Technical Topics > The RF Workbench

Hellschreiber beacon - with some oomph!

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redhat:
IF the first waveform is the drive to the fet, the duty cycle is WAY too high.  It should be closer to 40-50%.  This has a large effect on your efficiency, and thus heat load on the final device.  This is one of the reasons I use purpose built fet drivers in PA applications, better control over drive.

+-RH

Stretchyman:
All good advice and here's some more.

Throw away those ceramic caps, they never were any good. Fine for random 'Sprinkle brothers' decoupling but utterly useless for anything else.

Hopefully, eventually you'll go SMT and see the light!

Please stick with modern NPO or COG caps and use 630V ones across the FET and in the output and filter circuits.

Waveform wise your looking for a half sine on the drain at 3.5 X VCC, the classic class E shape.

Get yourself a 2x365pF (or whatever) variable capacitor from an old A.M. radio, you can use it when tuning the output. Must show a video on that as it's very easy to see the different shapes of the drain voltage.

Str.

Charlie_Dont_Surf:

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I really don't understand how the duty cycle of the FET drive could be that far off from 50 %, but my first guess is that the feedback resistor (Rf below) that bias the input and output of the inverter is too low. It's usually something like 1 Meg Ohm. Either that or the load capacitors for the crystal (C1 and C2 below) are grossly mismatched. For the crystal you are employing (HC-6?) something like 18-27 pF is probably appropriate and their values should match.

syfr:

--- Quote from: Radiotech on February 27, 2022, 0751 UTC ---
Yes, the Attiny45 carries the code for keying the transmitter and sending the Hellschreiber message. The Attiny works well, despite beeing in close proximity with the 10W transmitter.

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The ATtinys are perfect for this. You can easily get the standby current down to a mA or less by putting it to sleep for 8 seconds at a time ...of course you can loop the sleep statement to sleep it for longer times as needed.  Code is cheap and you can really minimize burning mAh's by sleeping the micro whenever possible.

I've not played with the 45 , but I've spent a lot of time with the ATtiny85. Great little chips.

I'm getting ready to build another beacon with mine, and you can do neat stuff like lowering your transmit duty cycle based on battery voltage, etc in order to put some hysteresis in the charge time.  That's a powerful little chip for short money.

You've got a lot done with your project. Very cool!

Pigmeat:
You boys are losing your edge. Three pages to get to the fights. You used to be going at it after three posts. Pitiful.

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