Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

782 CHAPTER 14
RF detector probes can burn out if they absorb too much power. There therefore is
an upper limit on the power density that can be measured with any particular probe.
Several factors must be considered when choosing or using a microwave-radiation
monitoring instrument. First, introduction of the detector probe causes some distortion
of the field, and the power density that is measured while the probe is in the
field is not necessarily the power density at that point in the absence of the probe.
This source of error can be minimized by using a small probe. A second factor is
polarization. If we have a dipole-receiving antenna and if the dipole is oriented perpendicularly
to the plane of polarization of the electric field, then no voltage will
be induced in the dipole and no microwave energy will be detected. On the other
hand, aligning the dipole parallel to the plane of polarization results in an induced
voltage, and thus to detection of the microwave field. One method of accounting for
polarization is to use a probe that contains three mutually perpendicular dipoles.
The sum of the induced voltage in each of the three dipoles will be the same regardless
of their orientation. Third, microwave detectors are frequency-dependent,
and the frequency response of the probe must be matched to the frequency of the
field being measured. Finally, detector probes will burn out if they absorb too much
power. There is an upper limit, therefore, on the power density that can be measured
with any particular probe. Most survey meters read the average power level rather
than the power in an individual pulse. Damage to probes, therefore, occurs more
frequently from pulsed sources rather than from CW sources.
W Example 14.26
Measurements are to be made in a 2.45-GHz field that is pulsed at a rate of 1600
Hz, with a pulse width of 0.25 μ seconds. If the probe’s damage level is 1.75 ×
10 5 mW/cm 2 and full-scale reading on the instrument is 100 mW/cm 2 , will the
instrument reach full-scale reading before the probe burns out?
Solution
According to Eq. (14.23)
P0 =
average power
P
=
pulse frequency × pulse width n × τ ,
the average power, P , corresponding to a peak power, P0 = 1.75 × 10 5 mW/cm 2 is
P = P0 × n × τ = 1.75 × 10
= 70 mW
.
cm2 5 mW
pulses
× 1.6 × 103
cm2 s × 0.25 × 10−6 s
pulse
Thus, the probe will be burned out if the meter reads ≥70 mW/cm 2 .