![RF Power Divider](https://upload.wikimedia.org/wikipedia/commons/2/2f/RF_Power_Divider.jpg)
Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are
passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
to a
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
enabling the signal to be used in another circuit. An essential feature of directional couplers is that they only couple power flowing in one direction. Power entering the output port is coupled to the isolated port but not to the coupled port. A directional coupler designed to split power equally between two ports is called a hybrid coupler.
Directional couplers are most frequently constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other. This technique is favoured at the
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
frequencies where transmission line designs are commonly used to implement many circuit elements. However,
lumped component devices are also possible at lower frequencies, such as the audio frequencies encountered in
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
. Also at microwave frequencies, particularly the higher bands, waveguide designs can be used. Many of these waveguide couplers correspond to one of the conducting transmission line designs, but there are also types that are unique to waveguide.
Directional couplers and power dividers have many applications. These include providing a signal sample for measurement or monitoring, feedback, combining feeds to and from antennas, antenna beam forming, providing taps for cable distributed systems such as cable TV, and separating transmitted and received signals on telephone lines.
Notation and symbols
![Directional coupler symbols](https://upload.wikimedia.org/wikipedia/commons/3/34/Directional_coupler_symbols.svg)
The symbols most often used for directional couplers are shown in figure 1. The symbol may have the
coupling factor in
dB marked on it. Directional couplers have four
ports
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...
. Port 1 is the input port where power is applied. Port 3 is the coupled port where a portion of the power applied to port 1 appears. Port 2 is the transmitted port where the power from port 1 is outputted, less the portion that went to port 3. Directional couplers are frequently symmetrical so there also exists port 4, the isolated port. A portion of the power applied to port 2 will be coupled to port 4. However, the device is not normally used in this mode and port 4 is usually terminated with a
matched load (typically 50 ohms). This termination can be internal to the device and port 4 is not accessible to the user. Effectively, this results in a 3-port device, hence the utility of the second symbol for directional couplers in figure 1.
![Power divider symbol](https://upload.wikimedia.org/wikipedia/commons/0/02/Power_divider_symbol.svg)
Symbols of the form;
:
in this article have the meaning "parameter ''P'' at port ''a'' due to an input at port ''b''".
A symbol for power dividers is shown in figure 2. Power dividers and directional couplers are in all essentials the same class of device. ''Directional coupler'' tends to be used for 4-port devices that are only loosely coupled – that is, only a small fraction of the input power appears at the coupled port. ''Power divider'' is used for devices with tight coupling (commonly, a power divider will provide half the input power at each of its output ports – a divider) and is usually considered a 3-port device.
Parameters
Common properties desired for all directional couplers are wide operational
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, high directivity, and a good
impedance match
In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize sign ...
at all ports when the other ports are terminated in
matched loads. Some of these, and other, general characteristics are discussed below.
Coupling factor
The coupling factor is defined as:
where P
1 is the input power at port 1 and P
3 is the output power from the coupled port (see figure 1).
The coupling factor represents the primary property of a directional coupler. Coupling factor is a negative quantity, it cannot exceed for a passive device, and in practice does not exceed since more than this would result in more power output from the coupled port than power from the transmitted port – in effect their roles would be reversed. Although a negative quantity, the minus sign is frequently dropped (but still implied) in running text and diagrams and a few authors go so far as to define it as a ''positive'' quantity. Coupling is not constant, but varies with frequency. While different designs may reduce the variance, a perfectly flat coupler theoretically cannot be built. Directional couplers are specified in terms of the coupling accuracy at the frequency band center.
Loss
![Coupling loss graph](https://upload.wikimedia.org/wikipedia/commons/5/58/Coupling_loss_graph.svg)
The main line
insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels (dB).
If the power transmitted to the load before insertion ...
from port 1 to port 2 (P
1 – P
2) is:
Insertion loss:
Part of this loss is due to some power going to the coupled port and is called
coupling loss
Coupling loss, also known as connection loss, is the loss that occurs when energy is transferred from one electrical circuit, circuit, circuit element, or medium to another. Coupling loss is usually expressed in the same units of measurement, uni ...
and is given by:
Coupling loss:
The insertion loss of an ideal directional coupler will consist entirely of the coupling loss. In a real directional coupler, however, the insertion loss consists of a combination of coupling loss,
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
loss, conductor loss, and
VSWR
In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmi ...
loss. Depending on the frequency range, coupling loss becomes less significant above coupling where the other losses constitute the majority of the total loss. The theoretical insertion loss (dB) vs coupling (dB) for a
dissipation
In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form to a ...
less coupler is shown in the graph of figure 3 and the table below.
