A Rat-Race Coupler is a type of RF coupler that is manufactured using microstrips in the shape of a ring/circle. It is a four-port coupler, with each port placed at a distance of one-quarter wavelength (λ) away from the other around the top half of the ring. The bottom half of the ring is three-quarter wavelengths (λ) in length. As seen in the figure below it has three branches which are 90 degree phase shifted from each other and one branch that provides a 270 degree phase shift.

This coupler is quite versitile and can be used as a power combiner by combining two in-phase signals. Or can be used as a power divider to provide two equally split signals that are 180 degrees out of phase of two equally split in-phase output.

A Power Splitter/Combiner is a passive device that can be used for two reciprocal functions: a single signal may be divided into multiple outputs, or in the opposite direction, multiple input signals are combined into a single output. In case of an N-port splitter, the input signal will be divided into N output ports. When used as an N-port combiner, the N inputs will be combined into an output signal from a single port.

Consider a 2-way power splitter. A single signal is the source, which is split into two output signals, each with half the original power (-3 dB). There is no phase difference between the outputs since they are derived from the same input signal. The outputs are thus coherent. If we introduce a time delay in one of the signal paths, such as a section of transmission line, there will be a phase difference, but it will remain constant, so the signals are still coherent. With these coherent signals, the signal paths are balanced and virtually no power is dissipated by the load resistor.

In the other direction — as a power combiner — if the two inputs are coherent and identical in amplitude, the device is balanced and there will be no power in the load resistor. But if there are any differences in the input signals (non-coherent), the system is unbalanced, resulting in power being sent to the load resistor. The amount of power dissipated due to non-coherent combining must not exceed the ratings of the load resistor.

In general, non-coherent signals have differences in amplitude, phase and/or frequency that will result in significant unbalanced power being dissipated. The dissipated power may range from a minor amount to the total input power.

RF and Microwave PCB Design – Transmission Lines and Impedance – Altium Academy
https://www.youtube.com/watch?v=GuJVkZgAloI

This video illustrates how RF (radio frequency) standing waves are created in transmission lines – through the addition of the forward (transmitted) wave and the reflected wave that results from improperly terminating the line or matching the load or antenna to the transmission line impedance.

This video describes the basic properties and specifications for directional couplers, and shows their basic operation on an oscilloscope. Typical applications are discussed, along with a practical example showing how to sweep the VSWR, or Return Loss, of an antenna using a directional coupler and a spectrum analyzer with a tracking generator. The resulting return loss measurement is compared to the SWR measured on an MFJ-259B antenna analyzer.

In this video I take a look at a 778D Dual Directional Coupler that was part of a lot I won at auction – I really wanted 1 but ended up with 6 – 3 of which didn’t pass manufacturer perf test – So I thought I’d take it apart and have a look inside.

A discussion of circulators and a teardown of a 915MHz circulator.

RF Isolator Teardown & Explanation
https://www.youtube.com/watch?v=Whn3QjHUgIw

Ever wonder what’s inside an RF Isolator?

In this video, I took apart a 8 to 10 GHz microwave RF isolator and did some measurements. High resolution teardown pictures at

http://www.kerrywong.com/2016/03/01/teardown-of-an-8-10-ghz-rf-isolator/