Cecil Moore wrote:
Ian White GM3SEK wrote:
It's a persistent ham myth that an RF choke has specially good
properties when the total length of wire is a quarter-wavelength, and
specially bad properties at twice that frequency. When the wire is
wound into any kind of coil, neither of those claims is true (except
maybe by some rare coincidence).

Please don't imply that I said anything about the total
length of wire - I didn't.

In that case, I suggest you stop making constant references to "1/4WL
self-resonance" and "1/2WL self-resonance". If you don't mean it
literally, it's a very misleading metaphor.

What you say is true and I
never said otherwise. Well-designed coils can be modeled
as rough approximations to transmission lines.




The choke acts essentially as a parallel-tuned circuit, with its
inductance tuned by its own self-capacitance. There will be a series
resonance at some higher frequency, but not at twice the parallel-
resonant frequency (except, again, perhaps by a rare coincidence).

I didn't say exactly twice the frequency and I said it
was an approximation. The chokes at:

http://www.k1ttt.net/technote/airbalun.html

average close to double the frequency.

We're actually looking at exactly the same data (except that the
original reference quoted on K1TTT's site also includes a ferrite bead
choke for comparison).

I have graphed the |Z| data for all the chokes (see link to spreadsheet
below) and there is no consistent trend. In the following table, Fmax is
the frequency of maximum impedance, and Fmin is the frequency of any
minimum observable within the frequency range (the 8t 1 layer choke has
two very small minima). Ratio is Fmax/Fmin.

Choke Fmax Fmin Ratio
6t 1 layer 24 none -
12t 1 layer 15 31 2.1
4t 1 layer 21 34 1.6
8t 1 layer 12 19 1.6
12 32 2.7
8t bunched 6 36 6.0

Judging from the shapes of the graphs and the table above, I would say
that "twice the frequency" is not even valid as an approximation.


No one would expect a bunched coil to be very well behaved.
Everything I have said applies to a coax choke wound on
some kind of coil form with some care given to its design.


Across the whole 1-30MHz band, the bunched choke behaves as an almost
perfect L-C circuit, free from any unwanted resonances. The only problem
with that design is to reproduce the exact parallel-resonant frequency
from one example to the next.


There is NO series resonance at twice the parallel-resonant
frequency - that would be about 12MHz, and nothing at all "special"
is happening there. At 18MHz, where the total winding length is 0.25
wavelengths, there is a very small wobble in the data, but nothing
more.
The series resonance, where the phase angle flips from negative to
positive again, is at 31.5MHz, which is totally unrelated to any of the
other frequencies above. The winding length is 0.5 wavelengths at
35MHz (where the data runs out) but again nothing "special" is
happening there.

Again, no one would expect a bunched coil to be well behaved.

Thus there is no evidence whatever for the myth of the "resonant
length of wire in a choke".

You keep saying that as if I said otherwise. I didn't. The
length of the wire is irrelevant to this discussion.

Turning now to the solenoid-wound choke, the different method of
winding has increased the parallel resonance of the same length of
cable from 6MHz to 9MHz. This is consistent with simple L-C
behaviour, and with the solenoid having less distributed capacitance
than the bunched winding.
Once again, this choke behaves almost entirely as a parallel-tuned
circuit. There are slightly larger wobbles in the data at the
frequencies where the total winding lengths are a quarter-wave and a
half-wave, but these "transmission-line" effects are still very minor,
and completely dominated by the simple L-C behaviour.

The point is that there is a 1/4WL high impedance resonance
and a 1/2WL low impedance resonance that are roughly where
they should be. The 1/2WL low impedance resonance should
be avoided.

As shown above, "1/2wl self resonance" ceases to be a valid concept
once a length of wire is wound into a coil...

The 1/2WL self-resonance has little to do with the length
of wire. It is where the phase angle flips at a point of
low impedance. The 1/4WL self-resonance is where the phase
angle flips at a point of high impedance. The length of wire
is irrelevant, a moot point. I don't know why you brought
it up in the first place.

