how to measure dielectric permittivity

[ali0001]

hi, i need to design an UHF RFID tag antenna on 0.45mm thick Maylar the substrate is 450 micron Polyester, dose any one know how can i measure its dielectric permittivity at UHF (869mhz).
thanks

 

[Higgler]

What about looking up the value in a data sheet? It's hard not to find a value for plastics.

How accurate do you need the data? 1% or 10%?

Does dielectric value affect a super small (electrically speaking) antenna? Have you alot of experience in RFiD design?

kch

 

[ali0001]

hi, thanks for your reply,i need the data to be very accurate, the values given in data sheets are frequency dependent.
i am designing ultra thin platform independent antenna and accurate value of permittivity is important at UHF. i have been working on these antennas for about a year now.
do you know how to measure the permitivity for this material with 0.45mm thickness.
ali

 

[Comcokid]

The usual way to test the permittivity of a material would be to use a dielectric test fixture (example: Agilent 16451B or 16453A) with an impedance analyzer or network analyzer, and test around the frequency of interest.

The basic method uses the dielectric material between parallel plates to create a capacitor, which then can be analyzed. Additional software extracts the parameters of the material being tested from the data and corrects for the presence of the fixture.

 

[VEBill]

Would this work?

Guess the value, build a prototype antenna, measure the prototype antenna, and then adjust accordingly.

 

[Higgler]

I would have guessed that RFID tags would be less sensitive to dielectric constant. Can you explain why it's so sensitive at such a low frequency for an inefficient antenna (or is this a transmit antenna? which are larger).

If you have money, you can send it to Damaskos, they do an assortment of measurement techniques.

Resonator techniques are simplest. you make a transmission line with small cuts or breaks in the transmission line at spacing 1/4 lambda and another transmission line with cuts 3/4 lambda and measure S21. With accurate mechanical knowledge you have highest transmission at your proper wavelength. Since there is 1/2 lambda delta line length between the two samples, and the cuts are the same, you find the dielectric constant accurately. A coworker used this at my former workplace and got the answer that was higher than what Rogers corp. specified. I thought he was wrong until we made a microwave lens and proved he was right.

Rogers is now changing how they specify dielectric on many of their products from the "this is what we measured using a standard IPC measurement tehcnique" to "use this dielectric value for circuits"

kch

 

[ali0001]

Passive UHF RFID tag antennas, use backscattering technique to communicate with the reader, tag antenna is also directly connected to the ASIC RFID chip with high capacitive impedance. therefore the tag antenna is designed to have a high inductive port impedance to conjugately match the RFID chip impedance. placing the tag antenna on a substrate with high permittivity would change the antenna port impedance and therefore the power transfer ratio to the chip and consequently tag read range. i am designing tag which are insensitive to the objects it is attached to, as a thin substrate is part of the tag design, exact value of permittivity is important parameter in calculating the resonant frequency and power transfer ratio of the tag.