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POWER DIVIDERS /COMBINERS AND DIRECTIONAL COUPLERS
A directional coupler is a passive device which
couples part of the transmission power by a known
amount out through another port, often by using two transmission lines set close enough together such that energy
passing through one is coupled to the other. As shown in Figure 1, the device has four ports: input, transmitted,
coupled, and isolated. The term "main line" refers to the section between ports 1 and 2.
On some directional couplers, the main line is designed for high power operation (large connectors), while the
coupled port may use a small SMA connector. Often the isolated port is terminated with an internal or external
matched load (typically 50 ohms). It should be pointed out that since the directional coupler is a linear device,
the notations on Figure 1 are arbitrary. Any port can be the input, (as in Figure 3) which will result in the
directly connected port being the transmitted port, adjacent port being the coupled port, and the diagonal port
being the isolated port.
Physical considerations such as internal load on the isolated port will limit port operation. The coupled
output from the directional coupler can be used to obtain the information (i.e., frequency and power level) on the
signal without interrupting the main power flow in the system (except for a power reduction - see Figure 2). When
the power coupled out to port three is half the input power (i.e. 3 dB below the input power level), the power on
the main transmission line is also 3 dB below the input power and equals the coupled power. Such a coupler is
referred to as a 90 degree hybrid, hybrid, or 3 dB coupler. The frequency range for coaxial couplers specified by
manufacturers is that of the coupling arm. The main arm response is much wider (i.e. if the spec is 2-4 GHz, the
main arm could operate at 1 or 5 GHz - see Figure 3). However it should be recognized that the coupled response is
periodic with frequency. For example, a λ/4 coupled line coupler will have responses at nλ/4 where n is an odd
Common properties desired for all directional couplers are wide operational bandwidth, high directivity, and a
good impedance match at all ports when the other ports are terminated in matched loads. These performance
characteristics of hybrid or non-hybrid directional couplers are self-explanatory. Some other general
characteristics will be discussed below.
The coupling factor is defined as:
Coupling factor (dB) =
where P1 is the
input power at port 1 and P3 is the output power from the coupled port (see Figure 1).
coupling factor represents the primary property of a directional coupler. 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. For
example, a 10 dB coupling ± 0.5 dB means that the directional coupler can have 9.5 dB to 10.5 dB coupling at the
frequency band center. 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. Multiple quarter-wavelength coupling sections are used to obtain wide
frequency bandwidth directional couplers. Typically this type of directional coupler is designed to a frequency
bandwidth ratio and a maximum coupling ripple within the frequency band. For example a typical 2:1 frequency
bandwidth coupler design that produces a 10 dB coupling with a ±0.1 dB ripple would, using the previous accuracy
specification, be said to have 9.6 ± 0.1 dB to 10.4 ± 0.1 dB of coupling across the frequency range.
In an ideal directional coupler, the main line loss port 1 to port 2 (P1 - P2) due to power
coupled to the coupled output port is:
The actual directional coupler loss will be a combination of
coupling loss, dielectric loss, conductor loss, and VSWR loss. Depending on the frequency range, coupling loss
becomes less significant above 15 dB coupling where the other losses constitute the majority of the total loss. A
graph of the theoretical insertion loss (dB) vs coupling (dB) for a dissipationless coupler is shown in Figure 2.
Insertion loss (dB) =
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 output ports are terminated by matched loads, or:
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.
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 30 dB while the isolation between ports 2 and 3 can be a different
value such as 25 dB. If both isolation measurements are not available, they can assumed to be equal. If neither
are available, an estimate of the isolation is the coupling plus return loss (see VSWR section). 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 directional couplers will have the best isolation.
isolation is high, directional couplers are excellent for combining signals to feed a single line to a receiver
for two-tone receiver tests. In Figure 3, one signal enters port P3 and one enters port P2,
while both exit port P1. The signal from port P3 to port P1 will experience 10 dB
of loss, and the signal from port P2
to port P1 will have 0.5 dB loss. The internal load on the isolated port will dissipate the signal
losses from port P3 and port P2. If the isolators in Figure 3 are neglected, the isolation
measurement (port P2 to port P3) determines the amount of power from the signal generator F2
that will be injected into the signal generator F1. As the injection level increases, it may cause
modulation of signal generator F1, 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
by F1. Therefore the isolators are used in Figure 3 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.