If you say "the length of wire is irrelevant to this discussion" - with
which I most strongly agree - why do you persist in using these terms
"1/4WL" and "1/2WL" - what dimension of the choke are they referring to?

The Excel workbook at

www.ifwtech.co.uk/g3sek/misc/chokes.xls

contains three spreadsheets.

1. Original data
For all the coiled chokes (same data in the ARRL Antenna Book and on
K1TT's site) with graphs of |Z|. There are minor dips at higher
frequencies, but they are *minor*, and always in a region where the
impedance is so low that you wouldn't be using that choke anyway.

These graphs simply don't support the assertion of a series resonance at
"twice the parallel-resonant frequency" - not even as an approximation.

2. Three chokes compared
The solenoid-wound 8-turn choke, the bunched 8-turn choke, and the
ferrite choke for comparison. The graphs give details of the Z magnitude
and phase.

3. LC model
For the 8-turn solenoid choke. The inductance is calculated from the
physical dimensions of the choke, using the standard ARRL formula
(winding length assumes close-wound RG213). The self-capacitance is
calculated from the inductance and the choke's parallel-resonant
frequency. The dynamic resistance is the peak value from 12MHz, and is
assumed constant at all frequencies.

Those simple assumptions - a fixed L, C and R, all connected in parallel
- give a very good fit to the measured data at all frequencies (only one
point has been forced to fit, namely the peak at 12MHz). This shows that
the dominant behaviour of the choke is like a simple LC circuit, damped
by some loss resistance.

Much of the loss resistance is probably due to losses in the PVC jacket
of the RG213. If these losses are actually increasing with frequency
(rather than being constant, as assumed) then the fit at all frequencies
would be improved.

This very simple LCR model predicts almost everything that was measured.
However, it cannot predict any series resonance at some higher
frequency. If Cecil cares to produce a transmission-line model of the
same choke that can do better, I'm sure we'd all be interested to see
it.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
In that case, I suggest you stop making constant references to "1/4WL
self-resonance" and "1/2WL self-resonance". If you don't mean it
literally, it's a very misleading metaphor.

All of those concepts are explained at:

www.ttr.com/TELSIKS2001-MASTER-1.pdf

in an IEEE sponsored paper by KB1EUD and K1AON. The first high
impedance self-resonance point of a coil, where the phase angle
changes sign, is called "quarter-wave resonance". (The self-resonant
frequency for a 75m bugcatcher loading coil *IS* the 1/4WL self-
resonant point.)

Under "III. TRANSMISSION LINE MODELING", it says:
"By means of conventional distributed-element theory, a thorny
boundary value problem has been reduced to a very simple RF
transmission line. In fact, the entire design and tuning
exercise ... can now be performed conveniently on a Smith Chart."

"There are a great number of formulae for coil self-capacitance.
None are of particular value for quarter-wave helical resonators
anywhere near the 90 degree point."

A coiled coax choke operated at its self-resonant frequency *IS*
being operated at the quarter-wave (90 degree) point.

I doubt that the IEEE would publish a "very misleading metaphor".

I have graphed the |Z| data for all the chokes (see link to spreadsheet
below) and there is no consistent trend. In the following table, Fmax is
the frequency of maximum impedance, and Fmin is the frequency of any
minimum observable within the frequency range (the 8t 1 layer choke has
two very small minima). Ratio is Fmax/Fmin.

What we are looking for is the phase shift from negative to positive.
That would indicate the 1/2WL point.

Choke Fmax Fmin Ratio
6t 1 layer 24 none -

The 1/4WL self-resonant point is at 24 MHz. 48 MHz data is not
given. There is no phase shift from negative to positive in the
given data. The 1/2WL resonant point is not contained in the
data so this set of data is useless for finding the 1/2WL point.

12t 1 layer 15 31 2.1

2.1 is approximately 2

4t 1 layer 21 34 1.6
8t 1 layer 12 19 1.6

Round the 1.6 to a single digit - that's approximately 2

8t bunched 12 32 2.7

Bunched isn't well behaved enough to count.

beaded 6 36 6.0

Beaded isn't a coil so doesn't count.

