Tuesday, March 16, 2010

6M band pass filter

I recently lost the 6M front end transistor in my FT-897D. This is a tiny SM transistor nestled in the middle of the PA board.

I can't be sure if this was due to early failure of the component (The 897 was only a month old) or a stray RF field from my HF or other VHF transmitters. Initially I looked for a replacement transistor. I found out its a Japanese part (Doh!!) and although there are equivalents available the original part is only available from Japan. I gave up and went back to Martin Lynch & Son who had the part (and a competent technician to fit it).

Very soon they put the magic factory smoke back in my FT-897D (They did it while I waited) and while I waited I thought about ways keep it in there. I decided on a two prong approach -
- Fit a low pass filter on my HF amplifier

- fit a bandpass filter on my 6m station

Prong one was reasonably easy. Fitting a good quality low pass filter on my HF station was a good move and fairly easy. It helped reduce TVI and other unwanted nasties although my neighbors TV still dies if I TX on 12M. I assume that must be something directly unacceptable in his set to 24mhz as I have fitted an HF choke and 6M stub in his down lead. Maybe I will try a 24Mhz stub as well...

Anyway the 6M bandpass filter proved to be more tricky. I ordered one from ICE but after a few months nothing arrived and no reply to my emails (no charge on my card either) so I gave up on that avenue. I was looking through the ARRL Handbook and found a reasonable design for a 6m bandpass filter and decided to build that. I also downloaded the original article from 1968 QST as my membership of the ARRL gives me access to their archives (Are you listening RSGB?) The article gave a little more detail on construction such as doubling up the air spaced capacitors to achieve the 100+ pf required.

It took me about a month to scrape together all the bits (Enclosure, caps , copper line etc) but eventually I felt ready to build the filter. I could procrastinate no further.

It took a Saturday to complete the project. To ensure the comb lines were properly connected to the enclosure I cut it from a single sheet of copper and sandwiched it to the rear using a strip of aluminum (an off-cut from the Maplin box I had to cut down) fixing it all in place with about 15 4M brass nuts and bolts and washers..

I found that the filter worked very well but was far narrower than I expected. The article said it should be reasonably flat across about 4Mhz. Mine was about 300Khz wide... I could live with about 1.5Mhz as the UK 6M band is only 2Mhz wide and I never go above 51.34 (local repeater) so I started to explore what was wrong.

I had built the filter from 26awg sheet and the design used this may have had a small effect but I didn't think it would make that much difference?

It was suggested by G4MDC that to improve the bandwidth I would need to increase the coupling between the comb lines. Rather than pull the whole thing apart I tagged copper strips over the existing lines. This thickened the combs and increased the coupling.

The finished filter was still quite narrow (about 900Khz ) but its easier to tune and with a Maplin knob fitted to the central tuning capacitor I can easily adjust the filter to center on the FM (51.34Mhz) or SSB (50.15mhz) frequencies.

I'm still a little baffled by the design. Did the original filter cover 4Mhz as claimed? Had anyone else built this in anger?

that said I can now run full legal on 6m with not a spot of interference (The filter really does handle QRO as claimed) and so far my front end transistor hasn't blown again!


Blogger polstar101 said...

Hi G8UBJ, I have a FT 817 also deaf on 6. Your blog has given me hope that this is the same problem as you had. I live in Rochester Kent so Martin Lynch may be getting a visit soon :-) Thanks for the blog. 73 de Colin G6PBS

5:22 AM  

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