Tuesday, 22 March 2011

40 Meter Homebrew QRP Transmitter


QRP TRANSMITTER - Michigan Mighty Mite!

This is my first homebrew hf rig. Its a version of the Michigan Mighty Mite and built for 40 meters. I currently have a 7040 crystal and the rig seems to put out just about 500mw at 7.0405 Mhz. The tone isn't stable and neither is the frequency, but its very exciting to work cw on a rig that consists of 7 parts that you built in an afternoon! Its a great exercise for the novice homebrewer and can be assembled for pretty cheap.

A very quick and easy way to get on the air is to build a "Michigan Mighty Mite" CW transmitter for 160, 80, 40 or 30 meters originated by Ed Knoll, W3FQJ and developed by Tom Jurgens, KY8I. It can't get simpler than this! It has very few parts, costs almost nothing, and it works!

Output power is about 500 milliwatts with a 12-volt power supply. I have measured about 250 mw with a 9v battery as the power source.

Q1: 2N3053, 2N2222, SK3265 or similar inexpensive general-purpose NPN transistor. I use a plastic-case transistor that came in a bargain-pack from Radio Shack - works fine. Use heat sink - try an alligator clip if you don't have a heat sink handy. TANK COIL: use a 1.25" diameter form (35mm film canister, pill bottle, etc.) and #20 - #22 AWG enameled ("magnet") wire. To make tap, wind L1 to the "tapped at" number of turns (see table below). Make a loop about 1 inch long, twist it a few times and finish winding. Sand the insulation off the end of the loop. This is your tap. After winding L1, wrap it with a thin layer of masking tape and wind L2 on top of the tape in the same direction as L1. Secure L2 with more tape and finish by sanding insulation off remaining leads.

 L1: L2:
(primary/collector windings) (secondary/antenna windings)
160m--60 turns, tapped at 20 160m-- 8 turns
80m--45 turns, tapped at 15 80m---6 turns
40m--21 turns, tapped at 7 40m---4 turns
30m--15 turns, tapped at 6 30m---4 turns

XTAL: fundamental-mode crystal for desired frequency. About that variable capacitor - you can salvage one from an old transistor AM Radio or try a trimmer capacitor. Of course, a fullsize variable will work - but it will also be bigger than the rest of the transmitter! Tracking down variable capacitors at a good price is a noble challenge and part of the game. I got mine at Ocean State Electronics for just a couple of bucks. I got the xtal from them as well, but it was not cheap. At $9 it was the most expensive part of the rig!



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  2. A little close-up on the circuitry would be nice for me since I am a neophyte. I know what a bug looks like. Don't need help there. I know beggars can't be choosers but a little close-up on the board, for Pete's sake! Ranting?