Vectronics SWR-584C - Manual (Page 26)

SWR-584C Instruction Manual HF/VHF/220MHz SWR Analyzer
26
• Voltage at the center of the resonance becomes high, causing severe arcing.
• Current in the windings becomes very high, resulting in severe heating.
To test for self-resonance:
[ ] Install and test the choke in its normal operating location to incorporate stray capacitance.
[ ] Disconnect any choke leads leading to associated circuitry.
[ ] Connect the analyzer to both ends of the choke using a short 50-Ohm jumper.
[ ] Set the analyzer for its default R&X function in the Basic menu.
[ ] Tune the VFO to slowly sweep the choke's target operating range (specific band, etc).
[ ] Looking for impedance dips. These identify series-resonant frequencies.
When a low-impedance dip is detected, move a small insulated screwdriver blade along the
choke windings to find a point where the series-resonate impedance changes suddenly. This
jump identifies the location where the voltage peaks, and it's the spot where adding or
subtracting even a tiny amount of capacitance will have the greatest effect. To shift the
resonance off the critical frequency (or out of the band), try removing turns to reduce
capacitance -- or add a capacitive stub. Note that even a small change in capacitance has a
much more impact than making a small change in inductance because the L to C ratio is very
high.