MIL-DTL-24643C
e. Specimens incorporating multiple, unshielded, insulated conductor triad transmission lines: the mutual
capacitance and, if specified (see 3.1), the capacitance unbalance of every transmission line shall be determined,
using the same method as used in d, with the following change: The overall specimen braid (if any) and all
specimen conductors except the conductor triad under test shall be connected together electrically and considered to
be the shield of the transmission line under test.
4.9.2.4 Observation. Specimen failure shall be construed if the specimen exhibits a mutual capacitance or
unbalance which is greater than that specified (see 3.1).
4.9.2.5 Characteristic impedance and/or mutual inductance. If the characteristic impedance and/or mutual
inductance test is specified (see 3.1), then the specimen shall be retained for use in that test.
4.9.3 Characteristic impedance [when required (see 3.1) use MIL-DTL-17]. This test shall determine the
characteristic impedance encountered by signals propagating along completed cable. The mutual capacitance (see
4.9.2) shall be calculated for each transmission line within the completed cable prior to performing the characteristic
impedance test. This is necessary since values of mutual capacitance are used to calculate values of characteristic
impedance.
4.9.3.1 Specimen. The specimen shall be that which was previously used to determine capacitance
characteristics (see 4.9.2.1).
4.9.3.2 Special apparatus. Apparatus shall include an instrument for measuring inductance (such as an
inductance bridge, Q-meter or other instrument) which shall support the requirements as specified in 4.9.8.3. This
apparatus shall have a two terminal measurement port, which shall use sinusoidal wave of the specified frequency
(see 3.1) for making measurements. Measurements shall be made within an accuracy of ±1 percent.
4.9.3.3 Procedure. The appropriate characteristic impedance test shall be selected and performed from the
following:
a. Specimens incorporating any number of individually shielded, insulated conductor transmission lines shall
be laid straight. At one specimen end the exposed conductor of each transmission line shall be electrically
connected to the pigtail of its associated shield by tightly twisting both together. At the opposite specimen end, one
transmission line shall be selected for measurement, which shall then be connected to the inductance measuring
instrument. The inductance of this transmission line shall be measured. In an identical manner, the inductance of
every other transmission line within the specimen shall also be measured. The characteristic impedance of each
transmission line shall then be calculated by using the following formula:
Characteristic impedance, in ohms = (105L/Cm)1/2
Where:
Cm =
Mutual capacitance, in picofarads per foot, as calculated in 4.9.2.3.
L=
Measured inductance, in microhenries/10 feet.
b. Specimens incorporating any number of individually shielded or unshielded insulated conductor pair
transmission lines shall be laid straight. At one specimen end the two exposed insulated conductors of each
transmission line shall be laid adjacent to each other (to minimize superfluous inductance) and electrically connected
by tightly twisting both together. At the opposite specimen end, one transmission line shall be selected for
measurement, and its two conductors connected electrically to the inductance measuring instrument. Its shield (if
any) shall remain disconnected. The inductance of this transmission line shall be measured. In an identical manner,
the inductance of every other transmission line within the specimen shall also be measured. The characteristic
impedance of each insulated conductor pair shall then be calculated by using the formula shown in a.
4.9.3.4 Observation. Specimen failure shall be construed if the specimen exhibits a characteristic impedance
other than that specified (see 3.1).
4.9.4 Conductor resistance. This test shall determine whether or not conductors within the completed cable are
fabricated from the specified material.
4.9.4.1 Procedure. Conductor resistance shall be tested in accordance with ASTM B193 and shall be corrected
to 25 °C.
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