Directional Couplers, Microwave

Crane Aerospace & Electronics Microwave Solutions
100 khz - 65 ghz
An RF signal applied to the input port splits
unequally between the coupled port and the main line
output port. The degree of unequal power division is a
function of the coupling ratio of the coupler.
A directional coupler inserted in a transmission line
allows precise monitoring of the RF energy flow in that
line while introducing minimum perturbation of the main
line signal in the sampling process. Merrimac directional
couplers are precision devices carefully designed for
monitoring incident and reflected power.
Merrimac directional couplers are available in four
different classes:
The isolated port, commonly known as a ‘‘load port’’
is designed to absorb and dissipate reflected energy in
a ‘‘failsafe’’ mode. This termination will survive when
open or short circuit conditions occur at the main line
output port, while operating to the maximum average
input power rating. The power dissipation capability of
the internal termination is the major factor determining
maximum reverse power ratings.
A complete line of catalog three port directional
couplers from 100 kHz to 40 GHz is available from
Merrimac. Models below 1 GHz are primarily lumped
element designs. They consist of very small packages
such as PC mount (CRS and CBP series) and flatpack
(CBF series) and various surface mount packages (CBG-A
and CRG-B series). (See Figure 2.) Connectorized units
are also available in the CR/CRM series.
1. Three port directional couplers
2. Four port bi-directional couplers
3. Four port, dual directional couplers
4. Three port directional detectors
Three Port Directional Couplers
Three port directional couplers, as illustrated in
Figure 1, are four port networks where one port is internally terminated in a resistive load thus becoming the
isolated port. The other three ports are:
1. Main line input port
2. Main line output port
3. Coupled output port
Figure 2: Surface Mount Three Port Coupler
Above 500 MHz, stripline coupler designs are generally more appropriate either as connectorized units or as
Mini-Filmbrid caseless couplers (CSD and CTD series)
suitable for direct integration into microstrip mother
For transmitter and power amplifier systems
requiring high power monitoring and leveling, directional
couplers using an air dielectric are more suitable. For
applications up to 600 watts CW, or 10 kW peak, direc-
Figure 1: Three Port Directional Coupler
Crane Aerospace & Electronics
Microwave Solutions – Merrimac Industries
41 Fairfield Place, West Caldwell, NJ 07006
+ 1.973.575.1300 ext. 1309 • [email protected]
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Directional Couplers - Jan 23 2012
Crane Aerospace & Electronics Microwave Solutions
tional couplers are available in the CEN, CEN and CGN
Four Port Bi-Directional Couplers
The four port network (see Figure 3) has the isolated
port externalized with either an RF connector or pin. One
advantage of this type coupler is that a higher power
termination can be selected to suit higher input power
requirements. The four port network can also act as a
bi-directional coupler monitoring signals in both directions
provided the coupled ports feed into a reasonably constant
50Ω impedance.
Figure 4: Four Port Dual Directional Coupler
Occasionally, a requirement arises that requires a dual
directional coupler monitoring power in the same signal
direction. These can be readily supplied as special orders.
Coupling Values
Merrimac catalog couplers are available in 6, 10,
12, 15, 20 or 30 dB coupling values depending on the
product series. High power units are cataloged only in 30
or 35 dB values. Custom coupling values are available on
special order.
Figure 3: Four Port Bi-Directional Coupler
For low power applications four port couplers are available in various miniature packages, including TO-5 (C114/
C115), Meri-Pac (CRP series) and Flatpack (CRF series).
Dual Directional Couplers
Dual directional couplers (see Figure 4) are four port
networks that are distinguished from the bi-directional
types in that dual directional couplers are two independent four port networks connected in series. Their principal application is in monitoring signals simultaneously
in both directions. They are preferred over the simple
bi-directional coupler in that they provide higher isolation
between coupled ports.
Transmission Loss
To determine total loss through the main line of a
directional coupler, the coupling loss (power directed to
the coupled arm) must be combined with the insertion
loss. Coupling loss is a function of the coupling value
utilized. Coupling losses of standard couplers are shown in
Table 1.
