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