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? Well, in one word splendid. The resonse 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.