Evanescent-Mode WG Filters

[G4GUO]

After all the talk about filters at CAT13, I just wanted to draw peoples attention to Evanescent-Mode WG Filters.
There have been a number of articles in the Amateur press about these filters in the past, most
notably for the UK by Andy G4JNT. www.g4jnt.com/EVANFILT.pdf‎

All the articles I have seen have used surplus waveguide however I have managed to build these filters
using rectangular extruded aluminium tubing and M4 brass nuts and bolts and a hand drill. All parts
obtainable on eBay. The filters I built were for 5.8 GHz. My filter was a post mixer rejection filter
and consisted of 10 cms of aluminium tube, 2 sma connectors and 4 M4 2.5 cms brass bolts.
Andy has examples of filters with more resonators and much tighter bandwidths / less ripple than
mine.

Although Amateur designs have been done using empirical methods there has been a 2 part article in
Microwaves&Rf magazine (July/August edition) where a closed form method has been proposed
using mathematical equations. Unfortunately you have to subscribe to that site to read the article.
the maths is not complicated (relatively speaking) so I am sure someone with a bit of spare time
could knock up a basic programme to do it.

These filters are relatively lossy so are probably only suitable for low power filtering after a preamp
or driver stage.

Just a heads up.

- Charles GUO