This free directional couplers are passive devices used in the field of radio technology. 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 directional couplers provide non-equal power splitting of the incoming signal. 1) Port 1 is the input port where power is applied 2) Port 2 is the transmitted port where the power from port 1 is outputted 3) Port 3 is the coupled port where a portion of the power applied to port 1 appears 4) port 4, the isolated port, A portion of the power applied to port 2 will be coupled to port 4
A key difference between couplers and power dividers is that couplers create a phase shift between the output signals. where as power dividers provide equal amplitude and equal phase splitting The main difference between directional couplers and quadrature hybrids is that directional couplers provide non-equal power splitting of the incoming signal while quadrature hybrids have equal power splitting . Since directional couplers are most often used in power sensing applications, their phase information is usually not specified In contrast, the 90 degree phasing of quadrature hybrid couplers (Fig. 4) is always specified since the phase accuracy is critically important for many applications like IQ-modulation and demodulation, single-sideband up-conversion, and image reject down-conversion. In both directional couplers and quadrature hybrids, the best performance is obtained when the circuits are well matched to 50. Why matched at 50 Ohm? The arithmetic mean between 30 ohms (best power handling) and 77 ohms (lowest loss) is 53.5, the geometric mean is 48 ohms. Thus the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length, for air dielectric.
Forward couplers: the power is coupled at the coupled port. Backward couplers: the power is coupled to the isolated port Most waveguide couplers couple in the forward direction as they rely on multiple coupling holes; a signal incident on port 1 will couple to port 3. On a waveguide broadwall directional coupler, the coupled port is closest to the output port because it is a forward wave couple. Forward couplers are in-phase couplers. In this case the holes are spaced a quarter wave apart so that the reverse wave cancels out
Microstrip or stripline couplers are backward wave couplers because they rely on coupled lines: for a signal incident on port 1, port 4 is the coupled port (port 3 is isolated in our clockwise notation). The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a backward wave coupler.
Generally, power dividers and couplers are quantified using nearly identical figures of merit, with a few subtle differences. The following list details the various metrics and some of the nuances that are often encountered when designing and specifying performance in datasheets.
In power monitoring and leveling, the most desired specification is a highly accurate and flat coupling value, because the coupling factor directly affects the measurement data. where P1 is the input power at port 1 and P3 is the output power from the coupled port. In reflection measurements, coupling factor is less important than directivity and SWR, since both the forward and reverse coupling elements are usually identical, and so the variation of coupling factors match versus frequency. Q1. what would be coupling values when high power system is considered? ans: then there can be a need of about 40 db coupling values. Q.2 You said in reflection measurement coupling factor is less important to consider but what is the reason behind? Ans: forward and reverse coupling elements are usually identical so variation of coupling factor match with frequency variation.
Directivity is not directly measurable, and is calculated from the difference of the isolation and coupling measurement. Directivity is an important figure of merit because it measures the ability of a directional coupler to distinguish between signals traveling in opposite directions. The higher the directivity, the better the coupler can distinguish between forward traveling and backward traveling waves. Referring to Fig. 3, this implies that in an ideal coupler port 3 will only see signals entering port 1 (the forward going wave), and will never see signals that enter from port 2 (the backward going wave)
Swr: The metrics “voltage standing wave ratio” (VSWR) and return loss answer the same question: how well is the RF network matched to a given load and source impedance? Unless otherwise stated, all Marki Microwave components are designed to operate in 50 systems. For power dividers and couplers, one must work very hard to maintain a good 50 match over all frequencies to achieve the best performance, and to minimize reflections within the system.
main lineloss of a directional coupler includes both insertion loss and coupling loss. Transmission loss is usually not important at low frequencies
Depending on the frequency range, coupling loss becomes less significant above 15 dB coupling where the other losses become more dominant.
For power dividers and couplers, insertion loss refers to the additional loss . The additional losses are caused primarily by reflections, dielectric absorption, radiation effects, and conductor losses . Broadband designs tend to have higher insertion losses because they are physically longer devices, and thus accumulate more dielectric, radiation, and conductor losses .
