jawod wrote:
Can you direct me to more good information (as in NOT sales literature)
about ferrites and toroids as choke baluns?



Here are some actual measurements of coax and ferrite chokes. Tests
done with a Hewlett-Packard 4193A vector impedance meter.

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

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:

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.


Ian and Cecil,

I think you both should disclose any ties you might have to the cable
industry. (hi)

I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable. Do you calculate the
transmission length for voltage peaks?

Thanks,

John
AB8O

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

jawod wrote:
I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable.

Ideally, they should be placed at a common-mode standing-
wave current antinode (maximum). Aren't you glad you asked?

Most hams install the choke at the most convenient
place. However, at the feedpoint of a one wavelength
dipole is not a good place.

A common place is at a BALanced to UNbalanced junction
where the choke can perform the balun function.
--
73, Cecil http://www.w5dxp.com

jawod wrote:
Ian White GM3SEK wrote:
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.


Ian and Cecil,

I think you both should disclose any ties you might have to the cable
industry. (hi)


Somewhere in there is a very bad joke about "cable ties"...

Cable is much less expensive over here than large ferrite beads imported
from the USA; hence my interest in the coiled cable chokes.

I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable. Do you calculate the
transmission length for voltage peaks?


The place to start is where the common-mode current would be launched
onto the coax. In an antenna fed with coax all the way, that would be
the feedpoint. In an antenna fed partly with coax and then with parallel
line, it would be at the transition point.

But if the system is physically asymmetrical (eg if the feedline runs
back horizontally below the antenna) a single choke may not be enough to
solve the problem. In that case, the next place to think about would be
a quarter-wave closer to the transmitter.

....Doorbell...

Someone stopped by for a long Sunday afternoon chat, and in the meantime
Cecil has provided the rest of the answer.


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

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:
David G. Nagel wrote:
Your best choke is about 30 feet of coax rolled up into a 12" coil.

Seems to me, the best choke(s) would be two chokes, one designed
for 75m-15m and the other designed for 12m-2m. I cannot bring
myself to believe that a coiled up piece of coax could ever be
an effective choke over a 40:1 frequency range.
--
73, Cecil http://www.w5dxp.com

Why worry about VHF though? Most people wouldn't want to use
their HF wire for VHF. Or at least, I wouldn't. I always use other
antennas for those bands. Seems to me, if you were to use two
chokes, one for 160-40, or 80-30, and one for 20-10 would be more
practical. A good choke for 20-10 "tribander", is 8 turns of RG8 wound
on a 6 inch form. There is no real need to wind in an orderly military
manner,
although some do. I often just throw them together liking rolling up a
rope,
and they work fine. I had to place a small choke on my mobile for 2m.
I kept having problems tuning a 5/8 wave 2m whip I made a few weeks
ago.
Tried trimming, adjusting the coil, etc. No joy. Came to the conclusion

common mode currents were causing me grief, and this is using a hole
mount drilled in the trunk lid. I usually don't have problems with
those..
I wound a small coil from rg-58, maybe 4-5 turns on a small appx 2 inch
dia,
and all my problems were gone.
A coax choke will need to be pretty small to work well on VHF.
I don't really see one made to work the upper HF bands as being very
good. So, in designing one for upper HF, I'd use 30 mhz as the upper
limit. I've got some info somewhere, maybe in the ARRL ant book that
gives appx dimensions for chokes at various frequencies. 30 ft of coax
wound on a 12 inch from would more likely suit the lower bands more
than
the upper HF bands. So I'd use a 8 turn, six inch dia choke for 20-10,
and whatever choke for the low bands. I'd leave VHF out of it, being
most
HF wire antennas are fairly lame for VHF. Heck, on my 706mk2g, using an
HF wire antenna is inviting lots of image problems for receive when
using
6 meters. 2 meters, I use ground planes, yagi's, etc.. I use a 3 el
yagi on
6m.
MK

On Sun, 22 Oct 2006 18:01:24 +0100, Ian White GM3SEK
wrote:


Cable is much less expensive over here than large ferrite beads imported
from the USA; hence my interest in the coiled cable chokes.

The cost of cores from imported from the US in Australia is also very
high.

For those in Australia interested in W2DU style baluns, I roughly
measured the impedance of an inexpensive core from Jaycar using a
Mighty Fine Junk 259B and plotted the results at
http://www.vk1od.net/balun/index.htm . Less than A$20 is probably
sufficient for an adequate balun for 80m to 10m.... depending...

My own view is that the number of cores required for a W2DU style
balun is unrelated to the characteristic impedance (inner to outer
conductor) of the coax as is commonly held, but that the number of
cores / effectiveness varies with location on the feedline and is
highly dependent on the scenario (frequency, topology etc) and the
optimum solution may required more than a single choke.

Ian, perhaps low cost suppression cores are also available directly in
your country, candidates for measurement and reporting!

Owen

PS Interesting plots, better presentation isn't it, the Y axis label
on your phase graph needs a fix.
--