MM0474
(2) One component frequently used to extend the range of a low power
meter is an attenuator which absorbs and dissipates a large fixed percentage of
the applied power. The input power to this attenuator is therefore some known
multiple of the output power which is measured by the low level power meter.
(3) A wide variety of devices can be grouped into the class of power
dividers.
These divert a sample of power from a transmission line into a
measuring instrument. The major portion of the power in the transmission line
is delivered to a dummy load or the specified transmission line termination.
Power dividers have the advantage of permitting measurement of microwave power
while it is being delivered to a load. Power dividers include such devices as
probes, loops, bifurcations and T and Y junctions.
Directional couplers are
devices which are designed to produce an output signal proportional to either
the incident or the reflected power in a transmission line. Total power can be
measured with a bidirectional coupler (which provides two outputs, one
proportional to the reflected power and one proportional to the incident power)
by subtracting the reflected power from the incident power.
(4) The major advantage of these various power dividers and power
reducing units is that their power division ratio can be calibrated at
relatively low power levels since the power reducing characteristics are
independent of power level.
(5) The description just given indicates that microwave power meters can
be additionally classified as to application.
Meters of one class terminate
the transmission line and absorb all the power being delivered by the
generators; the other class of meters removes a small sample of power from a
line, and hence permits measurements to be made of power delivered by a
generator to a given load under actual operating conditions.
3. Within the calibration system there are numerous standards and accessories
used to measure power. Each of these standards will be covered in detail in
Lessons Two through Five. The equipment and lesson number is listed in Table
1.
Table 1
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