HF Radio Beacon – RF Pulsing Oscillator

Simple HF radio beacon with only one transistor. RF pulsing oscillator. Great to be used as a test beacon or ARDF ( fox hunting ) training sessions.
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Do you like Sputnik’s bleeps ? I was always fascinated about the pulsing beacon sound of the Sputnik and wanted to build something similar. Of course just for fun and testing purposes. I do not intend launching a satellite. Here you have a very simple and easy to build HF radio beacon. An RF pulsing oscillator using only one transistor. The original design by VK3AEC was for 27MHz. Initially I wanted to make it for the 2m band as it was easier to listen to it with my Baofeng. Unfortunately I do not have any 2m band crystals on hand.

THE SCHEMATIC

I was searching for a long time for a schematic of a simple radio beacon. I keep trying to get my nephew into the hobby with not much luck. Since Easter is close here in Romania, I remembered that he likes eggs hunting. This year I decided to replace the eggs and use a radio beacon instead. Basically is nothing but simplified ARDF ( fox hunting ), except that now we search for Easter beacon eggs ( ha ). The schematic is simple and the entire circuit can be built in less than half an hour. I had a crystal for the 80m band, so I modified the circuit a little to make it oscillate and sound good to my ear.

The transistor I used was a 2N3904, but I think you can use any NPN general purpose transistor. This also depends on the desired frequency you want to use the radio beacon for as well. The only thing I changed after running the RF pusing oscillator the entire night, was the 3,3uF electrolytic capacitor. I wanted the bleeps to slow down in speed a little, so I replaced the 3,3uF capacitor with 4,7uF instead.

RF PULSING OSCILLATOR SCHEMATIC

CIRCUIT DESCRIPTION

This simple radio beacon is a Pierce oscillator. The impedance of the LC tank, must be capacitive at the oscillator frequency. That means that the resonant frequency of the LC tank, must be slightly below the frequency of the crystal you are using. So as an example using a 7.030MHz crystal, the resonant frequency of the LC tank should be around 7.020MHz. This is important as it will help you modify the circuit for your desired frequency, depending on the crystal you want to use. For testing purposes a small thin wire antenna about 20cm long should be enough. Remember that to test and use any kind of radio beacons you need an amateur radio license. This if you use a larger antenna increasing the transmitting distance.

The power of this simple RF pulsing oscillator  is around 20mW. I actually measured 30mW when the battery was fully charged. I am using a rechargable one as is more convenient. I only tested it at about 1 Km away from my QTH and the reception of the beacon was still strong. 

CHANGING THE FREQUENCY

I wanted the radio beacon to work on the 80m band. I had a crystal around 3.5MHz. But if you want to use it for other frequencies is easy to modify it. In the video I explained how to do that really easy. Here you have a video on calculating resonant circuits. The only difference is that instead of a variable capacitor, you will calculate the resonant frequency for a fixed capacitor. You can then place a trimmer capacitor in parallel to make little adjustments.

If you don’t use an already made inductor and you decide to calculate yours, Coil32 Software may come in handy. The length of the antenna wire you are using, and the use of a counterpoise will also influence the simple RF pulsing oscillator. So you may have to play around a little with the value of  C4 until you get it to oscillate and pulsate as you like. Now you are ready to use it for tests or as an ARDF radio beacon. I plan on doing that by attaching a telescoping antenna to the direct conversion receiver I just built, and use it for Easter eggs hunting… radio beacon eggs ( ha ). Have fun with yours… as I plan on doing the same.

73 DE YO6DXE

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