KRYTAR Directional Coupler Primer

Directional Couplers are passive devices used for sampling incident and reflected microwave power, con-
veniently and accurately, with minimal disturbance to the transmission line. Directional couplers are used
in many different testing applications where power or frequency needs to be monitored, leveled, alarmed
or controlled. 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, separating transmitted and received signals on telephone lines, in addition to electronic warfare
(EW) systems and the emerging mmWave and 5G wireless communications systems. As coaxial systems
reach higher frequencies, it becomes critical to sample and level signals over broad instantaneous band-
widths.

For many RF, microwave and mmWave systems, a constant monitoring of frequency is necessary from the
output of microwave transmitters, systems or special test setups. A directional coupler is connected to the
output of the system or instrument. A small amount of power and frequency is coupled from the main line
and connected to a frequency meter. This provides a constant measurement of frequency. Power output
from microwave transmitters may degrade as a function of time. A directional coupler may be used to couple
a small amount of power from the main line. The power from the coupled port is then connected to a Power
Meter where power can be constantly monitored. The power from the coupled port could be connected to a
detector. The DC output voltage from the detector could be used in a feedback circuit to assure power output
is constant - or - the voltage could be used to trigger shut down of the transmitter if the power is either too
high or too low. The voltage could also be used to sound an alarm or trigger a data recorder as long as the
power was out of its operating window. If the transmitter is located in a remote location, the voltage could
cause an alert to be made to a technician that the transmitter is not performing to specifications.

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. They couple a
defined amount of the electromagnetic power in a transmission line to a port 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 direc-
tional coupler designed to split power equally between two ports is called a hybrid coupler. In designing di-
rectional couplers for practical bandwidths, computer modeling and attention to design details help minimize
causes of signal loss.

Four and Three Port Directional

Couplers Directional couplers typi-
Port 4
cally have four ports (Fig. 1). Port 1 is
(Isolated Port)
the Input Port where power is applied. Port 3
Port 3 is the Coupled Port where a (Coupled Port)
portion of the power applied to port 1
appears. Port 2 is the Output Port
where the power from port 1 is out-
putted, less the portion that went to
port 3. Directional couplers are fre- Port 2
(Output or
quently symmetrical so there also ex- Port 1 Transmited
ists a Port 4, the Isolated Port. A (Input Port) Port)
portion of the power applied to port 2
will be coupled to port 4. However, the
coupler is not typically used in this Figure 1
method and port 4 is terminated with
a matched load (typically 50 ohms). This termination can be internal to the device and port 4 is not acces-
sible. Effectively, this results in a 3-port coupler (Fig. 2).
KRYTAR • 1288 Anvilwood Avenue • Sunnyvale, CA 94089 • Toll FREE: +1.877.734.5999 • sales@krytar.com • www.krytar.com
Three-port directional couplers are designed to generate two signal paths from an applied input signal. Di-
rectional couplers are often designed with a pair of adjacent
transmission lines, such as microstrip or stripline transmis- 4-Port Coupler Schematic
sion lines. In this architecture, one of the transmission lines
is the main or through signal path with most of the power; the
transmission line alongside it is the coupled line with one ter-
minated port, often in a load resistor.

When microwave power is applied to the mainline circuit

(Ports 1 and 2) a certain amount of energy will be “coupled”
to the coupled circuit (port 3). In general, the closer the cir- 3-Port Coupler Schematic
cuits are together, the more power will be applied to the cou-
pled circuit from the main line circuit.

Ideally, the output-power level of the mainline signal would

be reduced very little—only by the amount of signal power
coupled from the input signal. However, normal insertion loss
accounts for some additional loss of mainline power. The
power level of the second output signal—the coupled signal Figure 2
path—is reduced by the coupling factor of the coupler along
with any insertion loss associated with that signal path. Signal interfaces, such as coaxial connectors, will
also contribute to some loss of initial signal power.

Designing Directional Couplers Directional couplers are designed to operate within a well-matched sys-
tem, such as 50 Ω, promoting smooth signal flow from a signal source, through the coupler’s input port,
and through the output port to additional components in a system, such as a signal analyzer in a test system.
Again, in real-life applications, some amount of impedance mismatch is inevitable, and signal power will
be reflected at impedance mismatches. This results in an increase in return loss as signal power is sent
back through the directional coupler, often dissipated as heat.

