DUMMY_LOAD_AND_DETECTOR

Published:2009/7/12 23:08:00 Author:May | From:SeekIC

Two 100-Ω, 1/2-W resistors in parallel connected across a 50-Ω cable with near-zero lead lengths will be very close to a 50-Ω termination with the SWR less than 1.1:1. However, adding the diode introduces capacitor loading that results in an SWR of 1.5:1 or more. In the circuit, the components on the 2-dB resistor pad preceding the diode/load compensate for this, as they are tailored to reduce the SWR to less than 1.1: 1. Although the power capability of this assembly is 5 W, forced-air cooling is required at the higher power levels, depending on the measurement period.Schematic and layout of the dummy-load/detector assembly: A. Input, 1 ft RG-58/U. Fan out braid on connectink end, twist in two segments, and solder to PC board with minimum lead lengths. B. Base, 3 in2 ×in×1/16 in PCB. C. Capacitor-90° circular sector, 1 in radius, 0.21-in-thick Reynolds sheet aluminum. Surface polish with 220-grit sandpaper to remove burrs. Dielectric, 2.7-mil polyethylene (Ziploc heavy-duty freezer bag). Feed through 2-56 screw with a plastic insulator on the back side. Hole is reamed on both sides with a large drill to prevent shorting to the foil. D. Peak readout diode, 1N34A selected to have a reserve resistance of less than 5 MΩ (RS 276-1123). E. Component tie-down pads, 1/4 in1/×8 in×1/16 in glass-epoxy PC board. Cemented to base with clear household cement (Elmer's). F. Output tie-down pad 3/8 in1×/4 in×1/16 in PC board.



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