Dummy load
| Dummy load | |
|---|---|
| Also called | Dummy antenna, load; "cantenna" for an oil-cooled type |
| What it does | Absorbs a transmitter's output as heat instead of radiating it |
| Impedance | 50 ohms, purely resistive |
| Main types | Dry (air-cooled) and oil-cooled |
| Ratings | |
| Power | From a few watts for QRP rigs to several kilowatts |
| Duty cycle | Full rated power usually for a limited time only |
| Frequency range | Stated by the maker; most cover HF, and many reach VHF or UHF |
| Key part | A non-inductive resistor |
A dummy load is a resistor that stands in for an antenna. It presents a transmitter with the 50-ohm impedance it was designed to work into and turns the radio-frequency power into heat instead of radiating it. Connected in place of the antenna, it lets an operator test, adjust or repair a transmitter or amplifier at full power without putting a signal on the air or interfering with other stations. It is one of the simplest and most useful pieces of test equipment in a ham shack.
How it works
A good dummy load looks like 50 ohms of pure resistance at radio frequencies, so the transmitter sees an SWR close to 1:1. That calls for a non-inductive resistor. Ordinary wirewound power resistors are coils of wire with too much inductance for RF work, so dummy loads use carbon, metal-film or thick-film resistors designed for RF, mounted in a shielded enclosure with a coaxial connector.
No load is perfectly non-radiating. A small amount of signal leaks out, enough to be heard on a receiver in the same room but far too little to be noticed on the band, which makes a dummy load handy for listening to one's own transmitted signal.
Every load has a usable frequency range. A good one stays close to 1:1 throughout HF, and its SWR rises at higher frequencies as the resistor's stray inductance and capacitance take over. Check the specification before using an HF load on the 2-meter or 70-centimeter bands.
Types and ratings
- Dry loads are air-cooled: a resistor bolted to a finned heat sink, sometimes with a fan. The smallest are little bigger than a connector and handle a few watts, suitable for QRP rigs and handheld radios. Larger models handle 100 watts or more.
- Oil-cooled loads immerse the resistor in oil, which carries heat to the walls of a metal container. The best known was Heathkit's "Cantenna," a resistor fixed to the lid of a paint can that the owner filled with oil, and the name is now used loosely for any oil-filled load. Use only the type of oil the maker specifies, and never run one without its oil.
Power ratings depend on duty cycle, the share of time the transmitter is producing full power. A load sold as a 1,000-watt model may accept that power for only a few minutes before it must cool, and its continuous rating may be a small fraction of the headline figure. CW and SSB are easy on a load because their average power is well below the peak, but FT8, RTTY and a steady tune-up carrier hold full power for the whole transmission. A load run past its rating overheats, drifts away from 50 ohms and may fail. Loads also get hot enough to burn fingers, so give them space and air.
Dummy loads are a popular homebrew project. Twenty 1,000-ohm, 2-watt non-inductive resistors wired in parallel make a 50-ohm load good for roughly 40 watts of continuous power at HF.
Uses
- Testing a transmitter. Measuring power output on each band with a wattmeter, or confirming that a radio works before suspecting the antenna. If a problem disappears when the load replaces the antenna, the fault lies in the antenna or feed line.
- Adjusting an amplifier. Tube amplifiers are tuned with tune and load controls. Making the first adjustments into a dummy load keeps long tune-up carriers off the air.
- Setting levels. Microphone gain, speech processing and the audio drive for digital modes can be set while watching the radio's meters.
- Repair and alignment. Service work and kit building often call for long periods of transmitting.
- Checking instruments. A known 50-ohm load should read 1:1 on an SWR meter or antenna analyzer, a quick check that the instrument is working.
Rules and good practice
The FCC rules ask amateurs to avoid unnecessary interference and to use no more power than needed. Brief test transmissions on the air are permitted, with proper identification, but extended tests and tune-ups belong in a dummy load. The license exams make the same point: the Technician question pool describes the purpose of a dummy load as preventing signals from being transmitted over the air while making tests.