Isolation
Isolation of a directional coupler can be defined as the difference in signal levels in dB between the input port and the isolated port when the two other ports are terminated by matched loads, or:
Isolation:
Isolation can also be defined between the two output ports. In this case, one of the output ports is used as the input; the other is considered the output port while the other two ports (input and isolated) are terminated by matched loads.
Consequently:
The isolation between the input and the isolated ports may be different from the isolation between the two output ports. For example, the isolation between ports 1 and 4 can be while the isolation between ports 2 and 3 can be a different value such as . Isolation can be estimated from the coupling plus
return loss
In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load conne ...
. The isolation should be as high as possible. In actual couplers the isolated port is never completely isolated. Some
RF power will always be present.
Waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
directional couplers will have the best isolation.
Directivity
Directivity is directly related to isolation. It is defined as:
Directivity:
where: P
3 is the output power from the coupled port and P
4 is the power output from the isolated port.
The directivity should be as high as possible. The directivity is very high at the design frequency and is a more sensitive function of frequency because it depends on the cancellation of two wave components. Waveguide directional couplers will have the best directivity. Directivity is not directly measurable, and is calculated from the addition of the isolation and (negative) coupling measurements as:
:
Note that if the positive definition of coupling is used, the formula results in:
:
S-parameters
The
S-matrix
In physics, the ''S''-matrix or scattering matrix relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory (QFT).
More forma ...
for an ideal (infinite isolation and perfectly matched) symmetrical directional coupler is given by,
:
:
is the transmission coefficient and,
:
is the coupling coefficient
In general,
and
are
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
, frequency dependent, numbers. The zeroes on the matrix
main diagonal
In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, major diagonal, or good diagonal) of a matrix A is the list of entries a_ where i = j. All off-diagonal elements are zero in a diagonal matrix. ...
are a consequence of perfect matching – power input to any port is not reflected back to that same port. The zeroes on the matrix
antidiagonal
In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, major diagonal, or good diagonal) of a matrix A is the list of entries a_ where i = j. All off-diagonal elements are zero in a diagonal matrix. ...
are a consequence of perfect isolation between the input and isolated port.
For a passive lossless directional coupler, we must in addition have,
:
since the power entering the input port must all leave by one of the other two ports.
Insertion loss is related to
by;
:
Coupling factor is related to
by;
:
Non-zero main diagonal entries are related to
return loss
In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load conne ...
, and non-zero antidiagonal entries are related to isolation by similar expressions.
Some authors define the port numbers with ports 3 and 4 interchanged. This results in a scattering matrix that is no longer all-zeroes on the antidiagonal.
Amplitude balance
This terminology defines the power difference in dB between the two output ports of a hybrid. In an ideal hybrid circuit, the difference should be . However, in a practical device the amplitude balance is frequency dependent and departs from the ideal difference.
Phase balance
The phase difference between the two output ports of a hybrid coupler should be 0°, 90°, or 180° depending on the type used. However, like amplitude balance, the phase difference is sensitive to the input frequency and typically will vary a few degrees.
Transmission line types
Directional couplers
Coupled transmission lines
![coupled line directional coupler](https://upload.wikimedia.org/wikipedia/commons/8/84/Coupled_line_directional_coupler.svg)
The most common form of directional coupler is a pair of coupled transmission lines. They can be realised in a number of technologies including coaxial and the planar technologies (
stripline
Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line.
Description
A strip ...
and
microstrip
Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as the substrate. Microstrip lines are used to convey microwave-frequ ...
). An implementation in stripline is shown in figure 4 of a quarter-wavelength (λ/4) directional coupler. The power on the coupled line flows in the opposite direction to the power on the main line, hence the port arrangement is not the same as shown in figure 1, but the numbering remains the same. For this reason it is sometimes called a ''backward coupler''.
The ''main line'' is the section between ports 1 and 2 and the ''coupled line'' is the section between ports 3 and 4. Since the directional coupler is a linear device, the notations on figure 1 are arbitrary. Any port can be the input, (an example is seen in figure 20) which will result in the directly connected port being the transmitted port, the adjacent port being the coupled port, and the diagonal port being the isolated port. On some directional couplers, the main line is designed for high power operation (large connectors), while the coupled port may use a small connector, such as an
SMA connector
SMA (''SubMiniature version A'') connectors are semi-precision coaxial RF connectors developed in the 1960s as a minimal connector interface for coaxial cable with a screw-type coupling mechanism. The connector has a 50 Ω impedance. SMA ...
