What does an RF coupler actually do?

Question

I've spent a few hours reading about the basics of RF directional couplers, but I am still having trouble understanding just the very basics of what a coupler does. To start with, I've had difficulty finding a good, simple explanation of what directional coupling is. I read through Radio Electronics, but the section on directional couplers doesn't give an overview of what it means to "couple."

Could someone offer a simple explanation of what "coupling" is and what a directional coupler actually does?

Answer 1

Here's a couple of ways of drawing a directional coupler in a block diagram.

(image source: Wikipedia)

Most of the signal incoming at Port 1 (P1) will pass through to P2, since they're connected by a transmission line. And most of the signal incoming at P2 will pass through to P1.

"Coupled" just means (partially) connected.

The key idea of a coupler is that a fraction of the signal coming in on Port 1 (P1) will be "coupled to" the output at P3. Meaning the signal will be output from P3. What makes it a directional coupler is that (ideally) none of the signal entering P2 will appear at P3.

Similarly, the output at P4 will be coupled from P2, but not from P1. Often, the P4 output is terminated internal to the coupler device, so you don't have access to it, and then you'd use the second style of symbol from the image.

Answer 2

ThePhoton explains What do they do, but Why? (excuse me if you understand this)

They let you (a) measure how much power is going out to an antenna, and (b) how much of that is reflecting back because the antenna is mismatched. You can do this by having separate ports, one with a sample of outgoing and the other with a sample of reflecting, which you can measure.

You can't measure transmit power by just looking at the voltage because you never really know what impedance antennas are, and the voltage varies along the wire, so you must use the coupler for (a).

You must measure reflection (b) because if B is close to A, nothing is leaving the antenna, its all bouncing back, and turning to heat in the transmitter.

How is this magic possible?

The current for outgoing will be opposite to the current for reflecting. If we sample the current with a current transformer, then the voltage will be opposite. Now we can arrange for the voltage on the line to cancel out the current sample in one direction, and add to it in the other. (this is not exactly right, but should let you get your head around the idea)

Why do you call it Magic?

Well it is isn't it? The waveguide component that does this is called a magic T and its just some pipe. How do they do that? There ain't half been some clever bastards

Answer 3

A directional coupler can be a splitter and a combiner as well, depending on how it is used.

REF Directional Couplers are design as tapped transformers with 1/4 wave reactance inverters to cancel the signal in the reverse direction.

When the coupled port is exposed instead of terminated with some R, then you have a 4 port bidirectional splitter with good isolation and low reflectance when match terminated.

To get wider bandwidths , there are various tricks.
A magnetic transformer is often used with bifilar wound hydbrid ferrite core is used. ( commonly used for TV signals) e.g. using 50 or 75 ohm port impedance.

Above is a 3dB 75 Ohm splitter is also a DC-3 (meaning ideal 3db Directional Coupler , but actually 3.5)

Depending on mutual coupling impedance, the coupler can be a -3dB tap ( - 0.5dB loss) down to a -50dB tap with low instertion loss. Also they can be equal power taps.

This one can split 5kW at one frequency.

Since they cancel forward power if terminated properly, with good isolation ( ~ -30dB ) it can be used to combine two sources or split one source. Isolation is the cancellation of Mutual coupling.