Power dividers and directional couplers

The directional coupler is a component of the high-frequency technology and serves to divert a part of the running therein electromagnetic waves depending on the direction of a waveguide. The technical structure strongly depends on the frequency range: In waveguides are satisfied with small connecting holes in pieces of wire coaxial cables or transformers are used.

Main applications are the control signal for transmitters and measuring the standing wave ratio. A directional coupler, which produces a uniform distribution, ie power divider. With two output ports, the signals appear attenuated by 3 dB ( splitter).


With a directional coupler can be coupled out separate signals according to their direction of propagation in a waveguide. A directional coupler always has four ports or gates. The crucial property of an ideal directional coupler is that a wave which is fed to a gate divides the two goals of the functional opposite side in the defined ratio and is not coupled to the remaining door on the side of the feed. This property applies to each port of the directional coupler. Conversely, this means that the outcoupled wave can be excited at one port of a directional coupler only an incoming wave from the functionally opposite side. Now if a directional coupler inserted into a line so that two functionally opposite gates connect the signals in the line are issued exclusively dependent on their direction of propagation in the waveguide to the other two gates.

In real devices, the output port of the coupled- reflected wave can be provided internally with a terminating resistor. Then the directional coupler has only three connections for input, output and decoupled input signal. In the picture you can see some examples of directional coupler. The entrance is in each case left, the right output. The output for the coupled- forward wave is respectively located at the top left; the output for the coupled- reflected wave is partially replaced by a terminating resistor. Although the outcome for the coupled- forward wave functional is opposite the entrance, it is mechanical in its vicinity.

Directional coupler as a coaxial cable

In a coaxial cable in the space between the inner and outer conductors of a parallel wire is carried, the length of which must not exceed λ / 4 of the wavelength to be measured. Occurs both inductive and capacitive coupling, whose strength is determined by the distance. In an ideal directional coupler inductive and capacitive coupling are exactly the same size.

A signal on line 1 (represented by the directional flow arrow I green) is on line 2

  • A common-mode -like inductive coupling component (IM, blue) result, which is opposite due to Lenz's law.
  • A push -like capacitive coupling component (IC, red) to the sequence which is not oriented.

The currents are added to each of the two measuring resistors in the correct phase (constructive or destructive interference ) and generate proportional to voltages which are a measure of the power flowing. If the characteristic impedance of the coaxial cable to the antenna impedance match ( VSWR = 1), no output appears on the right measuring output.

The two-sided completion of Line 2 must be done with relatively low resistances ( ≈ 100 Ω ), whose value depends on the geometrical dimensions. This stress results in short lines to quite low measuring voltages. For this reason, two separate couplings ( lines 2a and line 2b ) are often used, which are not charged at the measurement output and therefore deliver higher voltage.

Directional coupler at the waveguide

The energy of the microwave transmitter is guided in a waveguide to the consumer. For measuring purposes it is drilled so that a part of the energy can be introduced into parallel waveguide. At certain intervals of these wells, the energy can propagate in only one direction due to constructive interference. In the opposite direction coupled power ( reflected due to mismatching of the antenna ), is converted into a resistor material into heat.

Directional coupler in microstrip technology

In the microwave directional coupler technology for low power in microstrip technology are made, as they are very inexpensive. Here exists a variety of circuit concepts such as

  • Tapered Line Coupler, translated approximately as Tapered - line coupler
  • Branch Line Coupler, in German about branch line (eg, 90 ° hybrid coupler )
  • Lange coupler ( consists of toothed spur lines )

Are chosen according to the requirements of the application. Especially Tapered Line and Branch Line Coupler are relatively easy to size and simulating. Disadvantage especially for the Branch Line Coupler is the space consumption on the board, which grows with the wavelength of the center frequency in all directions.

Directional coupler with transformers

In the coaxial construction, the coupling is highly dependent on frequency, so with the wavelength (lower short-wave area ) also increases the required coupling length. Because the results in either unwieldy dimensions, or at very low voltages, use a structure with current transformers ( through transformer).

Breitbandrichtkoppler by Sontheimer - Frederick

Two identical power converters are used to

  • Herabzutransformieren and 1: T1 with the power of the inner conductor n in relation
  • 1 herabzutransformieren: T2 is the tension between inner and outer conductors in the ratio n.

Thus, the impedance V / I is preserved. The coupling constant is calculated to be C3, 1 = 20 * log ( n ). The two resistors R1 and R2 of the transformer T2 must have the same value as the characteristic impedance of the coaxial cable between P1 and P2.

Bruene Bridge

The " Bruene directional coupler " has a current transformer and two variable capacitors. is the measured voltage almost regardless of the wavelength. The principle also works for only 50 Hz and is used in electricity trading to measure the direction of transport energy ( see figure).


Directional couplers are used as cable networks for connecting a user exit. Also, a proper impedance additive mixing multiple sources is possible.

Are RF rectifier attached to the two outputs of a directional coupler, the performance of the forward and reverse traveling waves can be separately determined with a DC voltmeter. From the ratio of these voltages, the so-called standing wave ratio, ie the ratio of pre- to the returning wave can be determined. It can be drawn on the adaptation of the line, for example, inferences to the impedance of the antenna and transmitter. Such devices are called SWR meter.

A the directional couplers related component is the circulator, this is the incoming power from a goal each in a specified direction of rotation only at the neighboring port. They are used in transceiver systems such as radar equipment to separate the transmitted and received by the antenna signal ( diplexer ).

Characterizing data of directional couplers