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| Message 2008-10-12 18:33:23 |
a question about antenna couplingctownsend says... Hi, I have a question about the antenna coupling.When two antennas are strongly coupled with each other, and the input signal power ratio to these two antennas changed, what kind of characteristic will be changed correspondingly? I feel that the radiation pattern, input impedance should vary, but how about the resonant frequency? Is the resonant frequency of the antenna systems only depends on its geometry, and cannot be changed by the feeding phase and magnitude? Thanks~ harakiri says... Hi, Jayleung:For a linear structure (antenna or circuit), if it is one-port, the performance of the structure does not change with the excitaiton magnitude and phase. If it is a 2-port structure, then it is a different story. Changing the relative magnitudes and phases of the 2-ports will change the structure's performance. I believe the concept of resonance is relative. In the other word, it is hard to say which frequency is the exact resonant frequency because it is dependent upon the loading and how you view it. A typical example is a microstrip antenna. What is its resonant frequency? Some people say it is the Re(Zin) at max. Some people say it is at Im(Zin) = 0 and some people say it is at Min(Gamma). All those points are different even though they are close. Also, for Min(Gamma), it is dependent what Zc you will use. When you have 2-ports, I believe the definition of resonance needs to be clearly defined because it can be some value in a range depending upon how you view it and how the excitations and terminations at the 2-ports are. Best regards, septer012 says... Hi, Jian. Thank you very much for your information.I think most of the people define "resonant frequency" with return loss or VSWR minimum in antenna field. So I am wondering by adding some tuning feed network for antenna such as L-, Pi-, or T-lump component network, every antenna system can be tuned to match 50 Ohm or other standard terminal at specified frequency point. However, single antenna element has its best resonant frequency corresponding to its geometry, for example, monopole with lamda/4, rectangle microstrip antenna with lambda/2. Is it due to the fact that corresponding impedance bandwidth should be deteriorated, or the radiation pattern will be totally changed for tuned matching network added? |
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