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Author Topic: ML/B 20m beacon 14357 kHz  (Read 528 times)

Offline mikelima

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ML/B 20m beacon 14357 kHz
« on: June 16, 2020, 0752 UTC »
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 :)
« Last Edit: June 16, 2020, 0915 UTC by mikelima »

Offline mikelima

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Re: ML/B 20m beacon 14357 kHz
« Reply #1 on: June 21, 2020, 1343 UTC »
Hi everybody! :)

These days there are a lot of storms here in north Italy, so the temperature inside the transmitter (and oscillator) vary a lot between day and night, but I noticed it drifts only of few tens of hertz, which I think is not bad at all.

Also, since the power is very low (and the dipole is pretty close to ground, unfortunately), I decided to change the message structure.
The dot duration is around 200ms, so there are more chances of it being copied even in low SNR conditions.
The new short message is transmitted every 10 seconds and consists of a 3 seconds carrier followed by "ML".
Every 15 short messages (so every 4 minutes, roughly) a longer message is transmitted, with additional information. Right now it is just transmitting the locator, the output power and an email address where I hope to receive some reports.
Actually the timings are not precise at all, I should calibrate the clock, since as keyer I'm using an ATMega328P running on its 128kHz internal oscillator (to make it use less energy), which apparently isn't that precise.

I'm so hopeful about receiving reports that I also prepared some QSL cards, just in case :D

Offline ChrisSmolinski

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Re: ML/B 20m beacon 14357 kHz
« Reply #2 on: June 21, 2020, 2321 UTC »
I have tried for your beacon - no luck yet, but I will keep listening!
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 300 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline mikelima

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Re: ML/B 20m beacon 14357 kHz
« Reply #3 on: June 28, 2020, 0849 UTC »
Thank you Chris for the effort!
Propagation isn't the best these days, plus the dipole is really low, sorrounded by concrete buildings, the angle towards the sky is pretty steep, and I guess this doesn't help DXing at all. I think I found the right spot where to place it, I'm busy with the exams at university right now but in a couple weeks I should have much more spare time to give it a try.

Anyway, I can hear it almost all the time during daylight on the University of Twente WebSDR (there's of course a lot of QSB) and at the beginning of the week I heard it clearly from the "Arctic SDR" (http://arcticsdr.ddns.net:8073/), which is a whooping 3000km distant, hence I guess it can be heard pretty well throughout western Europe.
To make it easier to copy I'm thinking to sync it with GPS (a ublox clone can be found for less than 10 on Amazon), so I can transmit precisely every minute at 00, 20 and 40 seconds and every 5 minutes I can transmit the long message with all the information, I will make some tests!
« Last Edit: June 28, 2020, 1026 UTC by mikelima »

Offline syfr

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Re: ML/B 20m beacon 14357 kHz
« Reply #4 on: September 04, 2020, 2028 UTC »
Will keep listening too!

You might look at the Epson programmable oscillators. $4 programmed to your frequency and 4mW out according to my measurements.

Will turn the beam your way
NRD525/TenTec Paragon lotsa wires and some beams

Offline mikelima

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Re: ML/B 20m beacon 14357 kHz
« Reply #5 on: September 25, 2020, 0802 UTC »
Thank you syfr!
The beacon is still on air and has been running almost 100% of the time since June, I hope you'll have some luck listening to it! I noticed that the frequency drifted about +100 Hz, but it  may be caused by temperatures getting lower in this season.

I'm taking a look right now at those oscillators, they're awesome! I've been searching for things like these for weeks, but only came across custom grinded crystals that cost an order of magnitude more. Thank you for the hint, I will make some tests as soon as I've got some spare time!