MIL-DTL-23806B
100
=
(Difference in calibrated attenuator reading in dB)
A
L
Where:
A = Attenuation in dB per 100 feet.
L = Length of cable under test (in feet).
For measurement at frequencies of 400 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, ant the test cable shall be matched to the
same characteristic impedance. Both pads shall be high enough in the attenuation value to minimize the
error caused by 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 Impedance (see 3.4.6). The characteristic impedance of the cable shall be determined by
calculation from the capacitance and velocity of propagation measurements using the following formula:
101,600
Zo in ohms =
Velocity of propagation (%) x Capacitance (pF/ft.)
NOTE: The capacitance and velocity of propagation shall be as determined as specified in 4.5.7.1 and
4.5.7.2.
4.5.7.1 Capacitance (see 3.4.7). The capacitance of the cable shall be measured to 3 significant
figures, at any one frequency between 1 KHz and 1 MHz for each second and reported in pF/ft. An
electrically short piece, that is less than 1/40 of a wavelength of cable, shall be used for this test.
4.5.7.2 Velocity of propagation (see 3.4.8). The velocity of propagation is determined in terms of the
percentage of velocity of propagation along the cable to the velocity of an electromagnetic wave in free
space. The velocity pf 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 an 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.
10
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