In designing directional couplers for practical bandwidths, computer modeling and attention to design details
help minimize causes of signal loss. What makes the performance of any directional coupler unique is that
it’s consistent across any specified bandwidth — not just in loss behavior, but in coupling, VSWR, and other
critical parameters that gauge a directional coupler’s usefulness.

Standard Coupling Values Some standard values of coupling for KRYTAR directional couplers are 6 dB,
10 dB, 13 dB, 16 dB, 20 dB, 30 dB. Virtually any coupling value may be obtained through custom designs.

Coupler Specification Definitions

Frequency Range (GHz): The frequency bandwidth in GHz over which a particular model will perform
while meeting all its specification limits. KRYTAR directional coupler designs operate over the frequency
area beginning at 0.5 GHz up to 110 GHz.

dB: A unit of gain equal to ten times the common logarithm of the ratio of two power levels or 20 times the
common logarithm of the ratio between two voltages.

Nominal Coupling: The coupling factor, defined as dB, represents the primary property of a directional
coupler. Coupling is not constant, but varies with frequency. While different designs may reduce the vari-
ance, a perfectly flat coupler theoretically cannot be built. Directional couplers are specified in terms of the
coupling accuracy at the frequency band center.
KRYTAR • 1288 Anvilwood Avenue • Sunnyvale, CA 94089 • Toll FREE: +1.877.734.5999 • sales@krytar.com • www.krytar.com
Frequency Sensitivity: The maximum peak-to-peak variation in coupling (in dB) of a directional or hybrid
coupler over the specified frequency range. Also referred to as “flatness”. KRYTAR directional couplers have
frequency sensitivity tolerances of +/-0.3 dB for narrow band models to +/- 1.2 dB for broadband models.

Insertion Loss: The change in load power due to the insertion of a particular device into a transmission
system. The main line insertion loss from port 1 to port 2 is due to some power going to the coupled port.
The insertion loss consists of 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. KRYTAR Insertion Loss includes coupled
power. Dissipation in the circuit will raise the insertion loss by the power dissipation in dB.

Isolation: A unit of measure (in dB) that states the separation of signal levels on adjacent ports of a device.
The greater the isolation value, less interference from a signal on one port is present at the other. 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. 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.

Directivity: A measurement of the desired signal strength to the undesired signal strength. Determined by
taking the value of isolation and subtracting the specified coupling (including all variations). Directivity is a
measure of how good the couplers performance is. Port 4 (Isolated Port) is not completely isolated in KRY-
TAR directional couplers as there are no perfect terminations. A small amount of power will be present at
the isolated port. If power out of Port 4 (isolated port) is 20 dB below the power out of the coupled Port 3,
the directional coupler directivity is 20 dB.

VSWR – (Voltage Standing Wave Ratio) or Return Loss: The ratio of the incident signal compared to
the reflected signal in a transmission line. VSWR cannot be directly measured, so a return loss measure-
ment (expressed in dB) is taken of reflected power to incident power. Once it is measured, it can be con-
verted by equation to reflection coefficient which can be converted to VSWR. VSWR or Return Loss is
caused by mismatches and discontinuities within the circuits of directional couplers. A mismatch on either
port 1 (Input port) or port 4 (Terminated Port) will reduce directivity by an amount equal to return loss (in
dB) of the mismatch. This permits measurement of Return Loss. Note that measurement of accurate Return
Loss requires very high directivity (typically 30 to 40 dB).

Connectors: Standard 2.4mm Female connectors are used with optional 2.92 K Female connectors. SMA
connectors are designed for use from DC to 26.5 GHz and are most commonly used in microwave systems.
Higher frequencies call for 1.0mm or 1.85mm Female connectors.

Solutions from KRYTAR

KRYTAR specializes in the design and manufacturing of ultra-broadband, high-performance microwave

components and test equipment. KRYTAR, founded by Thomas J. Russell in 1975, is a privately owned
California corporation specializing in the manufacture of Ultra-Broadband mmWave, Microwave, and RF
components and test equipment for both commercial and military applications. The KRYTAR product line
includes directional couplers, directional detectors, 3 dB hybrids, MLDD power dividers, detectors, termi-
nations, coaxial adapters and a power meter. Our products cover the DC to 110.0 GHz frequency range.
The broadband design expertise at KRYTAR has created unique new designs, several of which are
patented. KRYTAR has applied these designs to consistently introduce technologically advanced products
with superior electrical performance and ruggedness.

KRYTAR • 1288 Anvilwood Avenue • Sunnyvale, CA 94089 • Toll FREE: +1.877.734.5999 • sales@krytar.com • www.krytar.com