Conversion of the Motorola® MICOR®
Helical Band-Pass Filter as used in the Unified Chassis.
For use in the 222 MHz ham band.

By Lee Woldanski VE7FET

To modify a TFD6371A high-band helical Band Pass Filter, for use in the 222 MHz ham band.  The modification of this filter on the exciter output will allow the filter to be used for its original purpose however it will then tune the 222 MHz ham band. This filter is a 3 pole, externally tuned unit mounted behind (beside) the exciter inside the Unified Chassis.

After modification, the helical filter on the exciter output can be retuned from the original 150 MHz to 222.700 MHz. It required the disassembly of the filter, which was accomplished by removing the 8 top cover screws, the 8 bottom cover screws and desoldering the RCA output connector (from inside the cavity through the top).

In order to move the pass frequency up to 222.7, 4 turns were removed from the center of each of the 3 helical coils in the filter. I started in the center, and removed one turn at a time until the center frequency was just above where I wanted it.

After reassembly, I tuned the center slug until I had the filter on my pass frequency with minimum insertion loss (cranking the center slug in lowers the pass frequency). I then tuned each of the other slugs until I had the steepest response (just before the insertion loss started to increase).

I ended up with an insertion loss of 1.88 dB. This was about the same as the original. The filter response also appears to be same characteristic as the original. This filter has a 3 dB bandwidth of about 1.75 MHz at 222.700 MHz, gotta like that part.

Here is a plot of the converted filter tuned to 222.700 MHz.

MICOR exciter filter tuned to 222.700

Copyright April 14, 2001 by: Lee Woldanski, Grad Tech., Telecommunications Technologist, VE7FET
HTML Copyright April 16, 2001 by: Kevin K. Custer  W3KKC

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