RF Waveguide Couplers

RF waveguide couplers are circuits that sample radio frequency (RF) transmissions by coupling signals asymmetrically. There are three basic types of products: unidirectional, bidirectional, and dual directional. Unidirectional couplers are three-port devices with two lines: a main line with a separate input and output, and a coupled line with a coupled output and internal termination. Power that passes along the main line is combined with the coupled output. Power that is reflected from the main line output is coupled with the termination.  Bidirectional RF waveguide couplers are similar to unidirectional devices, but remove termination from the fourth port. Main line power is coupled to the forward output of the secondary line. Reflected power is coupled with the reflected output. Dual directional couplers are also available. These four-port devices can be connected back-to-back in series or integrated into one device with a single main line and two secondary lines. The use a single main line reduces insertion loss and increases isolation. 

RF waveguide couplers are made of aluminum, brass, bronze, copper, or silver and plated with cadmium, nickel, rhodium, silver, or tin. Flanges are used to connect waveguide sections or terminate waveguide components. RF waveguide couplers use several different types of flanges. Union guide (UG) is the military standard MIL-DTL-3922 for a range of flange types. Cover or plate flanges are flat, square UG-style flanges. Choke flanges are UG-style devices with an O-ring groove and choke cavity. They are often used to minimize the effects of flange discontinuity. Connector pressurized rectangular (CPR) flanges meet Electronic Industry Association (EIA) specifications and are used in many non-military applications. CPR variants include miniature (CMR), grooved (CPRG), and flat (CPRF) flanges. Mill-back flanges are made with the opening of the flange face equal in size to the inside of the waveguide, which is soldered into a larger milled opening at the rear of the flange.   

Performance specifications for RF waveguide couplers include operating frequency range, EIA waveguide size, length or height, coupling, directivity, and voltage standing wave ratio. Operating frequency range is the range for which RF waveguide couplers meet all guaranteed specifications. EIA waveguide sizes have a prefix of WR and indicate the widest inside dimension of the waveguide in hundredths of an inch. For example, WR650 is 6.50 in. wide, WR90 is 0.90 in. wide, and WR75 is 0.75 in. wide. Length or height is also measured in inches. Coupling is a measure of the amount by which the signal in the main waveguide is greater than the signal coupled to the secondary waveguide. Directivity is a measure of the ratio between the power coupled to the secondary guide and signals traveling in the two possible directions along the main guide. Voltage standing wave ratio (VSWR) is a unit-less ratio ranging from 1 to infinity that expresses the amount of reflected energy at the input or output of the device. A value of one indicates that all energy passes. Any other value indicates that a portion of the energy is reflected.

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