MIL-DTL-22931D
Where:
A = Attenuation in dB per 100 feet.
L = Length of cable under test (in feet).
For measurement at frequencies of 400 megahertz (MHz), or less, the characteristic impedance of
the attenuator pads and connectors shall preferably be the same as that of the cable under test.
For measurement at frequencies of 1 GHz or above, the attenuator pads, connectors, and the test
cable shall be matched to the same characteristic impedance. Both pads shall be high enough in
attenuation value to minimize the error caused by the mismatch of the signal generator and
detector. For the majority of measurements, it is recommended that the attenuation of each pad be
approximately 10 dB. Tuning stubs may be used in the circuit for impedance matching purposes.
Any other method approved by the procuring activity may be used in lieu of that described herein.
When the attenuation of the cable under test is less than 1 dB at the test frequency, the attenuation
may be measured by the short circuit method.
4.5.7 Velocity (see 3.4.6). The velocity of propagation is determined in terms of the percentage of
velocity of wave propagation along the cable to the velocity of an electromagnetic wave in free space.
The velocity of propagation in the cable shall be found by resonating a length of cable at a frequency
between 10 and 200 MHz with one end short-circuited or open-circuited or by equivalent method subject
to the approval of the procuring activity. The same sample may be used for velocity and capacitance
measurements.
Fr. x length (ft)
Percent velocity =
2.46 N
Where:
Fr. = Resonant frequency in MHz
N = Number of quarter wavelengths in the cable
4.5.8 Capacitance (see 3.4.7). The capacitance of the cable shall be measured to three significant
figures, at any one frequency between 1 kilohertz (kHz) and 1 MHz reported in picofarads (pf) per foot.
An electrically short piece, that is less than 1/40 of a wavelength of cable, shall be used for this test.
4.5.9 Impedance (see 3.4.8). The characteristic impedance of cable shall be determined by
calculation from the capacitance and the velocity propagation measurements using the following formula:
101,600
Zo =
Velocity of propagation (%) * Capacitance (pf/ft)
Velocity propagation and capacitance values shall be determined as specified in 4.5.7 and 4.5.8.
4.5.10 VSWR (see 3.4.9). The VSWR of a minimum length of 100 feet of cable shall be measured
over the specified frequency range (see 3.1) using a swept frequency technique with capability of
measuring a VSWR of 1.04 or less. The measurement system may be in the form of directional
coupler(s), hybrid, or reflection coefficient bridge and shall have a directivity of at least 35 dB.
4.5.11 Pressure tightness (when applicable, see 3.5.1). The entire length of finished cable with
smooth or corrugated outer conductor shall be pressurized with an internal pressure of 30 psig and the
pressure drop shall be measured after a 24-hour period.
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