19 September 2014

The 40m BPF to improve the reception in MM/HP contesting


when You are contesting in MM/HP category then You know very well that filtering is most important on each band...

In our case the 40m band was the biggest problem mainly because of breakthru from 160m band.

Even although the band decoder/PA lines used the proper TX BPFs for each band the reception on the 40m band CW was nightmare when 160m band operator operated the RUN station.

I started to modifying the lowband setup step by step... At first I installed the front-end savers into all external RX inputs of transceivers and decided to build good more-order BPF for 40m band receiving path.

I prefer the rugged and well shielded box for this filter to avoid blow-by the signal so decided for Hammond alloy box. It is really expensive but "You have what You paid for..."

As I had not enough time (as always I started to solve this issue just 1-2 weeks before the CQ WW) so I did not spent time with PC analysis, PCB layout etc. Simply I found and used the good design from W3LPL (mni TKS guys) and though "Network analyzer will say more..."

If You are going to build Your own filters also then do not use unknown or low-end components for the filter. High quality NP0/Silver Mica capacitors and Amidon cores (Hi-Q) as same as high quality connectors (PTFE) is the goal.

After assembling the filter on the breadboard I realized that the PCB is installed too high in the box and the cover is just 1mm above the components itself. In order to avoid the unwanted influencing of the filter and it's final performance by the closed cover I decided to reinstalling it and drowning the PCB as low as possible in the alloy box. It was not easy as the connectors did not fitted to PCB as at the beggining...

Well finally I solved all issues and even I found the way how to properly grounding the components together with alloy box and connectors with minimum junction points.

The first shot on the Network Analyzer (thanks to Mr. Hewllet and Mr. Packard) was not so satisfying...

I was not happy with the ripple as same as the insertion loss was a bit higher then I expected.( Network Analyzer has been actually calibrated and used measuring coaxial cables has been compensated properly)

Although I kept all values exactly as stated in W3LPL cookbook (even the coils has been tuned on the RLC-meter for their proper values)the slight modification has been needed.

After short alignment the BPF seemed better. I had to remove some wire (1wdg) and modify the coupling capacitors between the LC stages to get better response without ripples.

Also the insertion loss went down for 0.5dB (although it is still not satisfying for me and it will need some time in lab) and I decided to keep it like that for the coming contest.

Final result after tweaking and then closing the box is not so bad. The main goal the stopband on 160m band looks like over -100dB and the insertion loss on 40m is slightly over -1.5dB.

73 - Petr, OK1RP

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