N5ESEs QRP Dummy Load With Built-in RF Detector

Posted on Feb 6, 2014

This is yet another variation on the `parallel resistor` dummy load. [Go -here- for a discussion on dummy load theory]. This is another one I built during my infatuation with copper pipe. It`s suitable for QRP HF operation of 5-watts or less average power, and should be adequate for continuous operation at that level. It`s light and compact, about

N5ESEs QRP Dummy Load With Built-in RF Detector
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5-inches in length overall. This one is unique in that it has a built-in RF detector, with scaling, that may be used with your DC Voltmeter to measure power. Dummy loads dissipate energy by generating heat. Heat generated in a small space translates to temperature rise, and temperatures can be hot enough (under the right circumstances) to burn people and ignite adjacent materials. Because of the thermal mass of the dummy load and its enclosure, that heat can stay around for a long time. Always locate your dummy load in a safe place, where there is no chance that it will burn people or catch something on fire. The 1/2" copper pipe provides a convenient, compact form factor, is an excellent shield, and helps to dissipate heat to the outside world. Copper end-caps, available at most any hardware or plumbing supplier, provide a means of mounting the two UG-1094 BNC jacks and closing the unit. This version uses 8 each 100-ohm 1-watt 5% metal-oxide resistors, available from Radio Shack for a mere 25-cents each (RS 271-152). All the detector parts (yes, all three of them, a 0. 01 capacitor, a 4. 7-Meg resistor, and a 1N34A diode) are also available at Radio Shack. You would be hard-pressed to spend much more than $5-6 dollars on this project. The resistors are good through the HF range, but don`t do particularly well at VHF. Here`s a schematic: We`ll fabricate a printed-circuit board from scrap double-sided copper board, cut to...

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