MIL-DTL-24643C
4.9.1.4 Observation. Specimen failure shall be construed if any transmission line exhibits an attenuation which
is greater than that specified (see 3.1).
4.9.2 Capacitance. This test shall determine the capacitive characteristics of completed cable.
4.9.2.1 Specimen. The specimen shall consist of a 122±½ inch length of completed cable, which shall have
overall components (such as armors, jackets, or overall binders) removed from each end for a distance of 2+½, -0
inches, thereby exposing the insulated conductors (and shields, if applicable) at each end of the cable core. Each
exposed shield (if any) at each cable end shall be unwrapped from the end of its enclosed insulated conductor(s) for
a distance of 1± inch; the unwrapped portion of each shield shall then be tightly twisted to form a pigtail for
electrical connection. The insulation shall next be stripped from each end of each insulated conductor for a distance
of ½± inch. If the characteristic impedance test is not required (see 3.1), then the insulation may be stripped, for
the specified distance from all of the insulated conductors at one specimen end only.
4.9.2.2 Special apparatus. Apparatus shall include an instrument for measuring capacitance (such as a
capacitance bridge, Q-meter, or other instrument) which shall be for supporting the requirements as specified in
4.9.2.3. This apparatus shall have a two terminal measurement port, which shall use a sinusoidal wave of the
specified frequency (see 3.1) for making measurements, and shall make measurements within an accuracy of ±1
percent.
4.9.2.3 Procedure. The appropriate capacitance test method shall be selected and performed from the
following:
a. Specimens incorporating any number of individually shielded, insulated conductor transmission lines: One
transmission line shall be selected for measurement, and each end shall be positioned such that its conductor and
shield shall not come into mutual electrical contact, nor into electrical contact with any other conductor or shield
which may be within the specimen. One end of this selected transmission line shall then be appropriately connected
electrically to the capacitance-measuring instrument, and its capacitance shall be measured. In an identical manner,
the capacitance of every other transmission line within the specimen shall also be measured. The capacitance per
foot of each transmission line shall then be calculated by using the following formula:
Capacitance in picofarads per foot = C/10
Where: C = Measured capacitance of the transmission line, in picofarads.
b. Specimens incorporating any number of individually shielded, insulated conductor pair transmission lines:
one transmission line shall be selected for measurement, and each end shall be positioned such that neither its
conductors nor its shield come into mutual electrical contact, or into electrical contact with any other conductor or
shield which may be within the specimen. Three capacitances shall then be measured at one end of this transmission
line, where one conductor of this line has been designated as conductor number 1 and the other conductor of this line
has been designated as conductor number 2 as follows:
(1) Conductor number 1 and the shield shall both be electrically connected to one terminal of the
capacitance measuring instrument, and conductor number 2 shall be electrically connected to the other instrument
terminal. The capacitance shall then be measured and designated as Ca.
(2) Conductor number 2 and the shield shall both be electrically connected to one terminal of the
capacitance measuring instrument, and conductor number 1 shall be electrically connected to the other instrument
terminal. The capacitance shall then be measured and designated as Cb.
(3) Both conductor number 1 and conductor number 2 shall then be electrically connected to one terminal
of the capacitance measuring instrument, and the shield shall be electrically connected to the other instrument
terminal. The capacitance shall then be measured and designated as Cc.
The mutual capacitance per foot and, if specified (see 3.1), the capacitance unbalance (sometimes called the
coefficient of asymmetry) shall then be calculated by using the following approximate formulas:
Mutual capacitance, in picofarads per foot = (Ca + Cb)/20 - Cc/40
Capacitance unbalance, in percent = 10[Ca - Cb]/Cm
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