Hi everybody, I'm new in this forum, but I used to read it for years
For the past year I've been working on a low power propagation beacon near the 20m band, I've been working on increasing efficiency (since it has to be powered from a solar panel) and finding the right xtal (I don't want to annoy ham radio operators, which I am also, nor create security problems transmitting in bands reserved for emergencies or so).
I came up with a not-so-good-looking class E transmitter, using a 74HC14 pierce oscillator as exciter and a BS170 mosfet as PA, the oscillator is always on (backwave is really low, but I'm still planning to build a shield around it).
The frequency it is currently transmitting at is 14.357 MHz. As I said I didn't want to annoy hams, but I also wanted to be spotted, so I planned to place it near the top end of the 20m ham band. Problem: there aren't commercially available xtals near this frequency, and I didn't want to spend money on a custom made xtal to put into something that will probably get stolen someday. So I bought six 14.31818 MHz xtals for a couple euros on eBay (the vintage type, with high profile and with soldered package) and I tried to grind them to increase frequency, testing the frequency using my multi-purpose pierce oscillator.
Something worth noting: I had already experience on grinding crystals, I did it with some 3 MHz ones, the quartz was pretty thick and sturdy and I had to scratch (with fine sandpaper) the two electrodes several times in order to shift the frequency of 20-30kHz. Turns out (it was pretty obvious though) that the 14 MHz ones are extremely fragile, the quartz layer is thin as a sheet of paper and smalllllllllllllll scratches on the electrodes shift the frequency a lot, so the first crystal turned out to resonate at 14.379 MHz (it's over 60kHz shift, I didn't believe it was possible).
If somebody wants to grind down these crystals, here are the tips (based solely on my experience, so they may be wrong): use the finer sandpaper you've got, don't apply pressure and don't bend the crystal, tiny scratches are not visible to the naked eye but they still shift the resonance frequency, try to make even scratches on both sides (I noticed that uneven scratches increase jitter by a considerable amount) and clean the surface with alcool before closing them.
Power output is a little over 1W (you can see from the picture above that it's a little over 0dBm with 30dB attenuator). Efficiency is about 73% (which I think is not bad at all, given this is my first class E transmitter, using a cheap mosfet running at 14MHz).
The output is pretty clean and jitter is negligible (PA connected to the scope via a 10dB attenuator, input was set to 10x by mistake). The FFT screenshot was taken before grinding the crystal (so the frequency is around 14.315 MHz), and it was not calibrated (so try to ignore the absolute dBm values, what I think is pretty good is the delta between the fundamental and the second and third harmonics, what do you think?), one of these days I'm going the take home my HP 8591 a friend of mine repaired, so I can make some real spectrum analysis and post some pictures as well
Some final notes (and thoughts):
I'm using an F connector and 75 ohm sat cable since they are very cheap (I'm always thinking of it getting stolen), and I have no problem matching them with the 50 ohm output of the PA.
Actually it is using a dipole.
I'm planning to put it "into the wild" powered by a solar panel, but right now it is not, so I won't publish pictures of the antenna nor the transmitting site for the moment. Everything I can tell you is that it is using a dipole, it is located in northwest Italy (in Piemonte), and it's on for most of the time, but not the whole time.
You can listen to it most of the time during daytime (daytime here in Italy) tuning
14.357 MHz on the University of Twente WebSDR. Messages sometimes change, but they always begin with 6 seconds continuous tone, followed by "
VVV ML/B" and are repeated every 40 seconds.
And also: I
really appreciate suggestions and reports
Topic: ML/B 20m beacon 14357 kHz (Read 2236 times)
