POPPET TOP BAND AM TRANSMITTER
I was looking for a low power transmitter to build to use with local amateurs as a talk back frequency. With top band AM nets becoming popular again I decided to build my own version of the Poppet 160M AM transmitter. The first poppet circuit was published in the sprat newsletter and was designed by Doug Gibson.
The transmitter is a simple design and cheap to build, The output power is about half a watt for a total consumption of around 100 ma at 12 volts. There are no modulation transformers to make which are normally associated with AM transmitters.
In the original design the oscillator was preset to a desired frequency by L1. I decided to build my transmitter with an air spaced tuning capacitor in the VFO. This gave a generous amount of tuning from 1.905MHz to 1.988MHz giving a frequency swing of 83KHz to play with. I have included a DPDT changeover switch in the the circuit so I can use one antenna between the transmitter and a receiver. There is an output indicator meter in my version which is a very handy tool when tuning up into an ATU as you can see which is the best position for maximum power output. Finally the other addition to my circuit was a homemade bridge rectifier which I used for reverse polarity protection, This can not be seen in the photo as it is mounted under the main printed circuit board to save space.
When looking for a suitable enclosure in my junk box I came across an old power reducer which I got from a car boot sale for £2, I decided to take out the circuit board, switch and wiring and build the poppet inside this box. It already had stand off brackets for the new PCB, a meter, a couple of so239 connectors and power leads so I was able to keep the cost and work down to a minimum. The box measures 90mm* 90mm* 40mm*, It is rather cramped inside with the meter, switch and extra wires but looks like a nice tidy project on the outside.
When the project was completed I arranged an on air test with a local station who gave me a good 59+20db and reported that the audio was very loud and clear. He could hear me ok even when standing at the other side of my shack. The microphone was a carbon insert used in the old type telephones, other stations have had good reports with cheap CB type microphones.
CLICK HERE TO VIEW CIRCUIT DIAGRAM
If you build the circuit and would like further reading I would recommend you check out Graham Ogles dedicated poppet website, there are some excellent hints and modifications.
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