High 'E'-ffiency low power AM transmitter

Low power, pennywhistle, flea powered.. Just some of the names for small transmitters that the hobbyist can build to broadcast around their home or neighborhood.

I liked the design that Will Snyder came up with when I first saw it, something simple that seemed to meet the requirements for good quality AM transmission, signal stability, audio quality and effiency. Stability is assured by using a pre-tuned JITO-2 oscillator unit in a can. It's crystal controlled, with a buffer stage and puts out a TTL signal level, a 5 volt square wave.

2N7000 N-channel FET
Drain-Source Voltage60 V
Maximum Drain Current - Continuous200 mA - Pulsed 500 mA
Gate Threshold Voltage2-3 V
Drain-Source On-Resistance1.2-5 ohms

Will Snyder's original post to the FRN, 10-15-2006

2N7000 full specifications pdf

3.5 volts at 7 ma = 24.5 milliwatts input to IRF510 final Plugging in a 510 gave about 15 percent more power than the 2N7000
schematic

breadboarded circuit

The antenna is connected to the circuit through a variometer, which is a type of tuning coil used in the early days of radio, especially receivers in the 1920s.

Variometers allow for a continuously adjustable high inductance, perfect for tuning a short antenna. A variometer is two separate coils connected in series, one inside of the other, and the inner coil is on a shaft so that it can turn in relation to the larger coil. When the windings of both coils are going in the same direction the inductances add together. When the inner bobbin is flipped and the windings are in opposite directions, the inductance of the coils subtracts. In between are various levels of inductance.

The great thing about a variometer is it's a single component to do the whole job of tuning. It's harder to make than a plain coil, but it does away with tapped coils and tuning capacitors, and it's just one part to adjust, peak it for maximum signal.

My variometer adjusts from 180 to 500 microhenries, which should cover most 10 foot antennas on the upper end of the broadcast band. The wire is #26 enamel, and the shaft is the ink tube from a pen, with the ink cleaned out. It fits snugly through the outer coil's form, so that the inner coil will stay in place after it's turned. The outer coil is wound on a kitchen spice bottle, the inner on a piece of the spout from a laundry detergent bottle. Some builders use PVC pipe for the coils, for example a 3 inch pipe for the outer coil, and 2 inch for the inner.