KR1S Milliwatt Transmitter

May 20, 2008

A simple CW transmitter, driven by a K6ESE DDS. This version operates on 20 M. Band changing involves swapping out the low-pass filter & T/R board, mounted vertically. The device lineup is 2N3904 Buffer, 2N3904 Pre-driver, 2N2222A Driver, 2N3725 PA. The 2N3725 was a core-switching device with Ft of 300 MHz. It will deliver 5 W at HF with a good heatsink. Worth watching the auction site to buy a bag.

Although this transmitter will run more power, the Drive control will let me adjust power into the milliwatt range. I chose 20-M because it's open to the West Coast at night, when I can operate from home. A schematic may be forthcoming, but there's nothing new there. The 2N3904s operate Class A, and are keyed. The 2N2222A and 2N3725 operate in Class C. Keying is via a 2N3906. A pushbutton keys only the DDS for now, to allow spotting with a separate receiver. When I build a receiver that will transceive with this transmitter, the pushbutton will key the transmitter, for easier power-output setting.

The next version of this transmitter will probably omit the 2N3725, with power limited to 100 mW maximum. I think one 2N3904 stage can be eliminated. I began with the idea of using a VXO signal source. When I hooked up the DDS I found I needed more drive, and it was easier to add a stage than re-engineer what I had. I need to get some confidence in QRPp power levels before I take that next step down!

Milliwatt transmitter buffer, driver, PA

Breadboarded Transmitter

Milliwatt transmitter final version

Transmitter, almost-final version!

Milliwatt transmitter final version

20-M Low-pass Filter and T/R Switch Board

Milliwatt transmitter final version

Transmitter PA with low-pass filter in place

Milliwatt transmitter final version, in the box

Together at last!

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