by Chuck Swedblom, WA6EXV July 1997

San Bernardino Microwave Society

The main reason for installing a filter in ourmicrowave transverters is to remove the image noise signal, which isgenerated by the mixing process in receiver and to prevent radiatinghalf of the transmit power in an unwanted sideband. A second reasonto install the filter is to remove the local oscillator signal, whichin some cases could be outside the ham band.

There are several types of filters that can beused to perform this job, but most are either difficult to constructor have losses that are unacceptable. The waveguide approach isattractive from a construction stand point and at 10 GHz and 24 GHzare relatively small.

Of the two types of filters discussed here, Iprefer the Post type since I have a small lathe and can make theposts the correct diameter. I have tried several iris type filterswhich had higher insertion losses, due most likely to fabricationerrors. The cuts in the waveguide must not be wider than the verythin shims used for the iris's.

I have two programs that I use to design waveguidefilters. The Iris filter was designed using "WEFILTER.BAS"; WAVEGUIDEFILTER DESIGN, by G3JVL and is on the ARRL Microwave SoftwareDiskette which is a companion to the ARRL Microwave Manual. The postfilter was designed using a program written by Dennis Sweeney, WA4LPRwhich he presented at the 1989 MICROWAVE UPDATE Convention.

CONSTRUCTION

It is very important to layout the filteraccurately and be sure that the posts are centered on the broad wallof the waveguide. If the dimensions are followed closely, very littletuning will be required. I have included tuning screws to assist ingetting the required performance. The tuning screws will only lowerthe frequency of the filter, so if you error do it on the highfrequency side by reducing the spacing of the posts.

Drill the post holes by starting with smallerdrills and 'work up' to the correct size and be careful not to deformthe waveguide. Remove any burrs inside of the wave guide with a finefile or internal deburring tool. The posts can be soldered in placewith a high wattage iron or propane torch, use the least amount ofsolder possible so that no build-up occurs inside of the waveguide. Iprefer to silver solder the posts, since silver solder flows muchthinner for me, also I can then soft solder the nuts that hold thetuning screws without fear of unsoldering the posts. If you plan touse the filter in a coaxial system, then brass end plates will haveto be soldered to the ends of the waveguide. Leave at least one guidewavelength of space between the last post and the end of the filter.I have included a drawing of a Waveguide to SMA connector that Idesigned several years ago that has very good return loss in the 10GHz ham band, of course the waveguide flange would not be used inthis case.

TUNE UP

If you have a sweeper, then adjust the filter forbest return loss (VSWR) over the pass band, this will also give youthe lowest insertion loss. If you do not have a sweeper, then tune upthe filter for maximum signal. The response will be quite good sincethe coupling is set by the post diameter and the pass band is veryflat.

73 Chuck Swedblom, WA6EXV

Please excuse the postmarks & text from other side, these were copied from a newsletter, K6LJM.