QRPp Transmitter – Homebrew

amateur radio license required to operate on the air!



500mW CW TRANSMITTER

Cost estimate: $15

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! I made my very first QSOs with a 40-meter version (photo). It has very few parts, costs almost nothing, and it works!

Output power is about 500 milliwatts with a 12-volt power supply. To operate, attach 50-ohm dummy load or appropriate 50-ohm antenna (do yourself a favor and use a half-wave dipole antenna to avoid tuners and assure good results) and ground, insert crystal and close the key. Adjust the variable capacitor for the cleanest signal that has the most power (compromise).That’s it! You’re on the air, and can confirm that it’s working with field strength meter. Power output can be figured with a common multitester by using a very simple wattmeter circuit.


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 – the unit in the photo is a discontinued item from Radio Shack, but NO PROBLEM – 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.

..need parts?…

 

Source:

Ingram, Dave, K4TWJ, “World of Ideas: QRP Fun – Part II”

(CQ, Vol.48, No.3, March 1992, pp.107-108)

WHAT NEXT?– Try a station controller that frees you from switching the antenna from transmitter to receiver and back again. I use the one described in “How To Build A Simple QRP Transmitter Control System” by Pat Bunn, N4LTA (CQ , Feb 1992, p 40). It provides semi-QSK operation, electronic keying with keying filter, and optional sidetone and receiver muting features. To make it work with the transmitter described above, just short the transmitter’s key terminals — keying is then provided by the controller via the +V input.

***HOME***
***
RECEIVER***
***
Q-MULTIPLIER***
***
2-BAND VFO***
***
3W 2-BAND TRANSMITTER***
***
15 METER CONVERTER***
***
15 METER VXO-TUNED TRANSMITTER***
***
FIELD STRENGTH METER***
***
WATTMETER***
***
CRYSTAL TESTER***
***
SUPPLIERS***

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