About a week ago, Joel KB6QVI, tipped me off to the fact that Cecil K5NWA was selling 7.28MHz ceramic resonators on his online site, The Parts Place. Enthused, we both ordered some, and both our orders arrived yesterday. Joel slipped his into a test circuit, which gave him coverage of 7.015 – 7.236MHz. He used a similar circuit to the one I used, though I don’t know his exact component values. Here’s the test circuit I used, which gave me a coverage of 7.080 – 7.268MHz –
Adjusting the values of the 470p and 270p capacitors to, say, 330 and 270, or both to 330, should give a higher maximum frequency. If I were using this in a simple rig, such as a Micro 40, Beach 40, or similar, I’d be using varactor tuning. The 1N4001 (the “poor man’s varactor”) can have a minimum capacitance of as little as 5pF with an 8V bias across it, which would help too. It looks like it could be a useful component for simple 40M phone rigs and with a bit of experimentation, I bet I could increase the maximum frequency and get it close to the top of the US 40M band at 7.3MHz. If you’re in a part of the world in which your 40M band ends at 7.2MHz, then some extra parallel capacitance should do the trick. It’s a 3-pin resonator. If the center pin is grounded, then the internal capacitors are placed in parallel with the resonator. Ignoring the center pin and connecting to the 2 outside pins only, leaves the internal capacitors out of circuit, increasing the maximum frequency.
If you have any interest at all, it wouldn’t hurt to stock up on a few. Resonators for frequencies like 3.58MHz seem to always be in plentiful supply, but ones with frequencies that are useful for the phone portion of 40M are harder to come by. Who knows how long these will be available for?
PS – as I mentioned in the previous post on this subject, these resonators are also available from Mouser, both in the US and the UK. Thanks to Steve G1KQH, for the tip. Here’s the link to them on Mouser’s site. If you set the site to the country you are in, then the link will show you the resonator on the Mouser site specific to your location.