MIL-DTL-24640C
b. Specimens containing more than four but fewer than 21 transmission lines: Four different transmission
lines shall be tested. Not less than one of these transmission lines shall be selected from each concentric cabling
layer within the specimen.
c. Specimens containing 21 or more transmission lines: The number of transmission lines tested shall be
equal to the square root (rounded, if necessary, to the nearest whole number) of the total number of transmission
lines. Not less than one of these transmission lines shall be selected from each concentric cabling layer within the
specimen.
An electrical connector, which shall be designed for use at the specified test frequency (see 3.1), and which shall be
appropriate for use with the specimen, shall be attached to each end of each transmission line selected for testing.
4.8.1.2 Special apparatus. Apparatus shall include the following, and shall be used at the specified test
frequency (see 3.1), and shall be chosen such that all electrical interconnections (see 4.8.1.3) shall be of the same
nominal characteristic impedance throughout (except that connectors and specimens in which a single conductor
plus surrounding shield is tested may be of a different impedance than this apparatus, if necessary).
a. A sinusoidal signal generator, which shall produce an unchanging signal of the specified test frequency (see
3.1) within a frequency accuracy of 0.25 percent. The signal generator may be amplitude modulated in a constant
and unchanging manner if the detector (see (b) below), requires an amplitude modulated signal for proper operation.
The signal generator shall generate power to produce a reasonable indication on the detector when used as specified
in 4.8.1.3; however, the signal generator shall not generate so much power that the specimen or any piece of test
apparatus will consequently experience a significant temperature rise.
b. A crystal diode, bolometer, or other detector, with its associated indicating device, which shall be used as
specified in 4.8.1.3.
c. A calibrated variable attenuator, which shall produce a maximum attenuation greater than that exhibited by
the specimen, and which shall resolve attenuation within an accuracy of ±0.25 dB.
d.
Two fixed attenuators, each of which shall exhibit an attenuation of not less than 10 dB.
e. Impedance matching transformers (required only when testing a conductor pair plus surrounding shield):
Two, two winding transformers; one winding of each transformer shall have an impedance equal to the nominal
impedance of the fixed attenuators; the second winding of each transformer shall be center tapped, and shall have an
impedance equal to the specified impedance of the specimen (see 3.1). When testing a single conductor plus
surrounding shield, impedance matching transformers may be used, if necessary, to match the nominal unbalanced
impedance of each fixed attenuator (see (d) above), to the specified unbalanced impedance of the specimen (see 3.1),
as shown on figure 11.
f. An adapter connector (may not be required, see 4.8.1.3) for joining the two fixed attenuators directly
together. This adapter connector shall exhibit an attenuation which shall be not greater than 2 percent of the
specimen attenuation. If desired, the adapter connector may be fabricated in the same manner and of the same
components as the specimen, except that the shortest practicable length of completed cable shall be used to join the
two connectors.
4.8.1.3 Procedure. The test apparatus shall be electrically interconnected as shown on figure 11 (when the
transmission line to be tested is a single insulated conductor plus surrounding shield) or as shown on figure 12
(when the transmission line to be tested is an insulated conductor pair plus surrounding shield), as applicable; one of
the selected transmission lines shall then be connected where shown. Following this, the calibrated variable
attenuator shall be adjusted to a low value, and the signal generator adjusted to produce a test signal of sufficient
amplitude to produce a reasonable indication on the detector. The setting on the calibrated variable attenuator and
the indication on the detector shall then be noted. Next, the transmission line shall be disconnected from the test
apparatus, and the two fixed attenuators shall be connected together either directly, or, if necessary, by means of an
adapter connector. The signal generator shall not be readjusted. The calibrated variable attenuator shall then be
readjusted until the detector produces the same indication as was noted when the transmission line was connected to
the test apparatus. The new setting on the calibrated variable attenuator shall then be noted, and the transmission
line attenuation shall be calculated by using the following formula:
Attenuation, in dB per 100 feet = 100 (A2 A1)/L
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