MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
20
7.8
Testing RF Chokes
For testing self-resonance in chokes, use the analyzer in basic SWR, Resistance (R) + Reactance (X) Mode. Large
RF chokes often have frequencies where distributed capacitance and inductance form a low impedance series-
resonance. Series resonance occurs because the choke winding acts like a succession of back-to-back L networks.
This condition can potentially result in three problems:
1.) The end-to-end Impedance of the choke becomes very low.
2.) The voltage at the center of the resonant point becomes very high, often causing severe arcing.
3.) The current in the winding becomes very high, often resulting in severe heating.
To detect troublesome series resonance, install the choke in its designated operating location and connect the
analyzer in an end-to-end configuration through a short 50-ohm jumper cable (with no other connections). Slowly
sweep the choke's operating range looking for impedance dips that identify low-impedance series-resonant
frequencies. When detected, move a small insulated screwdriver blade along the choke to find a point where the
series-resonate impedance changes suddenly. This is the region that has the highest voltage present, and the area
where adding or subtracting even a tiny amount of capacitance had the greatest effect. To shift the resonance out of
the desired frequency range, try removing turns to reduce capacitance -- or adding a capacitive stub. A small
change in capacitance has a much more impact than making a small change in inductance because the ratio of L to
C is so high.
8.0
TECHNICAL ASSISTANCE
If you have any problem with your MFJ-259C, first check the appropriate section of this manual. If the manual
does not reference your problem and the problem isn't solved by reading the manual, you may call MFJ Technical
Service at 662-323-0549 or the MFJ Factory at 622-323-5869. We can serve you best if you have your unit,
manual, and all pertinent information about your difficulty handy so you can answer questions the technicians may
ask.
You can also send questions by mail to:
MFJ Enterprises, Inc.,
300 Industrial Park Road,
Starkville, MS 39759;
by FAX to 662-323-6551; or by e-mail to [email protected]. Send a complete description of your
problem, an explanation of exactly how you are using your unit, and a complete description of your station.