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