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Mohammed
Antenna Theory Regular

Joined: 10 Nov 2009
Posts: 14
Location: malaysia

 Posted: Mon Apr 05, 2010 4:04 am    Post subject: how to explain the s-polar and smith chart of an antenna..:) Hello everyone  Im simulating a planar array antenna using CST  but l dont know how to explain the s-polar and smith chart graphs  and how should be my optimum graph  here is the graphs  Your help is highly appreciated  Thank you in advance  Best regards ;; Mohammed
bigSteve
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Tue Apr 06, 2010 7:22 pm    Post subject: So for the s parameter polar plot you are plotting s11? What frequency range are you scanning on the smith chart? My guess is that the start frequency is out of band, you travel closer to the center of the smith chart as the array becomes in band, and then back out to the edge of the smith chart as the frequency is too high.
Mohammed
Antenna Theory Regular

Joined: 10 Nov 2009
Posts: 14
Location: malaysia

Posted: Wed Apr 07, 2010 2:00 pm    Post subject:

 bigSteve wrote: So for the s parameter polar plot you are plotting s11? What frequency range are you scanning on the smith chart? My guess is that the start frequency is out of band, you travel closer to the center of the smith chart as the array becomes in band, and then back out to the edge of the smith chart as the frequency is too high.

hello bigSteve ..

firstly, thank you for replying ... well, l'm simulating the antenna within the range of 0.8 - 1 GHz .. and an operating frequency of 0.9 GHz ... but l couldnt get what you mean exactly .. do you mean that the impedance matching is not achieved ... and the antenna is not operating properly ? ..

thank you in advance .. and i hilghy appreciate your assistance ..
bigSteve
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Thu Apr 08, 2010 4:35 am    Post subject: On the Smith chart, the closer the curve gets to the center, the better the impedance matching is (to 50 Ohms usually). On the outer circle of the smith chart, all power is reflected so nothing is radiated. You will never get it exactly to the center, and typically you want it such that the VSWR is <2 or 3 at the most (reflection coefficient is < 0.5 ). This basically indicates the operating band of the antenna should be within a circle centered around the center of the Smith chart. The farther away you go, the worse the match and typically therefore the less power is radiated. For all antennas/arrays, the antenna will have a start and stop frequency. Below the start frequency, the antenna typically has an impedance that is very high and reactive. Hence, the antenna on the smith chart at this frequency will be on the edges of the Smith chart (far from the center, so most power is reflected). As the antenna comes into band, it loops around the smith chart, goes near the center, and then comes out of band (far from the center of the Smith chart). This is a typical response of an antenna. Dual band antenna usually have two loops on the smith chart, where it comes in and out of band.
Mohammed
Antenna Theory Regular

Joined: 10 Nov 2009
Posts: 14
Location: malaysia

Posted: Thu Apr 08, 2010 10:20 am    Post subject:

 bigSteve wrote: On the Smith chart, the closer the curve gets to the center, the better the impedance matching is (to 50 Ohms usually). On the outer circle of the smith chart, all power is reflected so nothing is radiated. You will never get it exactly to the center, and typically you want it such that the VSWR is <2 or 3 at the most (reflection coefficient is < 0.5 ). This basically indicates the operating band of the antenna should be within a circle centered around the center of the Smith chart. The farther away you go, the worse the match and typically therefore the less power is radiated. For all antennas/arrays, the antenna will have a start and stop frequency. Below the start frequency, the antenna typically has an impedance that is very high and reactive. Hence, the antenna on the smith chart at this frequency will be on the edges of the Smith chart (far from the center, so most power is reflected). As the antenna comes into band, it loops around the smith chart, goes near the center, and then comes out of band (far from the center of the Smith chart). This is a typical response of an antenna. Dual band antenna usually have two loops on the smith chart, where it comes in and out of band.

thank you very very much dear for those wealthy information ...it's too efficient and i really understood from those few lines what i counld not understand in a week of reading ...

anyhow, l got a VSWR of 1.13559 (as calculated by CST) ... also, l have simulted the antenna with 100 input impedance to get lower S11 dB value ... so what you suggest me to do now to improve the performance in general and smich chart in particular .... ??

your assistance and support are highly appreciated ....

best regards ..

mohammed
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