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rat race coupler unsymmetric outupts

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kazim

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dear friends

i connected my two antennas to the rat race coupler. one to sum port and other to delta port .. when i analyzed input reflection at input port A and input port B, it seems from results that one port is wideband port and other is narrow band port
does some one have the idea why is this so because the 90 degree gives symmetric bandwith and both input ports
the couplers are used as combiners in my design
 

biff44

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rat race couplers are very narrow band. Odds are it is mistuned, or your antennas have poor VSWR which are mistuning the coupler.
 

kazim

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even i tune them properly still one the delta port has narrow bandwidth than the sum port....i think it might be due to structure of this coupler. the 90 degree hybrid is symmetric in structure so it does not experience this sort of behaviour.
does any one have anything else in mind to explain this phenomena
 

vfone

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The rat-race couplers that I used in the past, they have about 30% bandwidth for a specific phase error between ports. Actually surprised me this kind of characteristic. The BW I've got, could be related to the PCB material that I used, I remember was not something special.

Also the bandwidth of Σ and Δ ports was different, which make sense because used as a combiner this is not a symmetrical structure.

To be sure that you are not mismatching the coupler ports, try to add a 3dB attenuator on each branch and you should get exactly 6dB improvement in return loss on each port. If you get more or less than 6dB, means the coupler ports are affected by the antenna impedance.
 

kazim

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is there any way i could improve the bandwidth of on the narrow band port...because one of the antenna will operate on wideband port and the other one will operate on narrowband which i dont want to happen
 

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