For too many weeks I've been tinkering with a small transmitter for the active AM frequency of 7.125Mhz and just now I called CQ and was kindly answered by Ross, VK3ARW. He reported good signal strength both where he is (west of Bendigo) and on the Ironstone ridge SDR in South Australia. The transmitter is home brew (although I used a Jaycar mic preamp kit).
I've been testing by listening to myself on the PKLoops SDR.
The transmitter uses an Arduino Nano that simply boots up and tells an Si5351 to put out 7.125MHz. There's an RF buffer using a 2N2222. The magic happens, the generation of AM, using a simple diode ring mixer that is set to be off balance. Mic audio, from the preamp, is fed in and out comes AM. (This technique was mentioned by Dave, VK3ASE, who also uses it). There's another 2N2222 gain stage then a low pass filter. Next I have the three stage RF power amp from Drew Diamond's 80m transmitter. Finally another low pass filter for 40m.
To answer Ross's question.. out of all this I get about 1.5W so, for the sake of getting some contacts I've fed this in to a little RF amp that gives about 25W peak.
I must say I'm astonished that I didn't get RF feedback or other instability given the construction. Despite Ross's kind words on the audio I can see that a peak limiter is needed to keep modulation up without over-modulating.
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