Smith Chart Reading 1

[m0ngoose]

Hi,

I have a smith chart measurement of input impedance and I'm not sure how to read it. It's for a 2.4 Ghz patch antenna. Any help of resources would be appreciated. Photo attached.

 

[m0ngoose]

So according to the only useful resource I could find (

http://www.antenna-theory.com/measurements/impedance.php),

based on the photo I put in my first post, this antenna has 4 resonant frequencies, two that are really bad (near the outer perimeter of the smith charter), one that is decent, and one that is the best (by the 0.5). the closer to the center and completely real (right on the equator), the better is the antenna impedance matching to the source... is that right? what else can i tell from this? what are those resonant frequencies? thanks

 

[VEBill]

There's a really good explanation of Smith charts on the following link:

http://www.ac6la.com/stss.html

On your plot, the two points along the outer edge are at + and - infinity ohms (more or less). They're not resonant frequencies and are certainly not of any practical use unless you're trying to make a quarterwave conductive 'insulator'.

If you want to know the frequencies of each point on the Smith chart, then you need to have the software that produced the Smith chart annotate it with frequency information. If that's not possible, then you need to examine the same data in tabluar format with the frequency as an index column.

 

[biff44]

Well, assuming the measurement plane is right at the antenna feed, the antenna impedance normalized is 0.6 - j.15, which corresponds to 30 -j7.5 ohms in a 50 ohm measuring system.

You can also tell something about the bandwidth. If you draw a straight line between where the curve crosses over itself and the origin, find the center of that line and draw a +45 and -45 degree line, that shows you where the bandwidth (3 dB down points) are in frequency. Another way would be to plot that reflection coefficient curve on a linear frequency log amplitude rectangular plot, and the bandwidth is where the S11 magnitude drops 3 dB from the most negative value.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[zappedagain]

biff44,

That's neat trick on the Smith Chart! Where did you learn that one?

Z

 

[biff44]

From the old ones, waveguide handbook, rad lab series, etc. It was a very common way to measure Q back then with a slotted line, measure 3 dB down points or +/- 45 deg phase shift points. You can tell all sorts of things about Q, loaded Q, unloaded Q, coupling factor, etc, with a simple swept measurement. With today's easy of making such measurements...it is a shame that the knowledge to interpret the screen face "at a glance" seems to no longer be taught!

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

here is one paper:

http://www.google.com/url?sa=t&source=web&cd=7&ved=0CDwQFjAG&url=http://

http://www.ee.olemiss.edu/darko/rfq..._73zDw&usg=AFQjCNF2aaYZrEURHdh9PdMXidhLfVrqQw

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[zappedagain]

Thanks. I loved all these tips and tricks for RF analysis. I haven't worked RF for 10+ years now so all of this is getting a bit fuzzy...

Z

 

[zzzjim]

Improvements in test gear , with a little help from software .
Pretty amazing in the last 30 years . Its even affordable !

Cheers Guys , enjoying the memories