. The
internal load power rating may also limit operation on the coupled line.
![Short line directional coupler equivalent circuit](https://upload.wikimedia.org/wikipedia/commons/5/58/Short_line_directional_coupler_equivalent_circuit.svg)
Accuracy of coupling factor depends on the dimensional tolerances for the spacing of the two coupled lines. For planar printed technologies this comes down to the resolution of the printing process which determines the minimum track width that can be produced and also puts a limit on how close the lines can be placed to each other. This becomes a problem when very tight coupling is required and couplers often use a different design. However, tightly coupled lines can be produced in
air stripline
Air stripline is a form of electrical planar transmission line whereby a conductor in the form of a thin metal strip is suspended between two ground planes. The idea is to make the dielectric essentially air. Mechanical support of the line may b ...
which also permits manufacture by printed planar technology. In this design the two lines are printed on ''opposite'' sides of the dielectric rather than side by side. The coupling of the two lines across their width is much greater than the coupling when they are edge-on to each other.
The λ/4 coupled-line design is good for coaxial and stripline implementations but does not work so well in the now popular microstrip format, although designs do exist. The reason for this is that microstrip is not a homogeneous medium – there are two different mediums above and below the transmission strip. This leads to
transmission modes other than the usual TEM mode found in conductive circuits. The propagation velocities of even and odd modes are different leading to signal dispersion. A better solution for microstrip is a coupled line much shorter than λ/4, shown in figure 5, but this has the disadvantage of a coupling factor which rises noticeably with frequency. A variation of this design sometimes encountered has the coupled line a higher
impedance than the main line such as shown in figure 6. This design is advantageous where the coupler is being fed to a detector for power monitoring. The higher impedance line results in a higher RF voltage for a given main line power making the work of the detector diode easier.
The frequency range specified by manufacturers is that of the coupled line. The main line response is much wider: for instance a coupler specified as might have a main line which could operate at . The coupled response is periodic with frequency. For example, a λ/4 coupled-line coupler will have responses at ''n''λ/4 where ''n'' is an odd integer.
A single λ/4 coupled section is good for bandwidths of less than an octave. To achieve greater bandwidths multiple λ/4 coupling sections are used. The design of such couplers proceeds in much the same way as the design of
distributed-element filter
A distributed-element filter is an electronic filter in which capacitance, inductance, and resistance (the elements of the circuit) are not localised in discrete capacitors, inductors, and resistors as they are in conventional filters. Its pu ...
s. The sections of the coupler are treated as being sections of a filter, and by adjusting the coupling factor of each section the coupled port can be made to have any of the classic filter responses such as maximally flat (
Butterworth filter
The Butterworth filter is a type of signal processing filter designed to have a frequency response that is as flat as possible in the passband. It is also referred to as a maximally flat magnitude filter. It was first described in 1930 by the Br ...
), equal-ripple (
Cauer filter
An elliptic filter (also known as a Cauer filter, named after Wilhelm Cauer, or as a Zolotarev filter, after Yegor Zolotarev) is a signal processing filter with equalized ripple (equiripple) behavior in both the passband and the stopband. The a ...
), or a specified-ripple (
Chebychev filter) response.
Ripple
Ripple may refer to:
Science and technology
* Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid
** Ripple, more generally a disturbance, for example of spacetime in gravitational waves
* Ripple (electri ...
is the maximum variation in output of the coupled port in its
passband
A passband is the range of frequencies or wavelengths that can pass through a filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenn ...
, usually quoted as plus or minus a value in dB from the nominal coupling factor.
