MIL-DTL-28830E
In the block diagram, a suitable length of cable with an attenuation within the measurement range
accuracy of the equipment is inserted between the connector. The signal generator and calibrated
attenuator are adjusted to produce a reasonable indication at the detector, when the detector is tuned.
The detector reading is noted, and the calibrated attenuator output level is recorded. The cable under
test is then withdrawn and the circuit completed with the connectors (or a very short length of cable).
With the detector tuned, the calibrated attenuator is readjusted to reproduce the original reading at the
detector and the attenuator output level is again recorded. Attenuation is then computed as follows:
100
A=
(Difference in calibrated attenuator reading in dB)
L
Where:
A = Attenuation in dB per 100 feet (30.5 meters).
L = Length of cable under test (in feet).
For measurements 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 1,000 MHz or above, the attenuator pads, connectors, and test cable
shall be mated to the same characteristic impedance. Both pads shall be high enough in attenuation
value to minimize the error caused by any 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. An alternate method may be used upon approval by the Government.
4.7.8 Velocity of propagation (when specified, see 3.1) (see 3.5.7 ). 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 MHz 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.
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.
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