Judging from the shapes of the graphs and the table above, I would say
that "twice the frequency" is not even valid as an approximation.

Someone needs to explain to mathematicians that rounding 1.6
to an integer isn't equal to 2. If I said it was an extremely
rough approximation, would that be better?

Across the whole 1-30MHz band, the bunched choke behaves as an almost
perfect L-C circuit, free from any unwanted resonances.

Which means it is not behaving as a slow-wave coil structure.
One might say it is misbehaving and is a very poor design.

If you say "the length of wire is irrelevant to this discussion" - with
which I most strongly agree - why do you persist in using these terms
"1/4WL" and "1/2WL" - what dimension of the choke are they referring to?

I'm using them because the IEEE uses them. I keep telling you
that they do not refer to a physical dimension! They refer to
a measurable condition. The first self-resonance is obviously
the 1/4WL point.

I did make a mental slip-up in my previous posting. I forgot
that the VF of the coil changes with frequency. That would
help explain the deviation away from the times two value for
1/2WL resonance. To illustrate the transmission line
characteristic of the choke, the frequency needs to remain
constant while the number of turns is varied.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:

[Snip]

Only one part of that posting grabbed my attention:


beaded 6 36 6.0

Beaded isn't a coil so doesn't count.


The word "beaded" did not exist in my original posting... but there it
is in Cecil's reply, complete with the double that attributes it to
me, so that Cecil can knock the straw-man down.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
beaded 6 36 6.0
Beaded isn't a coil so doesn't count.

The word "beaded" did not exist in my original posting... but there it
is in Cecil's reply, complete with the double that attributes it to
me, so that Cecil can knock the straw-man down.

Ian, I think I corrected an obvious typo of yours. It
appeared that you had gotten off by one row. If that
was wrong, I apologize. Here's what you posted:

Choke Fmax Fmin Ratio
6t 1 layer 24 none -
12t 1 layer 15 31 2.1
4t 1 layer 21 34 1.6
8t 1 layer 12 19 1.6
12 32 2.7
8t bunched 6 36 6.0

There were six chokes. The fifth was bunched and the
sixth was beaded. The bunched ratio is 2.7. It seemed
obvious that you had made a typo in your chart which
I corrected.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:
beaded 6 36 6.0
Beaded isn't a coil so doesn't count.

The word "beaded" did not exist in my original posting... but there
it is in Cecil's reply, complete with the double that attributes it
me, so that Cecil can knock the straw-man down.

Ian, I think I corrected an obvious typo of yours. It
appeared that you had gotten off by one row. If that
was wrong, I apologize. Here's what you posted:

Choke Fmax Fmin Ratio
6t 1 layer 24 none -
12t 1 layer 15 31 2.1
4t 1 layer 21 34 1.6
8t 1 layer 12 19 1.6
12 32 2.7
8t bunched 6 36 6.0

There were six chokes. The fifth was bunched and the
sixth was beaded. The bunched ratio is 2.7. It seemed
obvious that you had made a typo in your chart which
I corrected.

Apology accepted. What you had missed was the statement immediately
above the table, that:
(the 8t 1 layer choke has two very small minima).
Therefore that choke has two lines of data.

The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6.
A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1
and 2.7 are all approximately equal to 2... so how about 6?


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
Apology accepted. What you had missed was the statement immediately
above the table, that:
(the 8t 1 layer choke has two very small minima).
Therefore that choke has two lines of data.

I apologize. I jumped to conclusions. The obvious typo
wasn't so obvious after all

The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6.
A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1
and 2.7 are all approximately equal to 2... so how about 6?

As I already stated: What I have said applies to well-
designed coils that can be modeled as transmission lines.
That includes properly wound chokes and bugcatcher loading
coils. A bunched choke is known not to be well-designed.
Quoting WA2SRQ again: "Don't bunch the turns together. Wind
them as a single layer on a form. Bunching the turns kills
the choking effect at higher frequencies."

Bunching also tends to kill the transmission line effects.

