Material for "Shielding" RFID Tag
Pretty much nothing is better than air. Materials with a higher permittivity will only increase the E-field strength inside the dielectric, making it appear more "conductive" at RF.
What material does Flexield use then?...
I haven't explored their offerings in-depth, but they appear to be lossy magnetic materials. I believe they are more meant to attenuate the potentially radiated fields. I'd guess that if you stuck something like their products to the back of your RFID, then you'd attenuate a lot of your desired signal.
I think I'm gonna use glass... Simulations through HFSS show that it has the least de-tuning effect... at least according to the available cheap materials in the HFSS package...
You didn't exactly tell, what "RFID Tag that is not for near-metal use" means in your case. I guess, you are still working an your previous 900 MHz topic. If it's an E-field antenna, e.g a dipole, only distance or "air", as enjunear told, will help. Plastic foam will be better than glass, and air is better than plastic foam. Preferably, a RFID tag should use an "one-sided" antenna in this situation, e.g a patch antenna.
Oh sorry. Hehe. It IS a patch antenna, and I'm supposed to determine materials that will be suitable to put between it and a vehicle plate (as in license plate). So I need something rigid. I've simulated glass, rubber, FR4, and polystyrene (using HFSS), and FR4 gave the best result. But it's the most expensive, so I chose glass because it's rigid and it's the 2nd best performer. =)
How about designing a patch antenna to work WITH the license plate instead of fighting against it?
I also expect, that a patch antena can perform well in this situation.
Well... We did think about that. But we figured it's going to take a lot of time, it would be a master's thesis at least.. I'm just an undergraduate student. hehe. But yeah, that would be a great idea.