Directional Coupler Calculator

What is a directional coupler?

A directional coupler is a 4-port passive device with an input port (port 1), output port (port 2), coupled port (port 3), and an isolated/terminated port (port 4). The purpose of a directional coupler is to sample a small amount of microwave power for measurement purposes. such as measuring incident power, reflected power, voltage standing wave ratio (VSWR),etc. When an input signal travels from port 1 to port 2, a part of this signal is coupled to port 3. The portion of the power coupled to port 3 depends on the coupling value of the coupler being used. For example, if we use a 3 dB coupler, the power split between port 2 and port 3 would be 50%, however, if we use a 10 dB coupler, then this power split would be 9:1. The coupling is one of the key parameters on the basis of which a directional coupler is selected. Typical coupling values vary from 6 dB to over 30 dB.

Directional couplers are signal processing devices. They are used to acquire a sample from the output signal for test and measurement. They can also be used to deliver a portion of the output signal to feedback circuitry.

How to calculate coupling?

In a directional coupler, the coupling is the ratio of input power (p1) to the power at the output of coupled port (p4).

The formula used to calculate the coupling is given below.

Where,

P1 = Input power (port 1) in dBm/watt(s)

P4 = Coupled power (port 4) in dBm/Watt(s)

How to calculate Coupling loss?

Coupling loss (also called connection loss) occurs at the connection between two circuits or between two elements when the energy is transferred from one circuit/element to another circuit/medium. 

The coupling loss of the directional coupler is calculated by using the following formula.

Where,

P1 = Input power (port 1) in dBm/watt(s)

P4 = Coupled power (port 4) in dBm/Watt(s)

How to calculate Insertion loss of a directional coupler?

Insertion loss of the directional coupler represents the reduction in the output power (in port 2) due to power supplied to port 3 for measurement purposes and a small amount of power/reflected from port 2 to port 4. For an ideal directional coupler, the reflected power is zero, but in a real-life coupler, a small amount of reflected power is always observed in port 4. 

The formula used to calculate the insertion loss of the directional coupler is given below.

Where,

P1 = Input power (port 1) in dBm/watt(s)

P2 = Output power (port 2) in dBm/watt(s)

P3 = Output power at isolated port (port 3) in dBm/Watt(s)

P4 = Coupled power (port 4) in dBm/Watt(s)

How to calculate the Directivity of a directional coupler?

Directivity of the directional coupler represents the difference between the power level of the coupled and isolated ports. In other words, it is a measure of independence of the coupled and isolated ports.