difference between 3dB coupler and directional coupler?

Hi,

You can read some theoretical articles below:

hxxp://www.ee.bilkent.edu.tr/~microwave/programs/magnetic/dcoupler/theory.htm

replace x with t

Basically 3dB couplers are used in application where you need 2 (quasi )identical outputs (power level) with 90/180 degrees phase difference between them (typical apps: balanced amplifiers, mixers, etc). Important figure of merit for these apps is the phase balance between the outputs of the coupler.
while directional couplers are used in apps where you need the main output with low insertion loss, and the coupled output with a small percentage of the power level. (for example output power level measurement, AGC loop, distributing/summing different signals, etc)

The couplers are also known as quarter wave couplers, and if you want to increase their bandwidth, you have to use multiple sections.

rgds, Al

I think you are referring to Branchline coupler, if you are talking about equal power split (and 90 deg out of phase).

directional coupler is a 4 ports network.
Power supplied from port 1 is transmitted to port 2 and a small amount of energy is coupled to port 3.

Power supplied from port 2 is transmitted to port 1 and a small amount of energy is coupled to port 4.

What is the phase difference between differnt ports of a Directional coupler?

the phase differences are:

0 degrees for a wilkinson
90 degrees for a branch line or a lange coupler
180 for a balun or for a race

ask also in www.uwaverf.altervista.org/forum

you are 1/2 right

A directional coupler doesnt have to be 4 port it can be just 3 port as stated in the first part of your statement
it is still a directional coupler just a single direction rather than bi-directional

Dave