40m AM Transmitter (120 watts, PWM/Class D)

This AM solid state Class D single band transmitter was assembled over a three year period. Started in 2018, it’s first configuration used a 100 watt push pull RF module published by Drew Diamond VK3XU in Amateur Radio magazine, modulated by a 200 watt linear power amplifier driving a reversed mains transformer, available as a kit from local supplier Jaycar. I built up the RF board, 50 volt power supply (using a stock 300VA toroids mains transformer, no regulator) and proceeded to destroy half a dozen power FETs (STW20NM50) in the RF power stage. Realising I didn’t really know what I was doing, I wisely put it aside.

Low-band AM Tx: Chassis and finishing

This post closes off the 160 and 80m ‘Low-band’ AM transmitter series, with a description of the chassis construction, fitting the assemblies into the chassis, and front panel finishing.

Low-band AM Tx: Pulse Width Modulator and Preamp

This post describes the modulator section of a 200 watt dual-band (160 and 80m) AM transmitter, built during the COVID-19 first-wave lockdowns in Melbourne. The transmitter is fully solid state, runs cool to luke-warm at 85 to 90% efficiency, and produces high quality AM on the two lower amateur bands.

Low-band AM Tx: RF Deck

‘RF deck’ is a term that describes the RF power modules in a transmitter or linear amplifier. In this 200 watt 160 and 80m AM transmitter, the RF deck consists of two identical FET RF modules each with its own driver, an RF power combiner, the band switched Low Pass Filter module, a Transmit/Receive switching relay, and an SWR and RF power sampling module.

Low-band AM Tx: Digital VFO/Controller

Lockdown has made 2020 a year unlike any other. Melburnians were dealt a long and painful period of isolation with a CoronaVirus second wave, from around July to September, still in force, and looking like continuing to (at least partially) keep us housebound for some months yet. Days merge together, work and leisure time is largely indistinguishable. People are rediscovering reading, knitting, and endlessly bingeing TV. Makers are melting solder and stringing wires in the air.

Digital SWR and Power Meter

This module is an analogue and digital SWR and power meter/monitor, designed to replace analogue SWR and power metering in an AM Transmitter project. Of equal importance was the ability to detect high SWR and raise an interlock (a control line) to inhibit transmitter power in the presence of unreasonably high SWR. I make no claim to either SWR or power accuracy; my version is an indicative tool and an interesting experiment, not an accurate test equipment, although it could be made into same with some skill and patience.

200 watt Class D and Pulse Width Modulated AM Transmitter

This post wraps up a series describing the construction of a 200W Class D AM transmitter for 160m, built, tested and put on air over 6 months from August 2019. The transmitter consists of a digital PLL VFO and driver circuit, an Arduino controller, a microphone amplifier, a Pulse Width Modulator, two H-bridge Class D RF modules each capable of at least 100 watts of carrier, an RF power combiner, Low Pass Filter, a 300W power supply and a switching regulator. At the Australian legal limit of 120W carrier, all parts of the transmitter run cool due the use of switching designs. Previous posts describe the switching regulator, 300 watt DC power supply and dummy loads for this project.

8 Band Superhet AM Receiver

Amplitude Modulation holds a fascination for me. It dates back to those hours spent as a teenager listening to the big broadcast-like amateur AM stations in the 1970s and 80s, on 160, 80 and 40 meters. Signals that seemed as wide and loud as medium wave commercial broadcast stations, bearing sonorous, paced voices that projected a wealth of wisdom and experience, and in many cases, a grandfatherly manner. Back then, AM was known as ‘The Gentleman’s Mode’.

Dummy loads for an AM PWM transmitter build

My Pulse Width Modulated AM transmitter project required dummy loads to match the load impedance of the RF power stage (50 ohms) and the Pulse Width Modulator (12 to 16 ohms). Surely any self-respecting amateur radio station would have a decent 50 ohm dummy load, I hear you ask incredulously. I do have one at VK3HN … or I did.

A 300 Watt DC Power Supply for a QRO AM Transmitter

This post describes construction of a 300 watt DC power supply for use in a 150 watt Class D AM transmitter. The transmitter design calls for about 150 volts DC at 2 amps continuous and up to 4 amps peak. This post describes the 240v mains DC supply. The switch mode regulator is described in another post. The other modules in this QRO transmitter project are to follow.