Category Archives: AM

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.

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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’.

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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.

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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.

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A 300w switching regulator for a QRO AM transmitter power supply

Laurie VK3SJ is an accomplished RF designer and homebrewer in Melbourne’s 160m scene. You can hear his excellent AM signal regularly on the 160m AM Coffee Break net most weekdays. Laurie has spent a great deal of time experimenting with Class D, E and Pulse Width Modulators. He has an interesting Class D AM transmitter design with a pulse width modulator. Laurie’s design has been built by Wayne VK3ALK, who I met online in the Class E Forum on AMFone. After lots of emails and a few calls with these homebrew AM experts, I decided to proceed with my build of a VK3SJ AM transmitter for 160m.

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160m 100W Class-E AM transmitter (FAT5)

The FAT5 project is a 100 watt Class E AM transmitter from Dave GW4GTE and Eric GW8LJJ, dating back to 2011. The name derives from Dave’s desire to design a 100 watt AM workhorse using contemporary FETs to rival the ubiquitous AT5, a widely used war surplus boat anchor for low band AM.

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