A Simple UHF Dummy Load

A very simple and effective dummy load can be made from several resistors, a connector, and a small metal plate or piece of PC board stock. Use six 1/4 or 1/2 watt carbon resistors whose paralled value equals 50 ohms. These should be CARBON only, not metal film. Six 300 ohm resistors will give 50 ohms, or you can use 270 ohms or 330 ohms (Will give 45 or 55 ohms respectively) Using three 270 and three 330 ohm resistors alternatively will yield 49.5 ohms and this is within 1 percent of 50 ohms.

The coax cable acts as an attenuator via its high losses and feeds a diode detector a sample of transmitter output with reduced level that the detector can handle. It also terminates the transmitter in a 50 ohm load that is better than anything you probably can make or buy cheaply. The 10 pf cap and RF choke remove RF components and the 1K resistor provides a load for the detector. Use a hot carrier diode if possible (HP5082-2800 series, etc) or else a 1N82AG, 1N21, or 1N23 type if you can get one, as the 1N914, 1N34, or 1N270 types used at lower frequencies are not as effective at UHF. A mixer diode salvaged from an old UHF TV tuner will work well. The 47 or 51 ohm resistor is not critical but should be a carbon, not film type, or you can use a chip resistor for lowest inductance. Detector output depends on input power and cable loss but will be 0.1 to 1 volt with a 1 to 2 watt ATV transmitter assuming about 15 DB cable loss. If you wish to observe detected video use a scope with 10 MHz BW and a low capacity probe, or connect scope to detector output with very short cable, six inches or less, to avoid loss of high frequency video components. For DC metering purposes these precautions are not necessary.

Caution: Do not use this circuit below the frequency where cable attenuation is less than 6 DB or more needed for reasonably good attenuation and decent 50 ohm termination of transmitter. Above 1300 MHz errors may result as well.

 
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