small logo SETI League Technical Manual -- Amplifiers

Water-Hole Band Pass Filter

Efforts continue to provide SETI League members with low cost Water Hole receive equipment. In 1995 we developed an edge-coupled microstrip bandpass filter, using the now-familiar no-tune hairpin layout popularized by Rick Campbell and Jim Davey. You may click here to view the general circuit topology and computer-modeled performance.

The filter prototype was discussed at the Microwave Update conference in October of 1995. Initial plans called for this design to be supplied commercially. Recently, however, a number of amplifiers and downconverters have been developed which incorporate internal filtering. Notice, for example that this converter circuit board includes the microstrip filter described above. Additional photos and performance plots of related modules appear here. We consider integral filtering to be a superior approach, in that it reduces the number of interconnecting cables required in an amateur SETI system. However, those users desiring a stand-alone microstrip filter module have found it easy to purchase the commercial converter printed circuit board from Down East Microwave, Inc., cut it down with tin snips, and connectorize just the bandpass filter sections.

In another approach to bandpass filtering, SETI League member and Eastern Australia regional coordinator Noel C. Welstead has built a coaxial cavity for his station, and finds that inserting it in the feedline between his first and second preamplifier stages decreases radio frequency interference. The filter is depicted here.

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