MIL-DTL-15370J
6.4.1.1 Unidirectional coupler. A unidirectional coupler is a directional coupler so designed as to provide a nominal
response in the secondary line to propagation in the primary line, in one direction only.
6.4.1.2 Bidirectional coupler. A bidirectional coupler is a directional coupler so designed as to provide separate
and simultaneous nominal responses in the secondary lines to each of the two directions of propagation in the
primary line.
6.4.2 Insertion loss. The loss produced by adding (inserting) a device into a signal transmission path (excluding
coupling power loss).
6.4.3 Primary line. The primary line of a directional coupler is the line designed to receive the principal flow of RF
energy in the transmission line to which the directional coupler is adjunct. The input end of the primary line of a
unidirectional coupler is the end into which power must flow in order to produce the maximum power at the output of
the secondary line. The other end is called the output (antenna) end.
6.4.4 Secondary line. The secondary line of a directional coupler is the line that is coupled to the primary line by
means of the coupling structure. In a unidirectional coupler there is one secondary line. In a bidirectional coupler the
secondary lines have a nominal response to a different direction of propagation in the primary line. In some
bidirectional couplers the secondary lines are physically separate; in others they run physically together.
6.4.5 Mid-frequency. The mid-frequency of a directional coupler is defined as the arithmetic mean of the limits
of its frequency range.
6.5 Conditions for use of level B preservation. When level B preservation is specified (see 5.1), this level of
protection should be reserved for the acquisition of directional couplers for re-supply worldwide under known
favorable handling, transportation, and storage conditions.
6.6 Subject term (key word) listing.
Couplers, bi-directional
Flanges
Pressurization
Couplers, unidirectional
Insertion loss
RF connectors
Effective directivity
Line, primary
VSWR
Electromagnetic interference
Line, secondary
Waveguide
6.7 Expanded discussion of power dissipation of secondary-line termination (see 3.5.6 and 4.7.10). Paragraphs
3.5.6 and 4.7.10 refer to a situation that may not apply to all directional couplers covered by this specification.
Furthermore, this discussion may be correct only for some directional couplers under this specification.
All directional couplers have at least three external ports: input, output and coupled. The input and output ports
are the two ends of the "primary line" of the coupler. The coupled port is one end of the "secondary line" of the
coupler. But the secondary line cannot have only one end; at some point the physical structure that comprises the
remainder of the secondary line must stop. It might stop at a connector that makes this "other" end of the secondary
line explicitly available to the user, as in a four-port coupler (eg. MIL-DTL-15370/3, Figure 1). It might instead stop at
an energy-absorbing external load (eg. MIL-DTL-15370/3, Figure 8). Finally, it might stop internally with or without
some construction that would act as an energy-absorbing internal load.
In whatever way the "other" end of the secondary line terminates, that is the "secondary line termination"
referenced in 3.5.6 and 4.7.10. The coupler specification is not concerned with the power-handling capability of
whatever the user attaches to the coupled port of the secondary line. Nor is the intent to permit lab testing by
simulating rated RF input power to primary in the form of a reduced power applied to the secondary - where the value
of the power applied to the secondary is determined from the formula. Applying reduced power to the secondary line
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