Power Dividers & & Directional Couplers
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How to Analyze the Properties of Power Dividers and Directional Couplers.

Power dividers / Power splitters when used in reverse, power combiners & directional couplers are passive microwave components used for power division or power combining in the field of radio technology.

In power division, an input signal is divided into two (or more) output signals of lesser power.

A power combiner accepts two or more input signals & combines them at an output port.

The coupler or divider may have three ports, four ports, or more. Three-port networks take the form of T-junctions while four-port networks take the form of directional couplers and hybrids.

Power divider is used for devices with tight coupling (commonly, a power divider will provide half the input power at each of its output ports – a 3 dB divider) and is usually considered a 3-port device.

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.

Coupling factor

Insertion Loss

Isolation

Directivity

S-parameters

Amplitude balance

Phase balance

Coupling factor
The coupling factor is the fundamental characteristic of a directional coupler. It is a negative value and cannot exceed 0 dB for a passive device. In practice, it typically does not surpass -3 dB, as exceeding this limit would lead to more power being output from the coupled port than from the transmitted port, effectively reversing their roles. While it is a negative value, the minus sign is often omitted. Additionally, coupling is not constant but varies with frequency.

Insertion Loss
The insertion loss of an ideal directional coupler will consist solely of the coupling loss. In a real directional coupler, however, the insertion loss comprises a combination of coupling loss, dielectric loss, conductor loss, and VSWR loss.

Isolation Loss
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:

Directivity
The directivity should be as high as possible especially at the design frequency. Directivity is directly related to isolation. It is defined as:
Waveguide directional couplers will have the best directivity. Directivity is not directly measurable and is calculated from the addition of the isolation and (negative) coupling measurements as:

Amplitude balance

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 difference.

Phase balance

The phase difference between the two output ports of a hybrid coupler should be 0°, 90°, or 180° 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.

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