MM0474
(2) The directional coupler used in the Waveguide Power Standard Set
consists of two waveguide sections, the main guide and auxiliary guide, bonded
together along their broad surfaces.
One end of the auxiliary guide has a
built in matched termination. The other end has a flanged port. An array of
holes in the common wall between the two waveguides transfers power from one
guide to the other. Power entering at the input of the coupler flows along the
main guide and divides at the coupling array.
Part is coupled into the
auxiliary guide, the rest continues along the main guide.
(3) The amount of power coupled into the auxiliary guide depends on the
coupling factor.
The directional coupler used in the coaxial power standard
assemblies consists of two quarter wave coaxial transmission lines capacitively
coupled.
One end of the auxiliary coaxial line has a built in termination.
The power flow through the coupler is analogous to that of the waveguide
coupler. Individual calibration reports are supplied for the couplers in the
power standard assemblies. The thermistor mount provided in the coaxial power
standard assemblies is designed for use with Model 431 and 432 Power Meters to
measure microwave power from 1 W to 10 mW.
Design of the mount minimizes
adverse effects from environmental temperature changes during measurement. For
increased measurement accuracy, effective efficiency and calibration factor are
measured for each mount, and at selected frequencies across the operating
range; the results are marked on the label of the instrument. The unit can be
used over the 10 MHz to 10 GHz frequency range.
Throughout the range, the
mount terminates the coaxial input in a 50 ohm impedance, and has a standing
wave ratio (SWR) of not more than 1.75 without external tuning.
(4) Each mount contains two series pairs of thermistors, which are
matched to cancel the effects of drift with ambient temperature change.
Thermal stability is accomplished by mounting the leads of all four thermistors
on a common thermal conductor to ensure a common thermal environment.
This
conductor is thermally insulated from the main body of the mount so the thermal
noise or shocks applied externally to the mount, such as those from handling
the mount manually, cannot significantly penetrate to disturb the thermistor.
This thermal immunity enables the thermistors to be used in the measurement of
microwave power down to the microwatt region.
(5) The thermistor mounts provided in the waveguide power standard
assemblies are designed for use with the Model 431 and 432 power meters in the
measurement of microwave power from 1 W to 10 mW in the range from 8.2 to 18
GHz. Design of power mater and thermistor mount is such that the measurement
system is temperature-compensated.
This feature permits microwave power
measurements that are relatively free of the drift in meter indication that
otherwise occurs with changes in ambient temperature.
(6) For improved accuracy of measurement results, calibration factor and
efficiency are measured at selected frequencies across the
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