Coupling Value
3 dB
Coupling “Loss”
3.010 dB
6 dB
1.250 dB
10 dB
0.460 dB
12 dB
0.283 dB
15 dB
0.140 dB
20 dB
0.044 dB
30 dB
0.004 dB
Table 1: Coupling Value and Coupling Loss
Crane Aerospace & Electronics
Microwave Solutions – Merrimac Industries
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Directional Couplers - Jan 23 2012
Crane Aerospace & Electronics Microwave Solutions
Phase and Amplitude Tracking
Frequency Sensitivity
Many microwave systems such as multi-channel
receivers, transmitter and antenna systems require
uniform phase and amplitude characteristics .Merrimac
catalog directional couplers yield good unit-to-unit phase
and amplitude uniformity. Merrimac can provide units
in matched groups to meet closely matched phase and
amplitude requirements either by careful stock selection
or by special design.
The peak-to-peak deviation in dB from the nominal
coupling over the specified frequency range.
Insertion Loss
The net unrecoverable power in dB dissipated within
the circuit at any frequency within the specified range.
The insertion loss is usually specified as excluding the
coupling loss, but to avoid a complex and perhaps critical calculation, it is expressed as including coupling loss
for dual directional and high power models.
Coupling ‘‘Loss’’
The main line signal loss in dB attributable to power
being sampled at the coupled port.
Transmission Loss
Figure 5: 500 W Fail-Safe Dual Directional Coupler for Cellular
Radio and TACAN
The sum of the maximum insertion loss and coupling
‘‘loss’’ in dB at any frequency in the specified range
Unequal Power Division
Merrimac directional couplers can be used as two
way unequal power dividers by specifying the ratio of
power division required.
Parameter Definitions
Nominal Coupling
The power ratio in dB by which the coupled output
port is decoupled from the input port when all ports are
terminated in reflectionless (matched) terminations. The
nominal coupling is specified as the arithmetic average
of the of the maximum and the minimum coupling within
the frequency band.
Coupling Tolerance
The specified allowable unit-to-unit variation in dB in
nominal coupling.
Crane Aerospace & Electronics
Microwave Solutions – Merrimac Industries
Expressed as a power ratio in dB, directivity is a
measure of the preferential coupling of RF energy from
the mainline to the coupled port in the ‘‘forward’’ direction compared to that in the ‘‘reverse’’ with all ports
terminated in matched (reflectionless) loads.
The voltage standing wave ratio specified for either
the main line path (input to output) and/or the coupled
output port(s).
Average or CW Power Capacity
The CW power handling capacity is the one-way
power transmission capacity through the device under
matched load conditions. For fail-safe power handling
capabilities not specified on the particular data sheet,
contact Merrimac.
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Directional Couplers - Jan 23 2012
Crane Aerospace & Electronics Microwave Solutions
1. Directional couplers are used to accurately sample
the directional power flow in a transmission line. In
conjunction with a calibrated detector or a power
bridge, an accurate, continuous measurement of
power flow can be obtained. In this function directional couplers can be an essential part of system
BITE (built in test equipment).
2. Power leveling can be performed when the coupled
output of a directional coupler is used in conjunction
with a modulator or a PIN attenuator as a part of a
leveling loop.
3. Frequency measurement can be made on a continuous basis when the coupled sample is fed to a suitable frequency counter or equivalent.
Figure 6: 0.5 to 40 GHz Directional Coupler
4. Frequency stabilization can be obtained when the
coupler output is used as the input to an AFC (automatic frequency control) loop.
5. Continuous power reflection measurements such as
might result from antenna misalignments can be
made using couplers as reflectometers. The power
source is fed to the main line output port and the
coupler main line input port is connected to the load
(antenna). Reflected power will thus be coupled to
the forward coupled output where it can be monitored.
6. Signal injection can be obtained by feeding the
injectable signal into the mainline via the coupled
port. The direction of the inserted signal power flow
depends on the coupler polarity in the transmission
AppNote_DirectionalCouplers_WC_MW_012312.doc. This revision supersedes all previous releases. All technical information is believed to be
accurate, but no responsibility is assumed for errors. We reserve the right to make changes in products or specifications without notice. Copyright
© 2012 Crane Electronics. All rights reserved.
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