![Stripline 5-section coupler](https://upload.wikimedia.org/wikipedia/commons/0/0e/Stripline_5-section_coupler.svg)
It can be shown that coupled-line directional couplers have
purely real and
purely imaginary at all frequencies. This leads to a simplification of the S-matrix and the result that the coupled port is always in
quadrature phase (90°) with the output port. Some applications make use of this phase difference. Letting
, the ideal case of lossless operation simplifies to,
:
Branch-line coupler
![Branch line coupler](https://upload.wikimedia.org/wikipedia/commons/3/3a/Branch_line_coupler.svg)
The branch-line coupler consists of two parallel transmission lines physically coupled together with two or more branch lines between them. The branch lines are spaced λ/4 apart and represent sections of a multi-section filter design in the same way as the multiple sections of a coupled-line coupler except that here the coupling of each section is controlled with the impedance of the branch lines. The main and coupled line are
of the system impedance. The more sections there are in the coupler, the higher is the ratio of impedances of the branch lines. High impedance lines have narrow tracks and this usually limits the design to three sections in planar formats due to manufacturing limitations. A similar limitation applies for coupling factors looser than ; low coupling also requires narrow tracks. Coupled lines are a better choice when loose coupling is required, but branch-line couplers are good for tight coupling and can be used for hybrids. Branch-line couplers usually do not have such a wide bandwidth as coupled lines. This style of coupler is good for implementing in high-power, air dielectric, solid bar formats as the rigid structure is easy to mechanically support.
Branch line couplers can be used as crossovers as an alternative to
air bridges, which in some applications cause an unacceptable amount of coupling between the lines being crossed. An ideal branch-line crossover theoretically has no coupling between the two paths through it. The design is a 3-branch coupler equivalent to two 90° hybrid couplers connected in
cascade
Cascade, Cascades or Cascading may refer to:
Science and technology Science
*Cascade waterfalls, or series of waterfalls
* Cascade, the CRISPR-associated complex for antiviral defense (a protein complex)
* Cascade (grape), a type of fruit
* Bioc ...
. The result is effectively a coupler. It will cross over the inputs to the diagonally opposite outputs with a phase delay of 90° in both lines.
Lange coupler
The construction of the Lange coupler is similar to the
interdigital filter with paralleled lines interleaved to achieve the coupling. It is used for strong couplings in the range to .
Power dividers
![Microstrip T-junction](https://upload.wikimedia.org/wikipedia/commons/7/74/Microstrip_T-junction.svg)
The earliest transmission line power dividers were simple T-junctions. These suffer from very poor isolation between the output ports – a large part of the power reflected back from port 2 finds its way into port 3. It can be shown that it is not theoretically possible to simultaneously match all three ports of a passive, lossless three-port and poor isolation is unavoidable. It is, however, possible with four-ports and this is the fundamental reason why four-port devices are used to implement three-port power dividers: four-port devices can be designed so that power arriving at port 2 is split between port 1 and port 4 (which is terminated with a matching load) and none (in the ideal case) goes to port 3.
The term ''hybrid coupler'' originally applied to coupled-line directional couplers, that is, directional couplers in which the two outputs are each half the input power. This synonymously meant a quadrature coupler with outputs 90° out of phase. Now any matched 4-port with isolated arms and equal power division is called a hybrid or hybrid coupler. Other types can have different phase relationships. If 90°, it is a 90° hybrid, if 180°, a 180° hybrid and so on. In this article ''hybrid coupler'' without qualification means a coupled-line hybrid.
Wilkinson power divider
![Wilkinson divider](https://upload.wikimedia.org/wikipedia/commons/4/48/Wilkinson_divider.png)
The
Wilkinson power divider consists of two parallel ''uncoupled'' λ/4 transmission lines. The input is fed to both lines in parallel and the outputs are terminated with twice the system impedance bridged between them. The design can be realised in planar format but it has a more natural implementation in coax – in planar, the two lines have to be kept apart so that they do not couple but have to be brought together at their outputs so they can be terminated whereas in coax the lines can be run side-by-side relying on the coax outer conductors for screening. The Wilkinson power divider solves the matching problem of the simple T-junction: it has low VSWR at all ports and high isolation between output ports. The input and output impedances at each port are designed to be equal to the characteristic impedance of the microwave system. This is achieved by making the line impedance
of the system impedance – for a system the Wilkinson lines are approximately
Hybrid coupler
Coupled-line directional couplers are described above. When the coupling is designed to be it is called a hybrid coupler. The S-matrix for an ideal, symmetric hybrid coupler reduces to;
:
The two output ports have a 90° phase difference (-''i'' to −1) and so this is a 90° hybrid.
