================

Dear Cecil, you know as well as I do, that pruning the coil is not
DESIGN. It is a procedure done by people who are floundering about in
the dark. Not by supposedly professionally qualified engineers who
are participating in this discussion.

Anybody, even a CB-er, can make an antenna with the top of the whip a
mile long and then severely prune it until the antenna resonates at
the pre-determined frequency, following a score of attempts to use
coils of different dimensions and numbers of turns. But even the
experimenting CB-er has to understand what he is doing.

Not so the so-called professionals.

It appears from this discussion the university-educated Ph.D
professionals are the ones who are floundering about in the dark.

Silly old-wives indeed.

As I have said before, the standards of education in Western schools
and universities are dropping to bits. Chinese, Japanese, Korean,
Iraqian, Iranian, Afganistan, Indian, Pakistan, Vietnam and
Indianesian school kids, not forgetting the half-starved sewer-rats of
Rio-de-Janerio, are better at arithmetic. And, what is more important,
what stems from it!

If you are interested I am on Spanish, Valencia Red tonight. It is
supposed to minimise cholesterol in the blood stream.
----
Your old pal, Reg.

Current through coils
 

Reg Edwards wrote:
Dear Cecil, you know as well as I do, that pruning the coil is not
DESIGN.

I dunno about that, Reg. The Texas Bugcatcher guy made a living
off of coils that were designed to require pruning. He even
sold a pruning kit.

If you are interested I am on Spanish, Valencia Red tonight. It is
supposed to minimise cholesterol in the blood stream.

Muy bueno. California Merlot here.
Makes me forget about my cholesterol. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

My 60 meter mobile antenna is 90 degrees long, 1/4 wavelength resonant
at 18 +j0 ohms [MFJ analyzer]. It is 10 degrees long from feedpoint to
base of coil. Current into the coil is 98% of feedpoint current [cos 10
degrees]. The antenna is 5 degrees long from top of coil to top of
antenna. The current at the top of coil calculates to 9% of feed current
[sin 5 degrees].

Conclusion: the coil, at 75 degrees of the circuit, has to be treated
differently from DC or LF models.

I don't know how you can reach that conclusion except by guessing.

The inductor has some effective equivalent series impedance that
includes resistance, reactance, and distributed capacitance.

I can easily build an inductor for your antenna that tunes the antenna
to resonance and when the top-whip is removed makes the lowest resonant
many times higher than 75 degrees plus 10 degrees at 60 meters would
appear.

On the other hand I can probably, given enough time, build an inductor
that might mislead us into thinking the coil inserted in the antenna
acts like it is about 75 degrees long.

Thinking the inductor or loading coil represents 60 degrees of
electrical length is EXACTLY where the big myth is at, and it can
easily be proven to be a myth!

73 Tom

Current through coils
 

Reg Edwards wrote:

SNIPPED A LOT

There's not one of the clever buggers who can design a coil-loaded
whip for a given frequency using a pencil, paper and a pocket
calculator.


Hmm ... The capacitance of a short vertical monopole above a conducting
plane is a straightforward EM problem [Em 101].

Hmm ... Resonance requires a series inductance where L is proportional
to N^2 times D^2. [Circuits 101]

Value of L can be adjusted using transmission line models for location
along the monopole. The Zo of the monopole is proportional to the
ln(len/dia minus a constant). [Antennas 101]

Given a little time to review the particulars and refresh 60+ years of
separation from EM 101 I think the method would work.

But, I don't have a calculator. Would my old Pickett do?

Does this mean I don't qualify as a "clever bugger"?

Current through coils
 

wrote:
Thinking the inductor or loading coil represents 60 degrees of
electrical length is EXACTLY where the big myth is at, and it can
easily be proven to be a myth!

Well then do it, Tom. But you are not allowed to use the
lumped circuit model. You must use the distributed
network model (or Maxwell's equations). I think the
distributed network model proves otherwise. Could be
you are the one spreading myths after using an invalid
model.

There's just no getting around it. The forward current
undergoes approximately a 90 degree phase shift from
the feedpoint to the end of a 75m mobile bugcatcher
antenna. It is reflected there (180 degree phase
shift) and becomes reflected current which undergoes
approximately a 90 degree phase shift from the tip
of the antenna back to the feedpoint. So from the
start of the forward current wave to the return of
the reflected current wave there is approximately
a 360 degree shift in order to put the forward current
and reflected current in phase so they can superpose
constructively. If the straight element part of the
antenna is 12 degrees, we can account for
12 + 180 + 12 = 204 degrees without the coil. The
coil is the only other thing in the whole system.
Where does the other 78+78=156 degrees of total phase
shift come from if not from the coil? I've explained
all of this to you many times. Might be a good time
to start listening.

You can prove this for yourself. Using current probes
and your o'scope you can measure the traveling wave
phase shift through a loading coil. That figure is
approximately the electrical length that is replaced
by the coil when it is installed in the antenna.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Tom, W8JI wrote:
"Thinking the inductor or loading coil represents 60 degrees of
electrical length is EXACTLY where the big myth is at and it can easily
be proven to be a myth!"

A vertical antenna is often driven against a reflecting ground system.
It is desirable that it be self resonant at nearly 1/4-wavelength
(90-degrees), in many instances, to eliminate reactive impedance to
current into the antenna, avoid loading coil loss, and avoid bandwidth
limitation which comes with high-Q coils. Even with its drawbacks, a
base loading coil is often the practical way to resonate a too-short
antenna.

Suppose the vertical is only 2/3 the height needed for self resonance,
or 60-degrees high. The loading coil must replace about 30-degrees of
missing antenna to bring the vertical to resonance.

30-degrees is not an inductance value. An inductor is impure because it
has resistance and capacitance in addition to inductance. Also, the
inductance needed to replace the missing 30-degrees of antenna depends
on where it is sited, high, low, or in-between.

Siting affects performance as it determines current distribution along
the antenna.

Where is the myth?

Best regards, Richard Harrison, KB5WZI