POST AND IRIS WAVEGUIDE FILTERS
by Chuck Swedblom, WA6EXV July 1997
San Bernardino Microwave Society
The main reason for installing a filter in our microwave transverters is to remove the image noise signal, which is generated by the mixing process in receiver and to prevent radiating half of the transmit power in an unwanted sideband. A second reason to install the filter is to remove the local oscillator signal, which in some cases could be outside the ham band.
There are several types of filters that can be used to perform this job, but most are either difficult to construct or have losses that are unacceptable. The waveguide approach is attractive from a construction stand point and at 10 GHz and 24 GHz are relatively small.
Of the two types of filters discussed here, I prefer the Post type since I have a small lathe and can make the posts the correct diameter. I have tried several iris type filters which had higher insertion losses, due most likely to fabrication errors. The cuts in the waveguide must not be wider than the very thin shims used for the iris's.
I have two programs that I use to design waveguide filters. The Iris filter was designed using "WEFILTER.BAS"; WAVEGUIDE FILTER DESIGN, by G3JVL and is on the ARRL Microwave Software Diskette which is a companion to the ARRL Microwave Manual. The post filter was designed using a program written by Dennis Sweeney, WA4LPR which he presented at the 1989 MICROWAVE UPDATE Convention.
It is very important to layout the filter accurately and be sure that the posts are centered on the broad wall of the waveguide. If the dimensions are followed closely, very little tuning will be required. I have included tuning screws to assist in getting the required performance. The tuning screws will only lower the frequency of the filter, so if you error do it on the high frequency side by reducing the spacing of the posts.
Drill the post holes by starting with smaller drills and 'work up' to the correct size and be careful not to deform the waveguide. Remove any burrs inside of the wave guide with a fine file or internal deburring tool. The posts can be soldered in place with a high wattage iron or propane torch, use the least amount of solder possible so that no build-up occurs inside of the waveguide. I prefer to silver solder the posts, since silver solder flows much thinner for me, also I can then soft solder the nuts that hold the tuning screws without fear of unsoldering the posts. If you plan to use the filter in a coaxial system, then brass end plates will have to be soldered to the ends of the waveguide. Leave at least one guide wavelength of space between the last post and the end of the filter. I have included a drawing of a Waveguide to SMA connector that I designed several years ago that has very good return loss in the 10 GHz ham band, of course the waveguide flange would not be used in this case.
If you have a sweeper, then adjust the filter for best return loss (VSWR) over the pass band, this will also give you the lowest insertion loss. If you do not have a sweeper, then tune up the filter for maximum signal. The response will be quite good since the coupling is set by the post diameter and the pass band is very flat.
73 Chuck Swedblom, WA6EXV