I recently ordered some replacement traps from Cushcraft/MFJ. Quality
was appalling, resonances were all over the place, and one didn't work
at all. After fixing the coil terminations and adding extra screws into
the plastic insulator to keep them from coming apart, all that was left
was to tune them.
The resonance can be adjusted by sliding the case back and forth, then
drilling a new hole. It is almost impossible to couple a dip meter to
the traps, as the coil is too far from the end of the case to allow
magnetic coupling, and capacitive coupling is weak. The method I settled
on was to attach a clip lead from a signal generator and a scope probe
on opposite sides of the case on the "hot" end, with their tips barely
protruding beyond the edge of the case. This results in very light
capacitive coupling and, I'm pretty sure, minimal disturbance of the
resonance by loading. The peak seemed to be about 50 kHz wide at the 3
dB points for the 15m traps. (Q of 400?) Has anyone else used this
method, or found a better one?
MFJ told me they measure the resonance by simply connecting the trap in
series with a 50 Ohm source and the input of a spectrum analyzer. I
tried that, and found only a very broad null.
BTW, the MFJ TB and TC traps (15m), unlike the originals, do NOT have
the same number of turns. The TC traps have 26 turns, the TB ones 25.
(The originals both have 31 turns on a slightly smaller form.) The MFJ
traps had varying numbers of plastic spacers on the outboard end,
apparently in an attempt to lower the resonant frequency. (One had a
shorted turn, and 4 spacers; I was able to remove one after unshorting
the coil.)
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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