2-4
Figure 2-1.—Ac voltmeter.
MEASUREMENT METHODS
At radio frequencies below the UHF range, power is usually determined by voltage, current, and
impedance measurements. One common method used to determine the output power of radio-frequency
(rf) oscillators and radio transmitters consists of connecting a known resistance to the equipment output
terminals. Current flowing through this resistance is then measured and the power is calculated as the
product of I2R.
Because power is proportional to the current squared, the meter scale can be calibrated to indicate
power units directly. A THERMOCOUPLE AMMETER can be used in this manner for measuring rf
power. The resistor used to replace the normal load is specially designed to have low reactance and the
ability to dissipate the required amount of power. Such resistors are commonly called DUMMY LOADS
or DUMMY ANTENNAS.
Q-4. What name is given to a resistor used to replace the normal load in a circuit?
In the UHF and SHF frequency ranges, accurately measuring the voltage, current, and resistance is
difficult. These basic measurements can vary greatly, depending on where in the circuit the measurements
are made. They are also affected by small changes in parts placement in the vicinity of tuned circuits.
