Directional Coupler in Microstriplines

Background

RF and microwave technology become an important part of day to day life, it is difficult to imagine life without microwave technology. These have commercial and military applications. With the rapid progress and improvement in mobile and wireless technology, microwave components such as filters, couplers, circulators, and power dividers are in high demand.
A microwave directional coupler in microstrip technology is an important component in the design of microwave communication systems. It is used to extract a small portion of the microwave signal from the main transmission line and to measure the power of the signal. This is accomplished by constructing a microwave circuit on a substrate material with a microstrip transmission line and a simple coupling mechanism. The directional coupler consists of two microstrip transmission lines that are separated by a narrow strip of insulating material. The incoming microwave signal is divided between the two transmission lines, with a portion of the signal being directed to the output port. The amount of signal that is coupled from the main line to the output port is determined by the geometry of the microstrip lines and the width of the insulating strip. This type of directional coupler is commonly used in microwave and millimeter-wave communication systems due to its compact size, low cost, and ease of integration with other components.

Introduction 

Coupler is a four terminal passive device, when power is applied to one of the transmission lines, an induced emf is developed across the other end due to the interaction of the electromagnetic field.Directional coupler is a passive four port device: port 1 is called input port, port 2 is called direct port or through port, port 3 is coupled port is where the forward power is received and port 4 is isolated
port where the reflected power is received.

            IMAGE

Microstrip Coupler
Microstrip lines consist of a ground plate separated by dielectric material and conductor above it. The microstrip line is well known as an open-strip line, although microstrip lines do not support Transverse Mode of Propagation (TEM) - both the electric field vector and magnetic field vector are perpendicular to the direction of propagation it is in fact it supports quasi-TEM mode of propagation the waves because microstrip lines are made up of two conductors separated by an electric material the waves are transmitted in two ways by the air above the
top pattern and through the dielectric substrate and resistivity are different in two media therefore wave travels with different speeds.

IMAGE

Designing
Now we are going to use one opensource tool to design the circuit and calculate its s-parameters, here we are using QUCS tool, the schematics are shown below. The microstrip line here is FR4 material having dielectric constant 4.4, it is to be noted that these kind of materials are not preferred for very high frequency waves because of high dielectric losses.

 

 

 

 

 

After we  get the dimension of the coupler it is time to design it in CST, although you have many tools available, but I have used CST student version because it was freely available so design it, simulate it, and take the layout, print it on a piece of glossy paper for next steps.

Steps Involved

Masking:

The layout is first printed on glossy paper using laser printer, it is then transferred in clean surface of PCB and ironed so that all the ink transferred to the surface of the copper.

Etching:

Corrosive solutions like FeCl3 is used for etching the undesired copper, the setup is continuously move to thoroughly remove the copper and the solution conc. should maintain constant value.

Soldering the Ports

The ports are soldered, use of good quality flux and solder is recommended.
Here I have used 25W solder iron.

 

3 section Directional coupler

Some additional couplers have been designed to fun.

[Three section is good for wider band, the difference is visible in the results]

 

Another design (fast prototyping using single sided PCB and copper tape)

Device under testing, two ports of VNA is connected at a time keeping other two ports connected with a load of 50 ohms, so that it is matched and no reflection is occurred.

After fabrication is done the device needs to be tested under VNA and s-parameters need to calculated, below is the graph of s-parameter for one section coupler.

It has been confirmed that when the resonating frequencies has been reduced it leads to bigger in size of the device.

The results of VNA is a CSV file which I cant upload here, apart from that, if you have any queries in the project feel free to drop a comment I shall try my best to answer.

Thank You.