3-23
The NONADIABATIC calorimeter uses an rf termination with a resistive film strip or LOSSY
dielectric materials (solids or liquids that are designed to efficiently dissipate the applied power) as a load.
Temperature indication can be accomplished with thermocouples, thermopiles, thermistors,
thermometers, bimetallic strips, and manometers. Calibration is against a power standard or known low-
frequency power.
Based on the above principle, a coaxial calorimeter of good sensitivity with a short, 50-ohm resistive
film on a lava (dielectric) center conductor, enclosed within a tapered, thin-walled outer conductor, is
used for frequencies between 0 and 1.2 GHz. The rf termination is electrically connected to, but thermally
isolated from, a massive mounting plate by a short section of silvered-lava coaxial line with a high
thermal resistance. The steady-state temperature rise of the outer casing of the load with respect to the
mounting plate is measured by a differential platinum-resistance thermometer in a Wheatstone bridge.
Low-frequency power applied to the termination provides a method of calibration. Power in the range of
0 to 2.5 watts may be measured. A 70-second time constant and steady-state temperatures are attained in
about 6 minutes. The small physical size of termination (to keep convective and radiative heat losses low)
provides high sensitivity. Calibration with lower frequency power is extremely accurate, because the
termination is broadband and should exhibit the same power distribution from dc to 10 gigahertz.
A twin calorimeter provides a method of using two calorimetric bodies thermally shielded against
ambient temperature variations and improves sensitivity. Figure 3-20 illustrates this type of calorimetric
device. The power to be measured is applied to one calorimetric body; the other calorimetric body acts as
a temperature reference. The steady-state temperature difference between the two calorimeters is used as
a measure of rf power. Calibration is performed by applying low-frequency power. A differential-air,
thermometer-type temperature difference indicator, shown in figure 3-21, is used with a twin calorimeter
to measure microwave power in the 0.1-mW range. This instrument consists of two similar glass cells
connected by a capillary tube containing a liquid pellet. Each glass cell contains a tapered, carbon-coated
strip; and the entire assembly is mounted in a rectangular waveguide. Balancing dc power heats one strip;
the other strip is heated by rf power. The liquid pellet, which indicates the differential expansion of the air
within the two cells, is viewed through an aperture in the waveguide wall, preferably with a microscopy
for highest sensitivity. This procedure permits a 2% accuracy at 10 mW.
Figure 3-20.—Twin calorimetric system.