Hybrid ring coupler
![Hybrid ring coupler](https://upload.wikimedia.org/wikipedia/commons/6/63/Hybrid_ring_coupler.svg)
The
hybrid ring coupler, also called the rat-race coupler, is a four-port directional coupler consisting of a 3λ/2 ring of transmission line with four lines at the intervals shown in figure 12. Power input at port 1 splits and travels both ways round the ring. At ports 2 and 3 the signal arrives in phase and adds whereas at port 4 it is out of phase and cancels. Ports 2 and 3 are in phase with each other, hence this is an example of a 0° hybrid. Figure 12 shows a planar implementation but this design can also be implemented in coax or waveguide. It is possible to produce a coupler with a coupling factor different from by making each λ/4 section of the ring alternately low and high impedance but for a coupler the entire ring is made
of the port impedances – for a design the ring would be approximately .
The S-matrix for this hybrid is given by;
:
The hybrid ring is not symmetric on its ports; choosing a different port as the input does not necessarily produce the same results. With port 1 or port 3 as the input the hybrid ring is a 0° hybrid as stated. However using port 2 or port 4 as the input results in a 180° hybrid. This fact leads to another useful application of the hybrid ring: it can be used to produce sum (Σ) and difference (Δ) signals from two input signals as shown in figure 12. With inputs to ports 2 and 3, the Σ signal appears at port 1 and the Δ signal appears at port 4.
Multiple output dividers
![Divider](https://upload.wikimedia.org/wikipedia/commons/0/03/Divider.svg)
A typical power divider is shown in figure 13. Ideally, input power would be divided equally between the output ports. Dividers are made up of multiple couplers and, like couplers, may be reversed and used as
multiplexer
In electronics, a multiplexer (or mux; spelled sometimes as multiplexor), also known as a data selector, is a device that selects between several analog or digital input signals and forwards the selected input to a single output line. The sel ...
s. The drawback is that for a four channel multiplexer, the output consists of only 1/4 the power from each, and is relatively inefficient. The reason for this is that at each combiner half the input power goes to port 4 and is dissipated in the termination load. If the two inputs were
coherent
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deri ...
the phases could be so arranged that cancellation occurred at port 4 and then all the power would go to port 1. However, multiplexer inputs are usually from entirely independent sources and therefore not coherent. Lossless multiplexing can only be done with filter networks.
Waveguide types
Waveguide directional couplers
Waveguide branch-line coupler
The
branch-line coupler
Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmiss ...
described above can also be implemented in waveguide.
Bethe-hole directional coupler
![Multi-hole waveguide coupler](https://upload.wikimedia.org/wikipedia/commons/5/5f/Multi-hole_waveguide_coupler.png)
One of the most common, and simplest, waveguide directional couplers is the Bethe-hole directional coupler. This consists of two parallel waveguides, one stacked on top of the other, with a hole between them. Some of the power from one guide is launched through the hole into the other. The Bethe-hole coupler is another example of a backward coupler.
The concept of the Bethe-hole coupler can be extended by providing multiple holes. The holes are spaced λ/4 apart. The design of such couplers has parallels with the multiple section coupled transmission lines. Using multiple holes allows the bandwidth to be extended by designing the sections as a Butterworth, Chebyshev, or some other filter class. The hole size is chosen to give the desired coupling for each section of the filter. Design criteria are to achieve a substantially flat coupling together with high directivity over the desired band.
Riblet short-slot coupler
The Riblet short-slot coupler is two waveguides side-by-side with the side-wall in common instead of the long side as in the Bethe-hole coupler. A slot is cut in the sidewall to allow coupling. This design is frequently used to produce a coupler.
Schwinger reversed-phase coupler
The Schwinger reversed-phase coupler is another design using parallel waveguides, this time the long side of one is common with the short side-wall of the other. Two off-centre slots are cut between the waveguides spaced λ/4 apart. The Schwinger is a backward coupler. This design has the advantage of a substantially flat directivity response and the disadvantage of a strongly frequency-dependent coupling compared to the Bethe-hole coupler, which has little variation in coupling factor.
Moreno crossed-guide coupler
The Moreno crossed-guide coupler has two waveguides stacked one on top of the other like the Bethe-hole coupler but at right angles to each other instead of parallel. Two off-centre holes, usually cross-shaped are cut on the diagonal between the waveguides a distance
apart. The Moreno coupler is good for tight coupling applications. It is a compromise between the properties of the Bethe-hole and Schwinger couplers with both coupling and directivity varying with frequency.
Waveguide power dividers
Waveguide hybrid ring
The
hybrid ring discussed above can also be implemented in waveguide.
Magic tee
![Magic tee](https://upload.wikimedia.org/wikipedia/commons/3/34/Magic_tee.png)
Coherent power division was first accomplished by means of simple Tee junctions. At microwave frequencies, waveguide tees have two possible forms – the
E-plane and H-plane
The E-plane and H-plane are reference planes for linearly polarized waveguides, antennas and other microwave devices.
