DMC WG Filters
DMC Waveguide Filters
The DMC Equipment contains waveguide filters on 22 or 23 GHz. Some can be tuned up to 24.048 GHz simply by adjusting the tuning screws, but many fail to get high enough. You can tell the approximate filter frequency by measuring the length between the outer faces of the input and output filter chambers: those with lengths of 27.5 mm tend to be nearer 22 GHz, whereas those with lengths around 25.0 mm tend to above 23.5 GHz and are suitable for modification. There are 2 known modification methods: screws in the narrow side of the waveguide, and squeezing the waveguide.
Screws in the Waveguide
By inserting tuning screws in the narrow side of the waveguide it is possible to pull these filters up by about 200MHz. The example shown below originally tuned up to 23.9 GHz, but was modified to work well at 24.048 GHz. Holes were drilled with the waveguide above the drill to minimise copper particles in the waveguide, and then the holes were tapped. The screws were 6ba silver-plated pulled from an old 8 GHz filter. Smaller screws might have been better!
Squeezing the Waveguide
Luis CT1DMK and Jac PE1KXH provided the following information: They raised the frequency of a DMC Filter by 200 MHz simply by squeezing each section inwards across the broad dimension of the waveguide by 0.5mm using a wrench. The indentations are clearly visible on this image
Finished result was very good:
