MIL-DTL-24640C
(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
Where:
Ca, Cb and Cc are as defined in (1), (2) and (3), respectively, in picofarads.
Cm
=
Calculated mutual capacitance, in picofarads per foot.
The mutual capacitance and, if specified (see 3.1), the capacitance unbalance of every other transmission line
shall also be measured and calculated in an identical manner.
c. Specimens incorporating multiple, unshielded, insulated conductor pair 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 for specimens incorporating individually shielded, insulated conductor pair
transmission lines, with the following change: the overall specimen braid (if any) and all specimen conductors
except the conductor pair under test shall be connected together electrically and considered to be the shield of the
transmission line under test.
d. Specimens incorporating any number of individually shielded, insulated conductor triad transmission lines:
one transmission line shall be selected for measurement, and each end shall be positioned such that none of its
conductors, nor its shield, come into mutual electrical contact, nor into electrical contact with any other conductor or
shield which may be within the specimen. Four capacitances shall then be measured at one end of this transmission
line as follows, where one conductor of this line has been designated as conductor number 1, another conductor of
this line has been designated as conductor number 2, and the remaining conductor of this line has been designated as
conductor number 3 as follows:
(1) Conductors number 2 and 3 and the shield shall 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 Ca.
(2) Conductors number 1 and 3 and the shield shall next 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 Cb.
(3) Conductors number 1 and 2 and the shield shall next be electrically connected to one terminal of the
capacitance measuring instrument, and conductor number 3 shall be electrically connected to the other instrument
terminal. The capacitance shall then be measured and designated as Cc.
(4) Conductors number 1, 2, and 3 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 Cd.
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:
Cm
=
[(Ca + Cb + Cc)/40] [Cd/120]
The capacitance unbalance or coefficient of asymmetry (K) of a shielded triad, expressed in percent, shall be
determined by the formula:
K1
=
10 [Ca Cb]/Cm
K2
=
10 [Ca Cc]/Cm
K3
=
10 [Cb Cc]/Cm
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