In waveguide systems, as in the electric circuits, it is often desirable to be able to split the circuit power into two or mor ...
. These two junctions split power equally, but because of the different field configurations at the junction, the electric fields at the output arms are in phase for the H-plane tee and are 180° out of phase for the E-plane tee. The combination of these two tees to form a hybrid tee is known as the
magic tee
A magic T consisting of four rectangular waveguides meeting in a single three-dimensional junction
A magic tee (or magic T or hybrid tee) is a hybrid or 3 dB coupler used in microwave systems. It is an alternative to the rat-race coupler. In c ...
. The magic tee is a four-port component which can perform the vector sum (Σ) and difference (Δ) of two coherent microwave signals.
Discrete element types
Hybrid transformer
![3 dB hybrid transfomer](https://upload.wikimedia.org/wikipedia/commons/8/83/3_dB_hybrid_transfomer.svg)
The standard 3 dB hybrid transformer is shown in figure 16. Power at port 1 is split equally between ports 2 and 3 but in antiphase to each other. The hybrid transformer is therefore a 180° hybrid. The centre-tap is usually terminated internally but it is possible to bring it out as port 4; in which case the hybrid can be used as a sum and difference hybrid. However, port 4 presents as a different impedance to the other ports and will require an additional transformer for impedance conversion if it is required to use this port at the same system impedance.
Hybrid transformers are commonly used in telecommunications for 2 to 4 wire conversion. Telephone handsets include such a converter to convert the 2-wire line to the 4 wires from the earpiece and mouthpiece.
Cross-connected transformers
![Transformer directional coupler](https://upload.wikimedia.org/wikipedia/commons/e/e6/Transformer_directional_coupler.svg)
For lower frequencies (less than ) a compact
broadband
In telecommunications, broadband is wide bandwidth data transmission which transports multiple signals at a wide range of frequencies and Internet traffic types, that enables messages to be sent simultaneously, used in fast internet connections. ...
implementation by means of
RF transformers is possible. In figure 17 a circuit is shown which is meant for weak coupling and can be understood along these lines: A signal is coming in one line pair. One transformer reduces the voltage of the signal the other reduces the current. Therefore, the impedance is matched. The same argument holds for every other direction of a signal through the coupler. The relative sign of the induced voltage and current determines the direction of the outgoing signal.
The coupling is given by;
:
:where ''n'' is the secondary to primary turns ratio.
For a coupling, that is equal splitting of the signal between the transmitted port and the coupled port,
and the isolated port is terminated in twice the characteristic impedance – for a system. A power divider based on this circuit has the two outputs in 180° phase to each other, compared to λ/4 coupled lines which have a 90° phase relationship.
Resistive tee
![Tee power divider](https://upload.wikimedia.org/wikipedia/commons/b/b3/Tee_power_divider.svg)
A simple tee circuit of resistors can be used as a power divider as shown in figure 18. This circuit can also be implemented as a delta circuit by applying the
Y-Δ transform
The Y-Δ transform, also written wye-delta and also known by many other names, is a mathematical technique to simplify the analysis of an electrical network. The name derives from the shapes of the circuit diagrams, which look respectively like th ...
. The delta form uses resistors that are equal to the system impedance. This can be advantageous because precision resistors of the value of the system impedance are always available for most system
nominal impedance
Nominal impedance in electrical engineering and audio engineering refers to the approximate designed impedance of an electrical circuit or device. The term is applied in a number of different fields, most often being encountered in respect of:
...
s. The tee circuit has the benefits of simplicity, low cost, and intrinsically wide bandwidth. It has two major drawbacks; first, the circuit will dissipate power since it is resistive: an equal split will result in insertion loss instead of . The second problem is that there is directivity leading to very poor isolation between the output ports.
The insertion loss is not such a problem for an unequal split of power: for instance at port 3 has an insertion loss less than at port 2. Isolation can be improved at the expense of insertion loss at both output ports by replacing the output resistors with
T pad
The T pad is a specific type of attenuator circuit in electronics whereby the topology of the circuit is formed in the shape of the letter "T".
Attenuators are used in electronics to reduce the level of a signal. They are also referred to as p ...
s. The isolation improvement is greater than the insertion loss added.