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 q.1: Using multiple holes allows the bandwidth to be extended by designing the sections as a Butterworth, Chebyshev, or some other filter class
For dividing a signal into two signals of equal amplitude and a constant 90° or 180° phase differential. This terminology “directional coupler”, “90° hybrid”, and “180° hybrid” is based on convention. However, the 90° and 180° hybrids could be thought of as 3 dB directional couplers. Despite these similarities, the parameters used to describe signal flow in directional couplers and the application, in actual use, is sufficiently different to warrant separate considerations
Since -3 dB represents half power, a 3 dB coupler divides the power equally (within a certain tolerance) between the output and coupled output ports. The 90° phase difference between the outputs makes hybrids useful in the design of electronically variable attenuators, microwave mixers, modulators and many other microwave components and systems shows the circuit diagram and truth table that will be used in explaining the operation of the RF frequency 90° hybrid. As can be seen from this diagram, a signal applied to any input will result in two equal amplitude signals that are quadrature, or 90°, out of phase with each other. Ports A and B and Ports C and D are isolated.
A 180° hybrid is a reciprocal four-port device which provides two equal amplitude in-phase signals when fed from its sum port (S) and two equal amplitude 180° out-of-phase signals when fed from its difference port (D). Conversely, signals input into ports C and D will add at the sum port (B) and the difference of the two signals will appear at the difference port (A). Figure 1 is a functional diagram which will be used in this article to represent the 180° hybrid. Port B can be considered the sum port and port A is the difference port. Ports A and B and ports C and D are isolated pairs of ports.
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. 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. The magic tee is a four-port component which can perform the vector sum (Σ) and difference (Δ) of two coherent microwave signals
This paper describes a D-band on-chip rat-race coupler using a novel phase inverter and developed in a standard CMOS process. The simple structural phase inverter which is based on the characteristic impedance and phase difference analysis has demonstrated its potential for wideband and compact millimeter-wave frequency range applications. The developed rat-race coupler using the proposed phase inverter, features a 40% size reduction and over 60% bandwidth improvement compared to conventional structures. high port-to-port isolation in the D-band frequency range. A proper design of the phase inverter can enhance the overall performance of the proposed coupler. Fig. 1 shows the geometry of the proposed phase inverter using standard CMOS process
MD ERFAN (33108908)
Masters in Electrical Communication Engineering
• Directional coupler
• Differences between couplers
• Key specification
• Types of coupler
• Recent development
•Passive device that divides and distributes power.
•Couples part of the transmission power by a known amount out
through another port.
•Uses two transmission lines set close enough together such that
energy passing through one is coupled to the other.
Couplers is a device having 4 port , which couples the power from
input port, to two out ports, while the fourth port is isolated.
• Symmetric Device
• Directional couplers
• Ports 1&2 are Primary ports
• Ports 3&4 are Secondary ports
•Power is defined to a specific direction,
while no power is reflected back at any port.
•Power is divided between transmitted port
and coupled port
• Energy that propagates down transmission line starts a parallel wave down to
transmission line 2 .
• Most common forward coupler is the multi-hole coupler realized in waveguide.
• In this case the holes are spaced a quarter wave apart.
• Asymmetric multi-section coupled structures provide forward-wave, in-phase
• Reverse coupler
• Energy that propagates down transmission line starts a reverse wave down
to transmission line 2.
• Single-section coupled transmission lines are always backward-wave
couplers (and outputs are in quadrature)
• Example: Lange coupler
• Symmetric multi-section couplers provide backward-wave, quadrature
The key specifications for a directional coupler relay on its purpose.
Each of them should be carefully evaluated to ensure that the meet
assemble its intended use.
• Indicates the fraction of input power that is coupled to
the output port.
• Coupling is not constant, but varies with frequency.
• Coupling values of 10 and 20 dB are most common.
• Larger the coupling value smaller the coupled power.
• Directional couplers are specified in terms of the coupling
accuracy at the frequency band centre.
• The accuracy is due to dimensional tolerances that can be
held for the spacing of the two coupled lines.
• Another coupling specification is frequency sensitivity.
• A larger frequency sensitivity will allow a larger frequency
band of operation.
