Voltage or power added
Voltage or power added
(OP)
Hi, I have a question about analyzing different coupler types. If you use a Wilkinson power combiner you add the power from the two input branches. That is clear so far.
For a directional coupler it is different. Let's name the branches as: port 1 is the one you input the signal on. Port 2 is the forward direction with just a small amount of attenuation (assume it is zero). Port 3 is the coupled port from port 1. If the coupling factor is C then P3=P1-C. (Note we talk in dB values here). Let port 4 be matched. If we have some missmatch at the port 2 (power reflection coefficient R2(dB)), we will get an extra signal to port 3 provided the directivity is not infinite. Contribution of the reflection from port 2 is P1-R2-C-D, where D is the directivity, defined as 10log(P3/P4) in matched case.
Now we have two signals at port 3. To calculate the POWER received in port 3 we need to add the VOLTAGES of the two signals rather than absolute powers.
I have some difficulties seeing this clearly. I suppose it has something to do with the physical coupling mechanism but I would be very happy to hear clarifying comments on this.
#jousto
For a directional coupler it is different. Let's name the branches as: port 1 is the one you input the signal on. Port 2 is the forward direction with just a small amount of attenuation (assume it is zero). Port 3 is the coupled port from port 1. If the coupling factor is C then P3=P1-C. (Note we talk in dB values here). Let port 4 be matched. If we have some missmatch at the port 2 (power reflection coefficient R2(dB)), we will get an extra signal to port 3 provided the directivity is not infinite. Contribution of the reflection from port 2 is P1-R2-C-D, where D is the directivity, defined as 10log(P3/P4) in matched case.
Now we have two signals at port 3. To calculate the POWER received in port 3 we need to add the VOLTAGES of the two signals rather than absolute powers.
I have some difficulties seeing this clearly. I suppose it has something to do with the physical coupling mechanism but I would be very happy to hear clarifying comments on this.
#jousto
Talk To Other Members
RE: Voltage or power added
Maybe a way to think of it physically is; you are really moving current/electrons on these metal surfaces in the coupler(which relates to voltage V=ri). Now if electrons from port 3 travel to port 4. If for example, 2 electrons from port 3 join 2 electrons from port 4, there is then 4 electrons, the voltage would double because your impedance didn't change instantly and the power would quadruple (V^2). If you think that the power only doubles, then some of your electrons would have to disappear magically and power would be lost since i^2*R would no longer apply.
That might apply in an antenna if it's radiating, but not in a coupler (although couplers do radiate and are a pain when trying to get low sidelobe antennas.)
kch