A.H. Systems active monopole series Посібник з експлуатації - Сторінка 11
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A.H. Systems Model Active Monopole Antennas
Measurement System Checks
MIL-STD 461E changed from the previous versions with the addition of making a pre-test
calibration (measurement system checks) as part of the test procedure. When testing to the
previous standards, one could go through an entire test suite with a faulty cable and never know it.
This cable could adversely affect the results for one or many tests, depending on how often it was
used.
The 10 pF capacitor (ECF-10) used with the rod antenna in the measurement system check
simulates the capacitance of the rod element to the outside world. With the rod antenna, the
electric field present induces a voltage in the rod that is applied to the preamplifier circuitry. One of
the functions of the preamplifier is to convert the high impedance input of the antenna element to
the 50 ohm impedance of the measurement receiver. The 10 pF capacitor ensures that the correct
source impedance is present during the check.
For measurement system checks, establishing the correct voltage at the input to the 10 pF
capacitor can be confusing. Since, the electrical length of the 104 cm rod is 0.5 meters, the
conversion factor for the induced voltage at the input to the 10 pF capacitor is 6 dB/m.
If the limit at the measurement system check frequency is 34 dBuV/m, the required field level to
use for measurement system check is 6 dB less than this value or 28 dBuV/m. The voltage level
that must be injected is:
28 dBuV/m – 6 dB/m = 22 dBuV
Since the input impedance at the 10 pF capacitor is very high, a signal source must be loaded with
50 ohms (termination load or measurement receiver) to ensure that the correct voltage is applied. A
"tee" connection can be used with the signal source connected to the first leg, the 50 ohm load
connected to the second leg, and the center conductor of the third leg connected to the 10 pF
capacitor.
A.H. Systems inc. – October 2005
REV A
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