rfid impedance matching

[touseef hayat]

hi. i am working on hfss 13. the problem i am facing is chip impedance matching between chip and antenna.

the impedance value value of chip is 20-485j and i have to assign its conjugate (20+485j) to antenna. but how am i supposed to assign these values ?

 

[WimRFP]

Did you read this recent thread: https://www.edaboard.com/threads/309539/ ? The example impedance almost equals yours.

 

[touseef hayat]

i checked that but nothing is discussed there about assigning these impedance values

 

[WimRFP]

You are right about "assigning", I didn't discuss that. It is not a case of assigning, but just a case of designing an antenna with 20+j485 Ohms impdance. When done, you have a conjugated match into the 20-j485 Ohms chip impedance.

if you want to normalize to a complex impedance (so that 20 - j485 is in the middle), you need to check your software very carefully to make sure you get what you want.

 

[touseef hayat]

but can you tell me what values am i going to specify for chip impedance, antenna impedance and port impedance

 

[WimRFP]

Port impedance is not of importance, chip and antenna impedance is.

Mismatch between a complex load an a complex source is tricky.

Similar to the normal reflection coefficient for real loads or real sources, you can define a power based reflection coefficient (RCpwr) for both complex load and source.

RCpwr = (ZL-Zs*)/(ZL+Zs) Where the loss due to mismatch = -10*log(1-(RCpwr)^2).

In case of receiving, ZL is the complex impedance of the chip and Zs is the complex impedance of the antenna (as the antenna supplies energy to the chip). You see when Zs* = ZL, you have zero RCpwr, hence maximum power transfer.

For a given ZL (here the chip), all values for Zs (here the antenna) given a certain mismatch are on an a circle on the Smith chart (with Real Zo). The center of the circle is not exactly at the position of ZL*. If you accept low mismatch, the circle becomes smaller. When ZL* is further away from the smith chart center, the circle becomes smaller also.

So if you accept 0.51 dB loss (equal to |RCpwr| = 0.3333), you can calculate for example 3 or 4 values of Zs that give |RCpwr| = 0.3333. When you plot these points on a normal smith chart, you can construct the center of the circle. Then you can draw the circle. All Zs (your antenna impedance) that are within this circle have a mismatch < 0.51 dB.

I am sure there is math to calculate the position and radius of this circle, but I don't have this at hand now. When I needed this, I did this graphically on the Smith Chart and with a spreadsheet that calculates RCpwr.

 

[touseef hayat]

my question is still there. which value am i supposed to assign to the chip and which one should be assigned to the antenna. please specify

 

[WimRFP]

Contact your manufacturer/supplier for chip values to "assign"

 

[touseef hayat]

i got those values both for the chip and antenna but i am getting positive results for s11 moreover antenna is not resonating at particular operational frequency

 

[flanello]

What do you mean with assign?
You probably will design an antenna with an impedance of (20+485j) Ohm. Then you should ignore the S11 of the simulator since these values are only for 50 Ohm systems valid.
You can calculate the reflection coefficient for your case according to the formulas given by WimRFP.