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What is the difference between Directional Coupler and two-hole directional coupler?
A Directional coupler is a device that samples a small amount of Microwave power for measurement purposes. The power measurements include incident power, reflected power, VSWR values etc. Directional coupler is used to couple the Microwave power which may be Uni-directional or bi-directional. It is a 4 port waveguide junction consisting of a Primary main waveguide and a Secondary auxiliary waveguide. The below figure shows the image of a directional coupler.
Properties
The properties of an Ideal directional coupler are as follows.
All the terminations are matched to the ports.
When the power travels from port 1 to port 2, some portion of it gets coupled to port 4 but not to port 3.
As it is also a bi-directional coupler when the power travels from port 2 to port 1, some portion of it gets coupled to port 3 but not to port 4.
If the power is incident through port 3, a portion of it is coupled to port 2, but not to port 1.
If the power is incident through port 4, a portion of it is coupled to port 1, but not to port 2.
Port 1 and 3 are decoupled as are port 2 and port 4.
Ideally, the output of port 3 should be zero. But, practically, a small amount of power called back power is observed at port 3. The below figure indicates the power flow in a directional coupler.
Two-Hole Directional Coupler
This is a directional coupler with same main and auxiliary waveguides, but with two small holes that are common between them. These holes are a λg/4 distance apart where λg is the guide wavelength. The below figure shows the image of a two-hole directional coupler.
A two-hole directional coupler is designed to meet the ideal requirement of the directional coupler, which is to avoid back power. The power while travelling between port 1 and port 2, some of it escapes through the holes 1 and 2. The magnitude of the power depends upon the dimensions of holes. This leakage power at both the holes is in phase at hole 2, adding up the power contributing to the forward power Pf. But it is out of phase at hole 1, cancelling each other and preventing the back power to occur.
Hence the directivity of a directional coupler gets improved with two-hole directional coupler.
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