John's homebrew pages

Simple band-pass filter for 23cm

The improved 23cm transverter had been tested in action but I wanted to add one final improvement - a bandpass filter.

There were two reasons for this. First, it is a Good Thing in that it significantly reduces the radiation of out-of-band signals if any are produced by the transverter. I don't think I really had any problems with out of band sugnals, there is already good filtering on the transverter main baord, and the driver and power amplifiers are both tuned, not broadband. However, it was also an interesting 'exercise for the student' since I hadn't built a proper metal filter(tuned lines in air are less lossy than microstrip) for these wavelengths before.

The second reason is that the transverter receive side is completely open, i.e. broadband - there is no filtering in front of the two MMIC devices at the front end. Again this had not given me any problems, but if I operate close to stronger microwave sources it could well do, and so filtering these out before they get into the transverter is also a good idea.

One problem was that the box containing the transverter, though quite large, did not really have room for full size quarter wave resonators at 23cm. I found a design for a short tuned line filter for 144MHz in the RSGB VHF/UHF Manual (3rd edition) and scaled the lines from this design to 1.3GHz, which gave a line length of 19mm - much shorter than a quarter wave. I had some 3mm brass tube, so calculated the inductance of 19mm of this, which resonates at 1296MHz with a 1.7pF capacitor.

I had found a few piston trimmers in some surplus equipment bought at the Galashiels rally. These trimmers have low loss for microwave use, and I picked out two from a two stage 1500MHz bandpass filter built with microstrip lines. I also found a design of a microstrip 1.3GHz filter in the RSGB Radio Communication Handbook (10th edition). I hoped - not having the specification, having tried to find piston trimmers on the web marked like the ones I had - that 1.7pF would be within range of the piston trimmers.

Here's the filter as built, without its covering tinplate lid (more Hula Hoops consumed!). The trimmers are screwed into the bracket I extracted off the board they came from; this is soldered to the back filter wall. The lines are soldered between the base groundplane and the trimmers, with the input and output connections tapped just below the line centres.

23cm bandpass filter

The design seems to work well; I tried a prototype with adjustable input taps, and found that the input and output tap positions on the lines were not too important. I don't have the test equipment to make accurate measurements of the insertion loss of the filter, and in fact once the filter is tuned no significant loss is detectable with my home-brew through-line power meter.

So that's the final addition to the new 23cm transverter which can now be given lots of exercise in the RSGB UKAC contests and from a few SOTA summits.