Ian White GM3SEK wrote:
With the right length of coax, number of turns and method of winding,
such a simple coil resonates with its own self-capacitance and is
unbeatable as a single-band choke.

Unfortunately, at approximately double that single-band
frequency, the choke is 1/2WL self-resonant and essentially
useless. Someone on QRZ.com quoted the 2006 ARRL Handbook
as saying the following: "A flat coil (like a coil of rope)
shows a broad resonance that easily covers three octaves,
making it reasonably effective over the entire HF range."

Such a coil is certainly NOT "reasonably effective over
the entire HF range" when used on a typical ladder-line
fed all-HF-band dipole. With a 50 ohm 75m dipole, the
SWR on 450 ohm ladder-line will be 9:1. Worst case, the
choke will see 9*450 = 4050 ohms. An effective choke of
five times that value would be 20K ohms or about 850 uH
of inductance. What do you reckon would be the 1/2WL self-
resonant frequency of an 850 uH coil of coax?
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ian White GM3SEK wrote:
With the right length of coax, number of turns and method of winding,
such a simple coil resonates with its own self-capacitance and is
unbeatable as a single-band choke.

Cecil makes several different points here.

Unfortunately, at approximately double that single-band
frequency, the choke is 1/2WL self-resonant and essentially
useless.

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).

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).

We don't have any performance data for the particular choke recommended
by MFJ (and I'll return to that later) but the ARRL Antenna Book does
have some measured data on two chokes, both made from 8 turns of RG213
wound into a coil of 6-5/8in diameter. The first choke is a bunched
flat coil, and the second is a solenoid. I took the time to import the
data (20th Edition, Table 3) into Excel and analyse it carefully.

The bunched choke has a sharp parallel resonance at about 6MHz, with a
maximum |Z| value of about 8500 ohms (could be higher because the data
are in 1MHz steps). The total winding length at this frequency is about
0.085 wavelengths - a very long way from a quarter-wave. At other
frequencies up to about 30MHz, the choke behaves like a classic
parallel-tuned circuit: the phase angle of Z is almost purely inductive
(+90deg) below the resonant frequency, and almost purely capacitive
(-90deg) above it.

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.

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

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 other difference is that the solenoid-wound choke has a much higher
parallel-resonant impedance - almost 16,000 ohms, compared with 8500
ohms for the bunched choke. Because of its higher resonant frequency,
the solenoid choke would be useful (Z 1000 ohms) from 7MHz up to at
least 18MHz, covering at least four amateur bands, while the bunched
choke would only hit 7MHz.


Someone on QRZ.com quoted the 2006 ARRL Handbook
as saying the following: "A flat coil (like a coil of rope)
shows a broad resonance that easily covers three octaves,
making it reasonably effective over the entire HF range."

That's in my 2005 ARRL Handbook also... but the claim of a "broad
resonance" is not supported by the more detailed information in the ARRL
Antenna Book. On the contrary, the parallel resonance is rather sharp.
It would be fair to claim that a carefully proportioned coil balun with
a resonance around 10-14MHz can have a usefully high impedance as low as
3.5MHz and as high as 30MHz... but the impedance won't be spectacular at
either end of that range, almost certainly less than 500 ohms. So I'd
agree that those claims need to be revisited.


Such a coil is certainly NOT "reasonably effective over
the entire HF range" when used on a typical ladder-line
fed all-HF-band dipole. With a 50 ohm 75m dipole, the
SWR on 450 ohm ladder-line will be 9:1. Worst case, the
choke will see 9*450 = 4050 ohms. An effective choke of
five times that value would be 20K ohms or about 850 uH
of inductance.
What do you reckon would be the 1/2WL self-
resonant frequency of an 850 uH coil of coax?

As shown above, "1/2wl self resonance" ceases to be a valid concept once
a length of wire is wound into a coil.... but I do see the point you're
getting at: such a large inductor would have too much self-capacitance
to be workable solution. If you were absolutely determined to tackle
this extreme problem head-on, the best choke balun would be one that
exploits its self-capacitance to give a parallel resonance on the
operating frequency. However, a far better solution to this problem
would be to avoid the extreme impedance by changing the feedline length.

As for the choke recommended for the MFJ-1622, that comes out at 12
turns of 9in diameter, which seems to be aimed at that antenna's lowest
operating bands. On the higher bands, the choke will be largely
ineffective because it's too big.

For a more broadband solution based on coiled coax, I'd agree with
Cecil's suggestion of cascading a large coil for the lower bands with a
smaller coil optimized for the higher bands. It would also be possible
to cascade a large coil with a small ferrite choke.




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

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. 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 don't have any performance data for the particular choke recommended
by MFJ (and I'll return to that later) but the ARRL Antenna Book does
have some measured data on two chokes, both made from 8 turns of RG213
wound into a coil of 6-5/8in diameter. The first choke is a bunched
flat coil, and the second is a solenoid. I took the time to import the
data (20th Edition, Table 3) into Excel and analyse it carefully.

The bunched choke has a sharp parallel resonance at about 6MHz, with a
maximum |Z| value of about 8500 ohms (could be higher because the data
are in 1MHz steps). The total winding length at this frequency is about
0.085 wavelengths - a very long way from a quarter-wave. At other
frequencies up to about 30MHz, the choke behaves like a classic
parallel-tuned circuit: the phase angle of Z is almost purely inductive
(+90deg) below the resonant frequency, and almost purely capacitive
(-90deg) above it.

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.

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.
--
73, Cecil http://www.w5dxp.com

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

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

On Sun, 22 Oct 2006 23:56:04 +0100, Ian White GM3SEK
wrote:

[ 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 ]

Hi Owen,

It is far easier to simply use OpenOffice which is an executable that
will translate to/from Windows Office (any spread sheet, document,
presentation, drawing...). The Open Document format is the mandated
standard of the European Community, if I recall correctly.

73's
Richard Clark, KB7QHC