How many 75m bugcatcher loading coils have you wound
in bunched mode? :-)
--
73, Cecil http://www.w5dxp.com

Ian White GM3SEK wrote:
The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6.
A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1
and 2.7 are all approximately equal to 2... so how about 6?

It is a known fact that the VF of these chokes changes
with frequency so they don't behave exactly like a
transmission line. Bunching probably changes the VF
considerably more than a helical wound choke since
the first turn and last turn may be on top of each
other.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:
The bunched choke is the one that has a Fmax/Fmin frequency ratio of 6.
A certain amount of mental gymnastics allowed you to claim that 1.5, 2.1
and 2.7 are all approximately equal to 2... so how about 6?

It is a known fact that the VF of these chokes changes
with frequency so they don't behave exactly like a
transmission line. Bunching probably changes the VF
considerably more than a helical wound choke since
the first turn and last turn may be on top of each
other.
--
73, Cecil http://www.w5dxp.com

I've never actually really compared bunching to helical wound.
The one they specify to build for my triband yagi is a helical
coil. On that antenna, I've used both methods, but not at the
same time to really compare. I didn't notice any real noticable
difference in operation though. I was just looking at the small
choke in my trunk for 2m. It hangs from the trunk lid, and is
tie wrapped. It was about 2-3 diameter, and about 4-5 turns
or so, bunched together. For the purpose I used it for, it
worked great. But it could well be possible what you say about
the higher frequency operation. I do know a bunched choke is
a whole lot better than no choke at all.
MK
MK

Ian White GM3SEK wrote:
Across the whole 1-30MHz band, the bunched choke behaves as an almost
perfect L-C circuit, free from any unwanted resonances.

One additional point. If the above were true, as the
frequency is increased, the phase angle of the coiled
choke impedance would drop from ~90 degrees to zero at
the self-resonant frequency, and then rise back to ~-90
degrees and stay there. But that's not what happens.

In every single case, the phase angle rises toward -90
degrees *and then decreases* as the 1/2WL self-resonance
point is approached. That is a clear indication of transmission
line effects. A lumped circuit simply doesn't act that way.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:
Across the whole 1-30MHz band, the bunched choke behaves as an almost
perfect L-C circuit, free from any unwanted resonances.

One additional point. If the above were true, as the
frequency is increased, the phase angle of the coiled
choke impedance would drop from ~90 degrees to zero at
the self-resonant frequency, and then rise back to ~-90
degrees and stay there. But that's not what happens.

In every single case, the phase angle rises toward -90
degrees *and then decreases* as the 1/2WL self-resonance
point is approached. That is a clear indication of transmission
line effects. A lumped circuit simply doesn't act that way.

The spreadsheet at
http://www.ifwtech.co.uk/g3sek/misc/chokes.xls
now includes graphs of the phase information for all the chokes.

[ Thank you, Owen - the phase axis label is now fixed .
For anyone interested who doesn't have Excel, Microsoft's free Excel
file viewer is at http://tinyurl.com/cup85 ]

Coming all the way back to the original question, the data confirms that
even though it may look like something held together with duct tape, a
coiled coax choke can be an excellent single-band solution. At its
parallel self-resonant frequency, it will have a much higher common-mode
impedance than a generic string of ferrite beads. It can probably
outperform or at least equal a ferrite choke over two or possibly three
adjacent HF bands; but being a resonant device, it cannot deliver
extreme broadband performance.

In the chokes we're looking at, the low-impedance series resonances of
which Cecil complains do not occur below 30MHz. Those resonances exist,
but not on the HF frequencies where the chokes would actually be used.

Within the practical working frequency range of all of these coiled-coax
chokes, the performance can be accurately described as that of a simple
parallel tuned LC circuit, which displays no transmission-line behaviour
whatever. Cecil complains that
One might say it is misbehaving and is a very poor design.

That sounds to me like the complaint of someone who has a pet theory to
hammer, and is disappointed when he can't find a nail.

I think that's it, really. The graphs themselves say the rest.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek