Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

HELP:UHF RFID antenna impedance measure

Status
Not open for further replies.
R

rf1008

Full Member level 2
Joined
Mar 30, 2012
Messages
129
Helped
7
Reputation
14
Reaction score
7
Trophy points
1,298
Activity points
2,219
hello, anyone could help me?

I measured my PCB meander antenna with VNA to get S parameters, and found that the data of S11 displayed on VNA varies while the possition of DUT(antenna) changes. I hold the rigid cable over head and change the possition and angles.Of course i also believe it's not a good method for antenna test,but u know, i have no access to chamber.

The input impedance of antenna is very important for rfid tag design, pls.help, thanks.
 

E Kafeman said:
If you have extended DUT ground plane with a measurement cable can this type of problem occur. Please check the section about how to reduce measurement cable related S11 problems.
For S11 measurement is normally not any chamber needed.
Click to expand...

Thanks to your advice. you know, the DUT i measured is a dipole which is differential feeding and no ground to be connected to VNA.Two rigid RF cables(pigtails)are used whose outer GND soldered together and two inner conductors soldered with dipole. S11/S22/S21/S12 can be measured with two ports single-ended VNA. Then,the differential input impedance could be calculated by equation with s11 and s21.

Two rigid cables have been calibrated used port extention function after standard OPEN/SHORT/LOAD calibration at port 1 and 2. So i believe my whole measure condition is correct enough.

Maybe sth.is still wrong somewhere,or not,pls.kindly advise more, thanks a lot!
 

Do really not understand if you have a dipole or two independent monopoles, and if dipole, why try to measure it in two halves.
Anyhow, during a part of the measurement are you measuring S11 for one antenna element and RF ground do then only consist of your cable shield?
If move your hand along coaxial outer shield, does then S11 at VNA fluctuate?
 
S11/S22/S21/S12 can be measured with two ports single-ended VNA. Then,the differential input impedance could be calculated by equation with s11 and s21.
Click to expand...
In this case, why you're complaining about S11 display variation? It's just an intermediate result, obviously affected by the common mode cable effects. The final result should be independent of these accidental variations - at least in theory.

Your measurement setup is however based on prerequisites. The parasitic common mode impedance must be constant during all measurements. In my opinion, the two port method serves a purpose, if common mode parameters need to be measured. If you're only interested in the differential dipole impedance, a wide-band balun or a good common mode chokes should give the intended result.
 
Thanks to Kafeman and FvM.

I made two types of antennas, one is general dipole and the other is meander dipole,of course the latter one is based on dipole model.

I found the data of S11 of the gereral dipole fluctuates much more than that of meander dipole which has better convergence in smith chart.

The data of S11 whether my hand contact the coaxial outer shield or not fluctuates not very much, and can be accepted.

I know at least three methods could be used to measure the differential impedance of these antennas,including your advice of monopole measurement based on a big conductive ground and then the result multipled by 2 to get the final impedance,and the third one is to measure with a good performance balun. But i doubt the reliabilities of the other two measurement just because the results rely on the size of conductive ground and the performance of balun. Anyway, i'll try and compare the results of them.

Thanks!
 

rf1008 said:
Thanks to Kafeman and FvM.

I made two types of antennas, one is general dipole and the other is meander dipole,of course the latter one is based on dipole model.

I found the data of S11 of the gereral dipole fluctuates much more than that of meander dipole which has better convergence in smith chart.

The data of S11 whether my hand contact the coaxial outer shield or not fluctuates not very much, and can be accepted.

I know at least three methods could be used to measure the differential impedance of these antennas,including your advice of monopole measurement based on a big conductive ground and then the result multipled by 2 to get the final impedance,and the third one is to measure with a good performance balun. But i doubt the reliabilities of the other two measurement just because the results rely on the size of conductive ground and the performance of balun. Anyway, i'll try and compare the results of them.

Thanks!
Click to expand...

so, can you share the results of all three measurements?

Do you know how to get the balun?
 

Not yet,it's difficult to measure the impedance by the other two method,i have not done it yet.
 

rf1008 said:
Not yet,it's difficult to measure the impedance by the other two method,i have not done

how about the accuracy of the first method? How do you calibrate the vna for two port measurement?
Click to expand...
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top