Bird Technologies 6156 - Manuale (Pagina 15)

Chapter 2
Theory Of Operation
The method of power determination used in the Termaline
Wattmeter may be expressed as W=E
2
/R where E is the
voltage across the resistance R, and W is the power in
watts. Figure 2 illustrates the E 2 /R power measurement
used in the wattmeters. RF energy flows from the transmit-
ter through resistance (R), producing a voltage drop across
this resistor. The RF voltage is rectified by a crystal diode
and voltmeter (E) records the drop. It is important that the
output impedance of the wattmeter be equal to the resis-
tance (R), and that the voltmeter is accurate at the operat-
ing frequency.
Voltmeter
Circuit
Basically, the voltmeter consists of two separate filtered
half-wave rectifier circuits. The active circuit is selected by
inserting the crystal diode into the desired socket and set-
ting the toggle switch to the selected scale.
RF Load
Resistor
The RF load resistor of this series wattmeter, consists es-
sentially of a cylindrical film type resistor immersed in a di-
electric coolant. The resistor, individually selected for its
accuracy, is enclosed in a specially tapered housing which
provides a linear reduction in surge impedance directly pro-
portional to the distance along the resistor. This produces
the uniform, practically reflectionless line termination over
the frequency range of the wattmeter.
The dielectric coolant is chosen for its desirable dielectric
properties and thermal characteristics. Cooling of the load is
accomplished by natural fluid and air convection. The di-
electric coolant carries the electrically generated heat from
the resistor to the walls of the cylindrical cooling tank. The
3
Transmitter E
R
Figure 2
Power
Measurement