Directivity is directly related
to Isolation. It is defined as:
where: P3 is the output power from the coupled port and P4 is the power output from the isolated
The directivity should be as high as possible. Waveguide directional couplers will have the best directivity.
Directivity is not directly measurable, and is calculated from the isolation and coupling measurements as:
Directivity (dB) = Isolation (dB) - Coupling (dB)
The hybrid coupler, or 3 dB directional coupler, in which the two outputs are of equal amplitude takes many forms.
Not too long ago the quadrature (90 degree) 3 dB coupler with outputs 90 degrees out of phase was what came to
mind when a hybrid coupler was mentioned. Now any matched 4-port with isolated arms and equal power division is
called a hybrid or hybrid coupler. Today the characterizing feature is the phase difference of the outputs. If 90
degrees, it is a 90 degree hybrid. If 180 degrees, it is a 180 degree hybrid. Even the Wilkinson power divider
which has 0 degrees phase difference is actually a hybrid although the fourth arm is normally imbedded.
Applications of the hybrid include monopulse comparators, mixers, power combiners, dividers, modulators, and
phased array radar antenna systems.
This terminology defines the power difference
in dB between the two output ports of a 3 dB hybrid. In an ideal hybrid circuit, the difference should be 0 dB.
However, in a practical device the amplitude balance is frequency dependent and departs from the ideal 0 dB
The phase difference between the two output ports of a hybrid coupler should be 0, 90, or 180 degrees
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.
The phase properties of a 90 degree hybrid coupler can be
used to great advantage in microwave circuits. For example in a balanced microwave amplifier the two input stages
are fed through a hybrid coupler. The FET 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 180E 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).
phase matched lines are used for an antenna input to a 180° hybrid coupler as shown in Figure 4, a null will occur
directly between the antennas. If you want to receive a signal in that position, you would have to either change
the hybrid type or line length. If you want to reject a signal from a given direction, or create the difference
pattern for a monopulse radar, this is a good approach.
OTHER POWER DIVIDERS
Both in-phase (Wilkinson) and quadrature (90°) hybrid couplers may be used for coherent power divider
applications. The Wilkinson's power divider has low VSWR at all ports and high isolation between output ports. The
input and output impedances at each port is designed to be equal to the characteristic impedance of the microwave
system. A typical power divider is shown in Figure 5. 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
multiplexers. The drawback is that for a four channel multiplexer, the output consists of only 1/4 the power from
each, and is relatively inefficient. Lossless multiplexing can only be done with filter networks.
power division was first accomplished by means of simple Tee junctions. At microwave frequencies, waveguide tees
have two possible forms - the H-Plane or the E-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 anti-phase for the E-Plane tee. The combination of these two tees to form a hybrid tee allowed
the realization of a four-port component which could perform the vector sum (Σ) and difference (Δ) of two coherent
microwave signals. This device is known as the magic tee.
Since hybrid circuits are bi-directional, they can be used to split up a signal to feed multiple low power
amplifiers, then recombine to feed a single antenna with high power as shown in Figure 6. 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 which is attached to an antenna. (See Section 3-4)
6. Combiner Network
Sample Problem: If two 1 watt peak unmodulated RF carrier signals at 10 GHz are received, how much peak
power could one measure?
B. 0.5 watts
C. 1 watt
D. 2 watts
E. All of these
answer is all of these as shown in Figure 7.
Table of Contents
for Electronics Warfare and Radar Engineering Handbook
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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