3-16
Figure 3-13.—Simple electronic wattmeter circuit.
Q-12.
For power measurements, what advantage does an electronic wattmeter have over an
electrodynamic wattmeter?
ABSORPTION POWER METERS
Absorption power meters absorb either all or part of the source power. They require means of
dissipating the absorbed power, sensing the power thus dissipated, and indicating the amount of power
absorbed by the sensing network. Output power meters, in-line wattmeters, and meters employing
bolometers are examples of absorption power meters used by the Navy.
Output Power Meters
Figure 3-14 shows a common output power meter used in vhf-uhf applications. It has a 0- to 150-
watt range covered in two steps: 0-50 watts and 0-150 watts. Attenuator AT1 provides a 50-ohm nominal
resistive (dummy) load and uses metal film on glass construction. This dummy load is tapped to provide
the proper operating voltage to the meter. Resistors R3 and R5 form a calibration network at 50 watts; R7
and R8 form a calibration network at 150 watts. Accuracy, at approximately 20º C, is
5% for frequencies
between 30 MHz and 600 MHz,
10% for frequencies between 0.6 GHz and 0.8 GHz, and
20% for
frequencies between 0.8 and 1.0 GHz. When radio-frequency (rf) power is applied to AT1, this attenuator
minimizes the effects of power factors generated by any reactive components. The rf energy is then
detected and filtered by CR1 and C1, respectively. The resultant dc voltage, which is proportional to the
input power, is applied to a sensitive microammeter via one of the calibration networks. This meter has a
scale provided with two ranges: 0-50 watts and 0-150 watts. To protect the meter, you should always try
the higher range first. If the value proves to be under 50 watts, a shift to the lower scale would provide
improved accuracy.
