[TowerTalk] A3 Trap Resonance

[K9MA]

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 at sdellington.us