How to Check the Outgoing Wattage on CB Radios

CB radio output wattage can vary if there are problems with the transmitter output in the radio and with other factors. Measuring the radio's output power with a wattmeter and "dummy load" checks the transmitter and helps ensure that the radio signal is strongest. There are several other ways to measure CB radio output power. While common electrical meters can't directly measure radio output power, basic RF (radio frequency) power measurements are not difficult with simple instruments and some math.


RF Power Meters

An RF power meter will measure output power with the CB connected to the transmitter side and a 50 ohm "dummy load" resistance attached to the antenna side. Both the meter and the load should be rated for the power output and frequency of the radio, usually five watts and 26-28 MHz. Instead of a dummy load, an antenna can be attached, but the power reading may vary according to antenna characteristics.

Alternate Measurement Techniques

Without a proper wattmeter, determining output RF power involves some math. Using an oscilloscope and measuring RF voltage across an accurate 50 ohm load will provide enough data to calculate RF power out. Without an oscilloscope, use a diode rectifier circuit with a dummy load. Measuring the voltage from it with a simple digital voltmeter (DVM) will give the same information (see Resources for more information.)

Factors Affecting RF Output

The CB radio's power output will be affected by a variety of factors. The power supply voltage for mobile radios can vary. The ideal is usually 13.8 volts, but in a car, it will change. A 12 volt bench power supply used for testing will produce different results than a 13.8 volt one. Transmitting with excessive antenna "mismatch" or standing wave ratio (SWR) will cause many radios to automatically reduce output power to avoid damage. Testing power output at the end of a long cable run will show reduced power due to cable losses.

Output Wattage and Signal Strength

The best communication comes from an antenna that can receive well and send the CB signal strongly. Increasing power can only affect the "reach" of the radio, but not its listening ability. A poor antenna or cable will waste output power. Both should be matched to the radio's output impedance, likely 50 ohms. On trucks and other vehicles, good antennas must be well designed, built and installed for the rugged environment. Grounding is key for CB antenna performance.



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