Friday, March 21, 2014

Coaxial Cavity Filter - Unit#2

The second try at building a cavity filter takes a slightly different approach.  This time I used 1/8" brass plate and the SMA connectors are mounted on the outside rather than the inside.  This avoids the problem with shortening the cavity with the SMA base protruding into it and the solder paste and flux running down the connector threads clogging them up. 

This filter is also constructed as a two section filter with each built independently and inter-section coupling done externally.  The following pictures capture the construction.  Starting at upper left and moving clock wise they are: a) second section SMA loop connections prior to mounting the outer tube, b) inside the filter after loops have been bent around and soldered, c) outside view of both sections, d) end view of filter.
The SMA connectors were soldered first, the loops constructed, then the outer tube soldered in place, tuning and then inner tube soldering.  The first section worked fine.  Indirect heat was used on the outer tube to minimize heat to the base and SMAs.  Amazingly enough this worked well.

The second section was a different story.  After initial tuning and alignment with the first section, the inner tube slipped during final soldering.  While trying to correct this one of the SMA connectors shifted and shorted the post against the side of the insertion hole.  At some point the filter was dropped breaking the solder on the outside tube of section 1. (hint: whats the visual difference between hot and cold brass - none, other than the blister after touching it)  When it was all said and done both sections were corrected and worked, unfortunately their response centers were too far apart to be pulled together using 1" 6-32 tuning screws. [ being off by 1/16" inner tube length difference is roughly 75MHz and the furthest I can pull with the fine tuning screws is about 28MHz ].

The SMA board edge connectors on the outside appear viable, proper soldering and construction order should help, and tapping the center tube will avoid some of the final assembly and adjustment problems.

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