Analyzing a Sinclabs SBP144-4 cavity filter

There used to be a little company in Newmarket, Ontario that made some very nice cavity filters. They got to be known as "intermod filters" because they knock down the response outside the 144-148MHz region, keeping commercial VHF transmitters from overloading the front end of your typically crappy ham radio transceiver. I used it back when I was doing 2m SSB weak signal work which is a bit challenging when you live in downtown Toronto like I do.

Here it is, centre frequency 146 MHz 2 MHz/div. The insertion loss is negligible, and it drops off about 10dB per MHz above 146 MHz and below 144 MHz. Hardly any ripple. Impressive.

Again centred at 146 MHz, at 1 MHz/div.

Again centred at 146 MHz 5 MHz/div. So around 5 MHz above and below it's -60dB.

Now, if you're going to be resonant at 146 MHz odds are you will also be at the 3d harmonic, and indeed it is. Here it is centred at 500 MHz with 5 MHz/div.

Here's the full spectrum from 0 - 2.05 GHz. I'm not sure what the crud is at the higher frequencies. But what I find interesting here is that the third harmonic bandpass is fairly broad (3x that of the 2m bandpass), but the higher peaks are not.

Click here for the start of the HP8555A analyzer tale