15 Meter Transmitter

Used With My Progressive Receiver

I built this transmitter to work with my Progressive Receiver. It is VXO controlled using 21.040 MHz and 21.060 MHz HC6U crystals that I bought at Ocean State Electronics. With the 21.040 MHz crystal the VXO tunes from 21.012-21.051 and the 21.060 MHz crystal provides a 21.037-21.072 range. There is no noticeable power change throughout the VXO ranges. The crystals plug into a socket on the front of the radio. An SPST toggle switch serves as a T/R switch and a DPDT switch serves as a TUNE switch. The TUNE switch toggles the transmit line to the antenna or to an internal 50 ohm dummy load, and when in the dummy load position, it also keys the radio. This serves as a SPOT function so I can tune the receiver to the transmit frequency. With the MRF 237 as the power amplifier transistor, the transmitter puts out nearly 3 watts. (First QSO was with AA0RQ on January 4, 2002.)

When I first started working on the transmitter I used the "Universal QRP Transmitter" published by W7ZOI in Solid State Design. As noted in Solid State Design, "chirp" was a problem with the 10 meter version. I also had a "chirp" problem with my 15 meter transmitter. I wanted to key the oscillator, however, to simply coordination with the Progressive Receiver. Otherwise, unless I muted the receiver on transmit, the oscillator was annoying to say the least. So I built a buffer stage and rebiased the oscillator, thinking that I could key the oscillator without "chirp" with a lower bias. I was wrong. The "chirp" was less noticeable but it was still present. W3TS, Mike, came up with the idea of keying a regulated voltage to the oscillator to lower the bias and stabalize it. It works great! What would I do without elmers?