Category Archives: QRO

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.

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Low-band AM Tx: Power supplies

Complete 500W variable regulated DC power supply assembly.

This post describes the power supplies for a 200 watt AM transmitter. There are four independent DC regulated supplies. The ‘high tension’ (HT) supply delivers 0 to 100 volts at 5 amps steady to the modulator, and is sized for a continuous 200 watt AM carrier (with 800 watts of peak power in the sidebands). Two regular 12 volt linear regulated supplies power the 12 volt circuitry (independent 12 volt supplies is a way of keeping digital and other noise off the modulator and driver stages).

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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.

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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.

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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.

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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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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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Listening to West Coast AM on 75 meters

Standing on  a crowded commuter train, going home at the end of the work day in Melbourne Australia, listening to a late night west coast AM QSO on 75 meters, 3870khz, using my Samsung smart phone, via W7RNA’s web SDR.  This is just one of the miracles of SDR and the Internet — the ability to use someone else’s station anywhere in the world, from almost anywhere in the world. My old elmer from my school days in the 1970s, Moss VK5TU, would turn his tuning knob in his grave if he could see this. KB6 and WB6es chewing the rag, on big AM transmitters.  You can almost smell the burning dust on the ultraviolet glass envelopes.  Pressing the tiny audio buds into my ear to shield some of the ambient train noise I hear the tone of the transmitter, rounded low-mids and bass.  It could be 1972. It’s a nostalgic and beautiful sound.

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