6 dB resistive bridge hybrid
![6 dB resistive bridge hybrid](https://upload.wikimedia.org/wikipedia/commons/5/59/6_dB_resistive_bridge_hybrid.svg)
A true hybrid divider/coupler with, theoretically, infinite isolation and directivity can be made from a resistive bridge circuit. Like the tee circuit, the bridge has insertion loss. It has the disadvantage that it cannot be used with unbalanced circuits without the addition of transformers; however, it is ideal for balanced telecommunication lines if the insertion loss is not an issue. The resistors in the bridge which represent ports are not usually part of the device (with the exception of port 4 which may well be left permanently terminated internally) these being provided by the line terminations. The device thus consists essentially of two resistors (plus the port 4 termination).
Applications
Monitoring
The coupled output from the directional coupler can be used to monitor frequency and power level on the signal without interrupting the main power flow in the system (except for a power reduction – see figure 3).
Making use of isolation
![Two tone Rx test](https://upload.wikimedia.org/wikipedia/commons/a/af/Two_tone_Rx_test.svg)
If isolation is high, directional couplers are good for combining signals to feed a single line to a receiver for
two-tone receiver tests. In figure 20, one signal enters port P
3 and one enters port P
2, while both exit port P
1. The signal from port P
3 to port P
1 will experience of loss, and the signal from port P
2 to port P
1 will have loss. The internal load on the isolated port will dissipate the signal losses from port P
3 and port P
2. If the
isolators in figure 20 are neglected, the isolation measurement (port P
2 to port P
3) determines the amount of power from the
signal generator
A signal generator is one of a class of electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typically used i ...
F
2 that will be injected into the signal generator F
1. As the injection level increases, it may cause
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
of signal generator F
1, or even injection phase locking. Because of the symmetry of the directional coupler, the reverse injection will happen with the same possible modulation problems of signal generator F
2 by F
1. Therefore, the isolators are used in figure 20 to effectively increase the isolation (or directivity) of the directional coupler. Consequently, the injection loss will be the isolation of the directional coupler plus the reverse isolation of the isolator.
Hybrids
Applications of the hybrid include monopulse comparators,
mixers, power combiners, dividers,
modulators, and
phased array
In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
radar antenna systems. Both in-phase devices (such as the Wilkinson divider) and quadrature (90°) hybrid couplers may be used for coherent power divider applications. An example of quadrature hybrids being used in a coherent power combiner application is given in the next section.
An inexpensive version of the power divider is used in the home to divide
cable TV
Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with broadc ...
or
over-the-air TV signals to multiple
TV set
A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
s and other devices. Multiport splitters with more than two output ports usually consist internally of a number of cascaded couplers. Domestic broadband internet service can be provided by cable TV companies (
cable internet
In telecommunications, cable Internet access, shortened to cable Internet, is a form of broadband Internet access which uses the same infrastructure as a cable television. Like digital subscriber line and fiber to the premises services, cable In ...
). The domestic user's internet
cable modem
A cable modem is a type of network bridge that provides bi-directional data communication via radio frequency channels on a hybrid fibre-coaxial (HFC), radio frequency over glass (RFoG) and coaxial cable infrastructure. Cable modems are primaril ...
is connected to one port of the splitter.
Power combiners
Since hybrid circuits are bi-directional, they can be used to coherently combine power as well as splitting it. In figure 21, an example is shown of a signal split up to feed multiple low power amplifiers, then recombined to feed a single antenna with high power.
![Power combiner phasing](https://upload.wikimedia.org/wikipedia/commons/4/4d/Power_combiner_phasing.svg)
The phases of the inputs to each power combiner are arranged such that the two inputs are 90° out of phase with each other. Since the coupled port of a hybrid combiner is 90° out of phase with the transmitted port, this causes the powers to add at the output of the combiner and to cancel at the isolated port: a representative example from figure 21 is shown in figure 22. Note that there is an additional fixed 90° phase shift to both ports at each combiner/divider which is not shown in the diagrams for simplicity. Applying in-phase power to both input ports would not get the desired result: the
quadrature sum of the two inputs would appear at both output ports – that is half the total power out of each. This approach allows the use of numerous less expensive and lower-power amplifiers in the circuitry instead of a single high-power
TWT. Yet another approach is to have each solid state amplifier (SSA) feed an antenna and let the power be combined in space or be used to feed a lens attached to an antenna.
