The ory of op era tion. 3, Volt me ter cir cuit. 3, Rf load re sis tor. 3 – Bird Technologies 6156 Manuale d'uso

Pagina 15: Theory of operation, Voltmeter circuit, Rf load resistor

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Chapter 2

Theory Of Operation

The method of power determination used in the Termaline

Wattmeter may be expressed as W=E


/R where E is the

voltage across the resistance R, and W is the power in

watts. Figure 2 illustrates the E


/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.



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


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





Figure 2



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