In the late 1980s I did a study of solenoidal RF chokes, for my own
curiosity and as a check against the theory from Termans Radio
Engineering books and other papers. I was faced with a system that
had continual problems in that it would overheat the B+ decoupling
capacitors. The system was an industrial RF dryer originally built by
Thermex, using 3CW30,000H3. It ran as a TPTG oscillator at approx 27
MHz unloaded, and dropped to approx 26 MHz loaded, with product in
the 'applicator' electrodes. Plate voltage was on order of 10 kV DC
and current was 5 Amps DC. The original stock RF choke was a three
section thing, with each choke wound seperate and (probably) in line
in a rectangular box. There had been several 1000 pF shunt Caps to
ground at the connections between the chokes. In short, it was full
of nasty resonances, some near the operating frequency. The first
ceramic capacitor near the RF end of the choke would sometimes
desolder itself.
I tried various OD to length ratios, and used both 8 AWG copper and
nichrome to try and get the lowest Q with highest resonant Z while
still maintaining a reasonable impedance over a couple of MHz of
operating range. The idea was to operate the system just on the low
(inductive) side of parallel resonance and use a single solenoid for
the choke, with a single cap to ground on the DC end.
I just found my plots from the vector impedance meter and Q meter
results. With a 1.75 inch diameter, 5.5 inch long choke, 0.187 inch
pitch, 24 turns, I got Q of 119 for Nichrome and 308 for copper.
First parallel resonance was around 23-25 MHz for both. Second
(series) resonance was at ~80 Mhz. I tried other diameters and turns
spacing, from 0.875 inch OD to > 2inch OD, and the optimal choke for
my application was the 1.75 inch as it had the lowest Q while still
holding reasonable Z near 10K ohms at resonance (in an aluminum
chassis). Had I wanted higher Q, as for a low loss RF inductor in a
tank circuit, I would have gone with large diameter short coils with
large turns spacing. When I connected two RFCs in series without
attention to right angles or reversing the turns between them, the
resonance just dropped to about 15 MHz and the series resonance went
down to 42 MHz. By moving the axis of the coils to 90 degrees, they
behaved more like individual inductors again. Here are some
references which shed a lot of insight into the design of chokes,
from early works. Most recent work has been on making tiny inductors
for microwave circuits, and on better modelling of the value of L and
distributed C for computer analysis of components in circuits.
My favorite references:
Prof J. Morecroft of Columbia University made comparisons of single
layers to banked windings, in his Feb 1922 paper in Proc IRE.
Harold Wheeler of Hazeltine expounded on the design and developed the
Wheeler approximation which gives inductance for a given winding
dimension. In Proc IRE, Oct 1928 he published "Simple Inductance
Formulas for Radio Coils". He discusses putting various chokes on
axis with their resulting resonances in his paper "The Design of
Radio Frequency Choke Coils" in Proc IRE, June 1936.
Reuben Lee of Westinghouse did some work characterizing RF chokes,
including sectionalized designs - Electronics, April 1934, "A study
of RF choke coils".
Herman Miller described his experiences with pie wound RF chokes, in
"Multi-band RF Choke Coil Design" in Electronics, Aug. 1935.
M. Scroggie wrote a series in Wireless World (May 17 and 24, 1935) on
"HF Chokes, Construction and Performance".
Dale Pollack of RCA also published two nearly identical reports, one
in RCA Review in 1938 and other in Electrical Engineering in 1937, on
the design of inductances for frequencies between 4 and 25 MHz. This
was more focused on receiver design, but most of the practices are
common for high power as well.
V. J. Cooper explains how to design single layer chokes to avoid
resonances near operating frequencies, in "The Design of HF Chokes",
Marconi Review, July, 1945.
C. Vernon Chambers. W1JEQ, wrote a very good piece for QST in May,
1954, "RF Chokes for High-Power Parallel Feed".
Jay bennett, AA6GK, designed a sectionalized single layer RFC for his
amplifier, but it looked extremely sensitive to layout and details.
Its in QST Technical Correspondence JUNE 1988, page 50 and 51.
There have probably been more recent articles in QST or QEX, and I
would welcome a chance to look them up if anyone cites them.
73 and Happy Thanksgiving (one of our big holidays in the States)
John
K5PRO
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