Phase difference
![combine2](https://upload.wikimedia.org/wikipedia/commons/6/62/Combine2.svg)
The phase properties of a 90° hybrid coupler can be used to great advantage in
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
circuits. For example, in a balanced microwave amplifier the two input stages are fed through a hybrid coupler. The
FET
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contr ...
device normally has a very poor match and reflects much of the incident energy. However, since the devices are essentially identical the reflection coefficients from each device are equal. The reflected voltage from the FETs are in phase at the isolated port and are 180° different at the input port. Therefore, all of the reflected power from the FETs goes to the load at the isolated port and no power goes to the input port. This results in a good input match (low VSWR).
If phase-matched lines are used for an antenna input to a 180° hybrid coupler as shown in figure 23, a
null
Null may refer to:
Science, technology, and mathematics Computing
* Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value
* Null character, the zero-valued ASCII character, also designated by , often use ...
will occur directly between the antennas. To receive a signal in that position, one would have to either change the hybrid type or line length. To reject a signal from a given direction, or create the difference pattern for a
monopulse radar
Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.
Monopulse radar avoids prob ...
, this is a good approach.
Phase-difference couplers can be used to create
beam tilt
Beam tilt is used in radio to aim the main lobe of the vertical plane radiation pattern of an antenna below (or above) the horizontal plane.
The simplest way is mechanical beam tilt, where the antenna is physically mounted in such a manner a ...
in a
VHF FM
radio station
Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
, by delaying the phase to the lower elements of an
antenna array
An antenna array (or array antenna) is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas (called ''elements'') are usually connected to a single receiver ...
. More generally, phase-difference couplers, together with fixed phase delays and antenna arrays, are used in beam-forming networks such as the
Butler matrix A Butler matrix is a beamforming network used to feed a phased array of driven element, antenna elements. Its purpose is to control the direction of a beam, or beams, of radio transmission. It consists of an n \times n matrix (n some power of two) w ...
, to create a radio beam in any prescribed direction.
[Fujimoto, pp.199–201]
Lo and Lee, p.27.7
See also
*
Star coupler
*
Beam splitter
A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wide ...
References
Bibliography
*Stephen J. Bigelow, Joseph J. Carr, Steve Winder, ''Understanding telephone electronics'' Newnes, 2001 .
*Geoff H. Bryant, ''Principles of Microwave Measurements'', Institution of Electrical Engineers, 1993 .
*Robert J. Chapuis, Amos E. Joel, ''100 Years of Telephone Switching (1878–1978): Electronics, computers, and telephone switching (1960–1985)'', IOS Press, 2003 .
*Walter Y. Chen, Home Networking Basis'', Prentice Hall Professional, 2003 .
*R. Comitangelo, D. Minervini, B. Piovano
"Beam forming networks of optimum size and compactness for multibeam antennas at 900 MHz" ''IEEE Antennas and Propagation Society International Symposium 1997'', vol. 4, pp. 2127-2130, 1997.
*Stephen A. Dyer, ''Survey of instrumentation and measurement'' Wiley-IEEE, 2001 .
*Kyōhei Fujimoto, ''Mobile Antenna Systems Handbook'', Artech House, 2008 .
*Preston Gralla, ''How the Internet Works'', Que Publishing, 1998 .
*Ian Hickman, ''Practical Radio-frequency Handbook'', Newnes, 2006 .
*Apinya Innok, Peerapong Uthansakul, Monthippa Uthansakul
"Angular beamforming technique for MIMO beamforming system" ''International Journal of Antennas and Propagation'', vol. 2012, iss. 11, December 2012.
*Thomas Koryu Ishii, ''Handbook of Microwave Technology: Components and devices'', Academic Press, 1995 .
*Y. T. Lo, S. W. Lee, ''Antenna Handbook: Applications'', Springer, 1993 .
*Matthaei, George L.; Young, Leo and Jones, E. M. T. ''Microwave Filters, Impedance-Matching Networks, and Coupling Structures'' McGraw-Hill 1964
*D. Morgan, ''A Handbook for EMC Testing and Measurement'', IET, 1994 .
*Antti V. Räisänen, Arto Lehto, ''Radio engineering for wireless communication and sensor applications'', Artech House, 2003 .
*K.R. Reddy, S. B. Badami, V. Balasubramanian, ''Oscillations And Waves'', Universities Press, 1994 .
*Peter Vizmuller, ''RF design guide: systems, circuits, and equations, Volume 1'', Artech House, 1995 .
{{DEFAULTSORT:Power Dividers And Directional Couplers
Radio electronics
Microwave technology
Distributed element circuits