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Homebrew Universal QRP Transmitter

Here's the homebrew Universal QRP transmitter adapted from work by Wes Hayward - W7ZOI. The circuit originally appeared in QRP Classics published in 1980 by the ARRL. The rig is sometimes refered to as the Little Joe. The transmitter pictured was built for 40 meters.

I added to the design three new features not originally incorporated; a QSK circuit for breakin operation, a variable frequency VXO crystal oscillator circuit, and a 7 pole harmonic filter. QSK allows automatic antenna changeover from transmit to receive, the variable crystal oscillator allows about two kilohertz frequency swing about the 40 meter crystal frequency, and the harmonic filter greatly attenuates spurious output. Additional harmonic suppression is considered good engineering practice by today's standards.

The transmitter was built using double sided circuit board for the chassis. On the front panel the variable frequency control knob can be seen. The seven pole Chebyshev filter is to the right of the transmitter circuit board. The QSK components consist of an inductor and coupling capacitor mounted to the rear of the transmitter board. The final transistor is a 2N3553 fitted with a black "stove pipe" style heat sink.

View the Universal QRP Transmitter Schematic and Parts List

Information about harmonic filters suitable for use with this transmitter can be found at this link courtesy of the G-QRP Club. Short Guide to QRP Harmonic Filters

A circuit board is available from Far Circuits, the board is called the "W7ZOI Little Joe Transmitter." Far Circuits

A kit with circuit board for the 40 meter transmitter, and parts to build the transmitter for the other bands are available from Dan's Small Parts and Kits. Dan's Small Parts

Measured power output on the 40 meter version is one watt after the additional harmonic filter. On 40 meters both the oscillator and final are keyed without chirp. That was not possible with the 10 meter version currently on the air with "...the Little Beacon that Could" the K0KP/B 10 meter beacon. To aviod chirp on 10 meters the oscillator is left running and just the final amplifier is keyed. This does result in a backwave signal which is down approximately 30 db from the fundamental. Referenced to the 500 MW power level of the 10 meter beacon, the radiated backwave is approximately 0.5 MW. The backwave at this low level has proved not to be a problem. At 10 meters I was able to easily obtain 15 kilohertz of frequency swing about the crystal frequency.

The Universal QRP transmitter does a great job and many QSOs have been made with it. It can be constructed for operation on any band between 160 and 10 meters. I would recommend the Universal QRP Transmitter to anyone looking for a nifty homebrew QRP project. This little rig will delight you!


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© 2002 - 2005 Rex Greenwell K0KP