2m Series Band Pass Filter

Well it took a few days longer thanks to some awful weather putting me off the trek to the workshop. But I finally finished the 3 pole series bandpass filter I alluded to in my last post. Here are some pictures of the filter:

What you might immediately notice is there are no trim caps. Since my last post I had removed another turn of wire and I had re-measured the inductors at 119nH. So I used this value in the design and since I'm confident in my technique I didn't feel the need to include trim caps in the first instance.

So how did it go? The response and losses were almost exactly as modelled. The model is shown below:

My centre frequency measured 141.7MHz, between the 3db points. And a peak at 140.5MHz. A few spot checks further down the response curve on higher and lower frequencies showed my filter had a very slight shift towards the left, or to the lower frequencies, compared to the predicted response. And I couldn't noticed any ripple in the response suggesting it wasn't worth trying to trim up the filter. With my loss measured at 1dB I adopted that old adage "If it ain't broke, don't fix it". I'm calling the filter finished.

You might notice I only fitted one RF connector. That's because I plan to mount this back onto the board the coils were removed from. I have a project in mind now that the filter worked so well.

Regards
Richard VK6TT

Footnote: This is the second most viewed page of my blog. While it shows what can be done with high Q inductors the results are not as good as a filter made with quality RG213 cable. While the RG213 filter is physically larger it would be a much better option for a homebrew project should space allow. The filter above had a bandwidth of 25MHz to keep losses around 1db. A 2 pole filter built with RG213 could have a bandwidth of 3MHZ for the same amount of loss.