Field Guide · hardware

Also known as: dummy load, RF dummy load, 50-ohm termination, matched load

A dummy load is a resistive termination that presents a radio’s nominal impedance — almost always 50 Ω — while absorbing the RF power delivered to it and radiating essentially none of it.1 It lets a transmitter be tuned, measured, and tested without putting a signal on the air, and more generally terminates any RF port cleanly so that energy is absorbed rather than reflected back down the line.

TX RF power 50Ω heat
A dummy load absorbs transmit power in a matched 50-ohm resistance, dissipating it as heat rather than radiating it.

Overview

A good dummy load is judged by three numbers. Its impedance must stay close to 50 Ω across the frequency range so its SWR is low (ideally under 1.2:1) and its return loss high — a poor match reflects power back and defeats the purpose. Its power rating must exceed the transmitter output, and that rating is tied to a duty cycle and time limit: a load rated “1 kW for 10 minutes” relies on its thermal mass and cannot dissipate that continuously. The resistive element must also be non-inductive so it looks resistive at RF, not just at DC.

Variants

  • Air-cooled resistor loads — one or more non-inductive power resistors on a heatsink; the common bench load for QRP through moderate power.
  • Oil-cooled loads — a resistor immersed in mineral oil (the classic “cantenna”) whose oil bath extends the power and duty-cycle rating.
  • Attenuating loads — a load combined with an attenuator tap so a small, calibrated sample can be measured on test gear while the bulk of the power is absorbed.
  • Termination “caps” — small connectorized 50-Ω terminations (a watt or less) for closing off unused ports on splitters, filters, and switches.

Relevance to SDR

The dummy load is a transmit-side tool, so it matters most where an SDR platform also transmits — full-duplex boards such as HackRF, LimeSDR, PlutoSDR, USRP, and bladeRF are routinely bench-tested into a load before an antenna is ever connected, both to protect the hardware and to avoid radiating spurious signals during development. Terminating a transmit port into a load while probing it with a power meter or spectrum analyzer is standard practice.

On the receive side, small 50-Ω terminations serve a quieter purpose: capping the unused outputs of a splitter, the isolated port of a circulator, or a spare antenna input so that reflections and stray pickup do not degrade the wanted path. A terminated input is also the canonical way to measure a receiver’s own noise floor, since it presents a matched, signal-free source.

GopherTrunk is a receive-only software decoder; it neither transmits nor contains any hardware, so it never drives a dummy load itself. The relevance is contextual — a dummy load belongs to the test bench and TX hardware around an SDR, and it is a useful reference point for the matched-termination and SWR concepts that govern how cleanly signal power moves through the analog chain feeding GopherTrunk.

Sources

  1. Dummy load — Wikipedia, on non-radiating resistive terminations used to absorb RF power for testing. ↩

See also