Saturday, July 20, 2013

Magnetic Loop Tuneup

Some bugs decided to take up residence in my magnetic loop's butterfly capacitor. They must have been misled by the name "butterfly" capacitor. Unfortunately this led to arcing, detectable by the SWR mysteriously jumping around up whenever I turned up the power.

I partially disassembled the capacitor and cleaned it. First I removed the mounting bracket and black plastic cover. Then I loosened the motor drive shaft set screw, and the four nuts holding the PCB, and removed the motor-drive PCB assembly. Remove the spacers as well; they may seem to be seized on and then fall off an an inopportune moment. Secure the motor drive assembly to the other end of the antenna, near the feed point, with tape or wire or something.

Next unscrew the plastic nut from the far end of the capacitor rotor. Orient the rotor so it's completely free of the stator plates and gently pull the shaft out from the hole in the PCB. Carefully remove the rotor. Be careful, these plates are aluminum and they are easily bent.

Finally remove the other PCB from the stator bolts.

To clean the plates, I used a putty knife to shove a thin brillo pad with a little dish soap in between the plates and gently scrub them. Then I rinsed everything off. Then I used a the putty knife again but this time with a damp microfiber cloth. I had to use a tool to remove a few cocoons the knife had missed; be careful not to scratch the plates, they're supposed to be smooth; you dont' want to create sharp edges where arcs could form. Finally I blew out any remaining crud, let it dry, and gave it a thorough visual inspection.

Reassembly is the reverse of disassembly.

I also took this opportunity to slightly tweak one of the limit switches; at the high end the rotor would turn so far as to start to go back into the stator, which is silly.

Now it takes power without arcing over, once again! Huzzah, back from the dead!

1 comment:

Herr Wabbaloo said...

Just what I was looking for!! I need to do this to my MFJ mag loop next month, and your step by step process is exactly what I need. Thanks very much and 73 from Texas.

Jeff, KF5KWO