No, I am not going to preach to you about deforestation and global warming, although I fully support "going green," and RFID can help reward consumers for recyling, but that is not what I am talking about when I say Environment.

Rather, this is part 2 in a series started a few weeks ago about choosing the right frequency for a given RFID application. Today we will discuss how environmental factors might influence your decision to go with either frequency band.

So, what I am talking about, environmental factors? Everyone knows that metal is bad news for RFID right? Well, sure, metal surfaces can reflect electronmagnetic fields, wreaking havoc on your system, and magnetic fields can induce eddy currents in metals that will in turn create other magnetic fields that can effectively cancel the signal, and blah blah blah, metal will cause headaches, good luck. There are ways around these issues, perhaps by using foam spaces between tags and the metal surface, or by using ferrite core sheets as isolators between an antenna and metal, and a lot of RFID applications become plausible using these kinds of solutions, as long as you aren't trying to read a tag through metal. But metal things aren't necessarily evil, and even could be essential when considering shielding. Every device containing a radio needs to be certified by some sort of regulatory body (like the FCC) before it can be sold, and some certifications don't allow you to be radiating electromagnetic waves all about, so a nice metal shield may come in handy (assuming you don't create any slot antennas.) There is no clear winner here in the "frequency war," both bands are affected and there is a lot to consider.

 Water and other liquids can also cause some issues. Liquids, and even plastics, have a higher relative permittivity than air, meaning the electric fields produced by radios tend to be absorbed by the medium, and the effects are greater with higher frequencies. This is why in submarine movies, they always have to break out the ultra-low-frequency radio to communicate when they are deep underwater. Luckily HF readers use magnetic induction, and they don't feel these kinds of effects nearly as much as their UHF bretheren. So when reading tags on liquid filled bags, HF might be your best bet, but don't forget about inductive UHF tags, because if range is not an issue, that could be an option as well.

But enough of the physics lesson, and environment isn't all materials. What about the place you might be using RFID? Is it a busy store, a crowded airport, or maybe a deserted (other than robots) packing facility. In a warehouse, you are probably more interested in picking up every item that moves through the dock door, and maybe in a store you want to know all of the people that step through the door, and provide each of them tailored service. But then again, you probably don't want to accidentily pay for the person's purchase in front of you at the counter because you have an RFID enabled credit card. Privacy is always a concern with RFID, and a lot of times it concerns where people are in relation to one another. If you want general information about people and things, and there are tons of them moving through gateways, longer range UHF applications work great. But if you want to be sure exactly who or what you are talking to, HF's proximity is your friend. Although, apparently the government isn't all that concerned with knowing what face goes with which passport as you move through customs.

I hope this gives you some idea of the factors that may impact your application. It can be a complicated business to fit RFID into every environment, but now you know some of the questions to ask when taking the RFID plunge.

In the final part of this series, we will talk about how your decision for HF vs UHF is influenced by the most powerful force on earth: money.