Hi Ed,Many thanks for your very interesting post, and your offer of writing it up for some kind of article in Lowdown. I for one enthusiastically support that and I hope you decide to do it.
Actually your post hit close to home for me. For a (very) long time I’ve had an on-again/off-again workbench project (work on it for a few evenings then not again for months), building a 178 kHz beacon for the LOWFER/1750 meter band, and was intrigued we’ve been using some of the same strategies – although I’m certain I don’t bring your level of sophistication to the project!
As you certainly know, for “Part 15” FCC-authorized transmissions in the 160-190 kHz band (1750 meter band), one is limited to one watt input to the final amplifier and to 15 meters (49.2 feet) for the TOTAL of post-amplifier transmission line plus antenna plus any ground run (if used). This basically forces your hand to have an inside driver feeding a remote amplifier at the antenna.
So far I’ve built the 178 kHz driver using an MPF-102 in a Hartley oscillator with 2 more MPF-102 buffer/follower stages plus a 2N3906 keying circuit. It’s quite stable on 178 kHz (although it would probably fall far short of what our QRSS LOWFER colleagues would require, single Hz stability!). I’ve also built a prototype remote amplifier using a push-pull strategy similar to your approach. I’m using two IRF-510’s for this stage. Feeding the amp board through coax from the driver board, and into a 50 ohm dummy load, all seems to work hunky dory up to 1 watt.
My plan (like yours) is to use a loop antenna strategy. To play at the workbench, I have a 50’ loop of wire hanging from the ceiling joists in my basement, and I can incorporate this into the post-amplifier circuitry and it works well at lower power (say, half watt or less), but when I increase the drive to achieve the full 1 watt, the amp goes into run-away oscillation. This does not happen with the dummy load. I’ve done my best to impedance match the 50’ loop to the output circuitry but clearly something is hot happy when I drive it to one watt with the loop. On the other hand, the amp itself sits pretty much inside the ceiling antenna loop, so plenty of opportunities there for feedback to drive the unwanted oscillation. Have played with input bias voltages to no avail. So that’s what I’m currently working on, fixing this oscillation. And when I get stumped (which is frequently), I ask myself “do I want to do this tonight, or just turn on the transceiver and have some fun HF QSOs?” Most often I choose the QSOs. So therein is the reason this project has become a multi-year endeavor!
Ed, I’ve got a bunch more work to do on this and again I doubt I am working at the same level of expertise you are, but I was struck by the similarities of our approaches (push-pull amplifier and loop antenna). So again returning to your offer to write up your work for a piece in Lowdown, I’d love to see you do that—and am certain others would enjoy it and learn from it as well. For now, happy tinkering!
Bruce