Thursday, 10 July 2014

Build a RF Dummy Load


A RF dummy load is quite useful when working on transmitters. It allows you to test and adjust the transmitter without an antenna, eliminating interference to other radios on your test frequency. It also presents your transmitter with a proper 50 Ω load so as to not cause any damage to its final RF amplifier stage.

A recent project required me to modify and align twelve UHF transmitters. The transmitters had a 25 watt output and the alignment session on each would be short. Rather than buy a dummy load for this project, I decided to build my own.

The central part of the dummy load is a resistor (or resistors) with a total resistance of 50 Ω and a wattage equal to or greater than your transmitter. The resistors also must be non-inductive which eliminates all the common wire-wound power resistors. Acceptable types of resistors include carbon composition and thick film.

For the resistor in this dummy load I chose a #32-1007 50 Ω flanged termination unit from Florida RF Labs with a rating of 40 watts. Other parts include an aluminum case, heatsink and SO-239 connector.

Dummy load parts

Construction is rather simple. First the heatsink and SO-239 connector are bolted to the case. Be sure to use heatsink compound between the heatsink and case.

Dummy load case

Next holes are drilled and tapped for the flanged termination unit. The flanged termination unit also gets a thin coat of heatsink compound before installation. Its lead is extended with a short piece of wire to reach the SO-239 connector.

Flanged Termination Unit

A quick check with a SWR Meter shows a VSWR of 1.1, an excellent reading.

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