HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: greator on July 15, 2020, 1619 UTC
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After having test run the mkI version in my garden with good results I have done a new construction of the transmitter, the basic design is the same but the source of the schematics is different(came with Chinese QRP TX from Ebay), some of the values are a bit different, and the source of the 74HC240 is other. I 3d printed a holder for the Arduino Nano, witch turned out nicely.
Now im just waiting for a few more components for the lowpass filter, according to the designer, the PA-stage will put out about 10W at 13.8v. Im expecting more lika 2-3W.
(https://i.postimg.cc/8zsgw7T4/bild.png)
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Looks nice! I hope to hear it on the air!
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This circuit gives a bit more than i expected, driven with 13v i get 5W out, with 55% efficiency! It also works surprisingly low in voltage. In standby mode the TX only uses 4mA. The frequency can be adjusted with about 2kHz, using the 22pf variable capacitor on the oscillator.
(https://i.postimg.cc/GmhrVKgs/bild.png)
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Testing "CQ TEST" at 7001,6kHz, fieldhell, at 5W once every second minute from southern part of scandinavia. I will have a look at the web-sdr to get a picture of the range. The antenna is a longwire with a reasonable ground plane.
(https://i.postimg.cc/sDsGk1br/bild.png)
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I havent seen my tx on any of the web-SDR, so i tried to switch over to slow-hell but discovered that the PA has a bit of an issue with frequency drift when transmitting longer than the short pulse of field-hell. The signal starts our at one frequency, but then drifts about 250Hz higher during that "dot".
This is a problem with fldigi, since the filter is very narrow. When using the shorter field-hell "dots" this isnt seen, but it wouldnt bee an issue either since the fldigi filter is a lot wider.
The TX didnt have any other issues, so i will take if off the air and start building its enclosure and work a bit more on the code.
(https://i.postimg.cc/tJ2symZY/bild.png)
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I did try checking a number of KiwiSDR receivers when you first posted it was on the air being tested, but could not get any reception, either.
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I accidentally overheated the circuit when it got stuck in transmission for a longer period of time(a wire short probably) whilst enclosed in a small airtight box, i think a couple of the components overheated. Since i didnt get the stability i wanted to be able to run slow hell i have started looking around after a better design of the PA-stage.
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That's a nice looking build BUT I think you'll find that getting enough frequency stability for good HELL images is going to be tricky .
BTW, design might have worked fine with a resonant antenna rather than random/long wire. Feed it into a dipole and I bet it's happy!
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Wondered what that HUGE heatsink was for!
You need to be running the PA in class E.
You won't need the H/S at all just use the PCB and an insulating pad. Efficiency is around high 80% min. Also using a single Xtal it will be nowhere near stable enuf and will pull noticibly under transmission, I can see you're already seeing this. There's no reason to use a Xtal as they're rather expensive! DDS's are under $2 and will generate any frequency upto 12 MHz (using the AD9833). Stability is improved massively.
Str.