Bird Technologies 6151A - Manual (Page 13)

Chapter 2
Theory of Operation
Travelling Waves The voltage and current present on a transmission line are expressed as the sum of
the voltages and currents of two waves; forward and reverse. Both waves are identical
except in the direction of travel. The Model 6151A measurement concept basically
samples the voltage and current only of the wave travelling from the source to load.
The forward wave travels (and its power flows) from source to the load, and has RF
voltage E and current I in phase, with E/I = Zo. Note that the component wave is
mathematically simple, and is completely described by a single figure for power;
Watts = E
/R = I R = EI. Zo is the characteristic impedance of the detector circuit
and the coaxial termination, simplifying matters by being a reasonably pure resis-
tance of 50 ohms.
Coupling Circuit The coupling circuit which samples the RF power from the standing waves is in the
Plug-In-Element. The circuitry of the element and its relationship to the other
components of the Termaline are illustrated in the schematic diagram in figure 2.
Energy will be produced in the coupling circuit of the element by both mutual
inductance and capacitance from the travelling waves within the line section. The
inductive currents will flow according to the direction of the travelling waves
producing them. The capacitive portion of these currents are independent of the
direction of the travelling waves.
The electrical values of the element circuits are carefully balanced and designed to
be sensitive to the current produced from only the forward wave. Also, due to the
practically non-reflective characteristics of the termination, almost no reflected wave
will exist.
RF Load Section The RF load resistor of the Model 6151A Wattmeter consists of a cylindrical film
type resistor immersed in a dielectric coolant. The resistor, individually selected for
its resistive film accuracy, is enclosed in a special tapered housing which provides
a linear reduction in surge impedance directly proportional to the distance along the
resistor. This produces the uniform, practically reflectionless line termination over
the stated frequency range of the wattmeter. Refer to figure 3.
The coolant is chosen for its desirable dielectric properties and thermal charac-
teristics. Cooling of the load is accomplished by natural fluid and air convection. The
RF Coaxial Line
XMTR
or
LOAD LOAD
or
XMTR
Directional Coupling
Detector Element
Diode
Meter
DC Contact
Bypass
DC
Connector
Figure 2
Coupler Circuit
Schematic Diagram
3