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raphael_spacecode AntennaTheory.com Newbie
Joined: 30 Sep 2015 Posts: 6

Posted: Mon Oct 12, 2015 3:28 pm Post subject: general procedure to design a 50ohms input impedance antenna 


Hello
I have a question:
Assuming we are trying to design an antenna that should be working at a particular frequency F0 with a 50ohms generator.
Is it better to:
1/ design such antenna with the goal of having a resonant frequency at F0, i.e. having the reactive part of the antenna input impedance equals to 0, but the real part of the input antenna is different from 50ohms. Then afterwards matching from any input resistor value to the 50ohms input generator
2/or having whatever complex input impedance at the F0 frequency and trying to match from any complex value for the input antenna impedance to 50ohms?
Thank you very much in advance for your reply
Regards 

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raphael_spacecode AntennaTheory.com Newbie
Joined: 30 Sep 2015 Posts: 6

Posted: Wed Oct 14, 2015 6:11 am Post subject: 


Hello
Is there somebody please to give me an answer?
Thanks
Regards 

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helix Antenna Theory Regular
Joined: 29 Jan 2015 Posts: 64

Posted: Wed Oct 14, 2015 2:44 pm Post subject: 


You're no doubt best off achieving both the desired resonant frequency and 50 Ohms simultaneously, and you often have enough degrees of design freedom to do so.
But, since you ask...
Option 1 is probably best if the resonant impedance is acceptably close to 50 Ohms, say between 25100 Ohms (real), to produce acceptable mismatch loss and impedance bandwidth. You can use a quarter wave transformer (or multisection transmission line matching network) to reduce mismatch loss from there when the two impedances to be matched are real. Actually, you can use a certain length of transmission line of some (non50 Ohm) impedance to match ANY complex impedance to 50 Ohms; the challenge is that it is not always easy to build/get the required transmission line (good luck buying 1 Ohm coax, which may well be required, depending...)
Option 2 is actually probably better if the mismatch loss of your antenna at the frequency of interest is unacceptable. The reason I say this is that to match from real to real on the Smith chart, two reactive "moves" (components) are required (look up Lsection matching networks). However if the (complex) antenna input impedance at the frequency of interest to you lands near the 50 Ohms impedance or 1/50 Siemens admittance circles, you can match with just a single component. Or, if it doesn't, you can still use smaller reactance (lower loss) components than would otherwise be required. One might even prefer the antenna input impedance at the frequency of interest to meet the above criterion on the inductive half of the Smith chart, so that small/cheap/lowloss capacitors can be used for matching. I believe this is often the lowest overall loss solution (produces the highest total radiation efficiency). When you match an antenna to a real impedance, what you're really doing is creating a composite resonant system that includes the antenna. You want the losses in that resonant system to all be radiation losses in the antenna and not Ohmic losses in the matching network. 

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raphael_spacecode AntennaTheory.com Newbie
Joined: 30 Sep 2015 Posts: 6

Posted: Thu Oct 15, 2015 10:00 am Post subject: 


Thank you very much for your reply. I am going to study it carefully.
Bye 

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