A Diminutive Beacon Transmitter
Written by Bryce Ringwood   

In "Experimeters Corner"  I had set up an Arduino to produce Morse Code from text. This simply flashed the built in LED to produce whatever message was coded into it and it was interesting, but not as much fun as hooking it up to a radio transmitter.

 

I rummaged around in my junk box and found some pristine 3A5 (DCC90) valves - these should have been just the job. I built a Colpitts oscillator using a 300kHz crystal, switched on - and nothing. I spent more time analysing the circuit than I had spent building it - but couldn't find anything wrong - except the valve refused to amplify (even). Here is the circuit:-

I suspected the valves and dragged out my valve tester (Another home made project that didn't quite work out - maybe I'll do an article when its sorted.) - but the valves tested fine - so its a mystery why the thing refused to cooperate.

After this setback, it was clearly time for Plan 9.

WW2 Beacon Transmitters

World War 2 Beacon transmitters used to be small single-valve transmitters operating on a set frequency.  You could pick them up at surplus stores, and the ones I saw had a single triode valve operating at around 40MHz. They had a smallish triode valve in a simple oscillator circuit.

If I had used the 3A5, it would have been too big anyway (sour grapes), so I scoured the local  valve emporia for a suitable subminiature valve. It was essential that the transmitter be not much bigger than the Arduino board, and should (preferably) operate from 5 Volts HT.

The Final Design

The dusty shelves of the store revealed a small quantity of DL70 and EC90 valves. The DL70 turned out to be a transmitting tetrode with a maximum anode voltage of 150 Volts and a filament voltage of 1.2 volts, making it suitable for use with a rechargeable nickel cadmium battery. I still prefer NiCADs over NiMH, because they seem to have a lower self-discharge. Be that as it may, the following circuit (A Pierce Oscillator) was assembled on Veroboard in about 30 minutes.Beacon Transmitter Circuit Diagram Power was applied - gradually increasing the HT votage from zero. At about 5 volts something started to happen and at 6 volts HT there was a strong steady carrier at the crystal frequency of 11.17 MHz. Since the supply to the Arduino was provided by a 9 volt battery, I decided to settle on that as the HT voltage. As you can see from the photo, the "transmitter" is about the same size as the Arduino - slightly narrower and a bit taller. I wouldn't be tempted to run the transmitter at a higher voltage - for one thing it would probably be illegal, and for another, the Pierce oscillator was always regarded as a "crystal cracker".  With 9 volts there should be plenty of power for you to be able to receive on your chosen frequency. Note that for higher frequencies, you may have to reduce the value of the grid resistor R1.

Parts List

V1    DL70 (Other valves to try are the EF732, or larger B7G pentodes/tetrodes.)Beacon Transmitter

R1   470k 1/4 Watt

R2   12k 1/4 Watt screen resistor

R3    5k 1/4 Watt anode resistor

X1     Junk Box crystal - I used 11.170 MHz

C1,C2  0.01uF 600 volts working (For appearance - those little transistor types look a bit daft)

Veroboard -  10 strips wide by 40 cm.

Construction

Cut the board using a hacksaw and smooth the edges with a file. Round the corners of the board so nobody can get hurt.

Cut away the unwanted leads on the valve - there are three(pins 1,3 and 6). Fan out the remaining leads in the order Screen grid, Anode. Control grid, filament and filament/g3. With the valve half way down the veroboard, solder the leads to every other strip. The strip with filament/g3 will be the ground. The strip furthest from the ground will be the "HT" supply, and the strip immediately adjacent will be the screen grid strip.

Next connect a 0.01 uF cap across HT and Ground, followed by a cap from Screen grid to ground. Now connect a 12k resistor from HT to screen, to complete the screen grid circuit.

Connect a 5k resistor from HT to anode. Connect the crystal from the anode to control grid strip.

Connect a 470k resistor from control grid to ground.

Clean the solder joints with solvent, otherwise it all corrodes and looks horrible in a few months time.

Finally, connect the power supply leads.

In order to limit the range, I did not attach an aerial to the transmitter.

Keying the Transmitter

The transmitter can be on/off keyed with a morse key in the "HT" lead. Unfortunately, it doesn't work reliably at 5.0 volts - so connecting pin 13 of the Arduino board isn't really an option (But you can try it, all the same.)

The two options that were considered were a MOSFET in the ground circuit, and Frequency shift keying using a varicap diode in the anode circuit to pull the crystal. In the end frequency shift keying was chosen, as shown in the circuit diagram.

Using the Arduino pin 13 results in a shift of about 600 Hz.

Conclusion

This is a really simple beginners project suitable for anyone to build in an evening. There are still a few DL70 valves available locally, but its probably better to try whatever you have available. You might find a FET, such as a 2N3819 works just as well (or better) - but of course you won't need the screen grid circuit.

You might be surprised that some valves operate with such a low HT voltage, but a range of valves was designed to operate with 12 volts HT for car radio applications. Somewhere, I have a circuit for a simple regenerative reciever using a 6J7, also with 9 volts HT, and for a more challenging cicuit there's the superhet designed by Joe Sousa at radiomuseum.org.

 

 

 

 

 

 

 
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