• 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.
• Also 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.
• The isolation among the input and the isolated ports may be
dissimilar from the isolation between the two output ports.
• For example, the isolation between ports 1 and 4 can be 30 dB
while the isolation between ports 2 and 3 can be a different
value such as 25 dB.
• Isolation can be estimated from the coupling plus return loss.
• Larger the bandwidth, higher the frequency, difficult to provide
• Higher the isolation, better the performance (Ideally Infinite)
• Indicates the coupler’s ability to isolate the forward and
• How well the forward travelling wave in the primary
couples only to a specific port in secondary.
where P3 is the output power from the coupled port and P4 is
the power output from the isolated port.
• The directivity should be as high as possible (Ideally
• The directivity is very high at the designed frequency
• More sensitive function of frequency because it depends
on the cancellation of two wave components.
• Waveguide directional couplers will have the best
Standing wave ratio for power dividers and couplers, 50
Ohm match over all frequencies.
• Best performance
• Minimize reflections
Main line loss
• Indicates the power loss as the signal travels
from Input port to Through Port.
• Ratio of input power through Input port (port1)
to power out of the Through Port(port 2).
• Indicates the portion of Mainline loss that is due
to coupling some of the input power into the
• This loss is unavoidable
• Very small value
• dielectric absorption,
• radiation effects,
• and conductor losses
Broadband designs tend to have higher
• accumulate more dielectric,
• Reflection loss
• radiation, and
• conductor losses 1) skin effect 2) surface roughness
One of the most common, and simplest, waveguide directional
couplers is the Bethe-hole directional coupler
• Two parallel staked WG
•Power pass through hole
•λ/4 a parted hole
Two couplers in series, in opposing directions, with the
isolated ports internally terminated.
This component is the basis for the reflectometer.
Using internal, well-matched loads helps remove errors
with poor terminations that might be present in real
A hybrid coupler is a special case, where a 3 dB split is desired
between the through path and the coupled path.
There are two types of hybrid
90 degree couplers
and 180 degree hybrids (such as
rat-races and magic tees).
• 3 dB coupler divides the power
equally (within a certain tolerance)
between the output and coupled
Quadrature hybrid couplers
90° Hybrids or hybrid couplers are basically 3 dB directional couplers in which
the phase of the coupled output signal and the output signal are 90° apart.
1) Variable attenuators
2) Microwave mixers
90° Hybrid Schematic
180° Hybrid Coupler .
Depending on the junction
Ring hybrid or rat-race
Tapered coupled line and
Application: combiner and
Hybrid CouplerHybrid Coupler
Four-port network with 180°
phase shift output
The output are in phase
• Four-port component
• Coherent power by means of
simple Tee junctions
• E-plane and H-plane
• split power equally
• Tends to compensate phase
Lange Coupler .
Coupling is too loose to achieve 3 or 6 dB
90°phase difference between port 2 and 3
Advantages: Small Size and large Bandwidth
Disadvantage: lines are very narrow and close
Application: power combiners and splitters
The Lange Coupler and Magic T
Wave guide-magic T 
• Power Measurements
• Frequency measurements
• Signal levelling
• Reflection coefficient measurements
• Signal sampling
• Signal injection
• Measure incident and reflected power
to determine VSWR
3D Phase Inverter Structure .
Top View Of Phase Inverter .
• Great features a 40% size reduction and
over 60% bandwidth improvement
• high port-to-port isolation in the D-band
• 90° and 180° output phase differences
• Phase inverters are based on CMOS process
• Performance dependent on phase inverter.
• Wide operation bandwidth
•Acceptable return loss,
•Good port-to-port isolation,
•Excellent amplitude and phase
imbalance in the whole D band
Based on novel phase inverter and developed in a standard CMOS process
• Couplers are often called directional couplers.
• Directional coupler is ideally matched at all ports.
• In waveguide technology, bethe hole couplers and multihole couplers are
• Microstrip couplers are hybrid couplers, coupled line directional couplers
and lange couplers.
• Phase inverter can simultaneously increase bandwidth and can reduce
size for the hybrid coupler application.