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.