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freqzt
Joined: 13 Sep 2006
Posts: 123
Helped: 2
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 18 Apr 2007 3:47 Input Impedance for UWB Microstrip Antenna |
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| I've design a UWB microstrip antenna. The S parameter is ok, but the input impedance is not. The input impedace is not constant at the range of frequency that is between 3.1 - 10.6 GHz. When I design this antenna, I set the input impedance at 50 ohm. The simulation result show that the input impedance is oscillation between 30 - 80 ohm. Anyone know how to make the input impedance is around 50 ohm, if there is a deviation, it's just 5 ohm at the band 3.1 - 10.6 GHz. Should I make an impedance matching? How to make it? I fed the antenna using CPW. Thank you for your help.
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fracosoe
Joined: 25 Jun 2006
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18 Apr 2007 7:23 Re: Input Impedance for UWB Microstrip Antenna |
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What do you mean by "The S parameter is ok"?
If your s11 is ok at frequency range you operate, then you don't need to be worry with the input impedance...
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freqzt
Joined: 13 Sep 2006
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18 Apr 2007 7:58 Re: Input Impedance for UWB Microstrip Antenna |
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| I mean the S11 at the frequency range is below -10 dB. Is the input impedance not important here?
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ananth
Joined: 29 Jun 2004
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18 Apr 2007 10:21 Re: Input Impedance for UWB Microstrip Antenna |
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| what facosoe meant was that if your S11 is better than -10dB, it means you input impedance for that frequency is a decent match. Are you saying your S11 is less that -10dB for the whole range. thats pretty darn good in itself for that huge band.
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freqzt
Joined: 13 Sep 2006
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19 Apr 2007 1:41 Re: Input Impedance for UWB Microstrip Antenna |
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| ananth wrote: |
| what facosoe meant was that if your S11 is better than -10dB, it means you input impedance for that frequency is a decent match. Are you saying your S11 is less that -10dB for the whole range. thats pretty darn good in itself for that huge band. |
yes it is. If S11 is less than -10 dB for the whole range, it is implied that the input impedance is must be match with 50 ohm?
The figure below is the graph of S11 and input impedance. The red line is input impedance, and the other is for S11.
Last edited by freqzt on 19 Apr 2007 8:26; edited 1 time in total |
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jallem
Joined: 23 Jul 2005
Posts: 477
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19 Apr 2007 3:04 Input Impedance for UWB Microstrip Antenna |
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It would better if you show the phase of the
impedance as well or the real+ imag part
of the impedance
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dongxl
Joined: 31 Oct 2006
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19 Apr 2007 11:13 Input Impedance for UWB Microstrip Antenna |
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freqzt:HI!
This UWB antenna function is in the 3 GHz GHzs-10s S11<-11.5 dbs, the importation resistance motion is big some.If continue excellent turn, the whole function would be better.
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freqzt
Joined: 13 Sep 2006
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19 Apr 2007 14:10 Re: Input Impedance for UWB Microstrip Antenna |
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This is the picture of S11 and input impedance in complex plane (smith chart). The blue line is input impedance and the other one is S11.
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dissa
Joined: 29 Mar 2007
Posts: 16
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20 Apr 2007 10:57 Input Impedance for UWB Microstrip Antenna |
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Freqzt,
Remember for 2:1 VSWR match with a 50 Ohm line , which is close to 10dB return loss or -10dB |S11|, you can have approximately 25-100 Ohm input impedance swing. Frankly, antenna can be matched according to the definition (i.e. 10dB return loss) even if it does not have 50 Ohm input impedance.
You have asked whether is it needed to bother about input impedance if S11 is fine. My answer is no, unless you are concerned about normalized transfer functions of the antenna.
Afterall, the matching of a UWB antenna is the least of the worries, go ahead and analyze the radiation patterns.
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Azulykit
Joined: 18 Feb 2007
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05 May 2007 6:49 Re: Input Impedance for UWB Microstrip Antenna |
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A -10 dB reflection coefficient over that much bandwidth is generally considered good.
Just a warning, it is easy to achieve with a -5 dB attenuator. Be careful and do not fool yourself with a lossy antenna. I think that is what dissa was getting at with the comment about analyzing the radiation patterns (be sure to measure gain, not just directivity).
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