The answer to these questions is that just about any asset, part, product or other item can be identified, tracked and managed using 

an ultrahigh-frequency RFID tag. But tagging methods vary, based on an item's shape, size, material and texture, 

as well as the environmental conditions in which it will be tracked.Among the key issues that must be considered, as below:

• the size of the asset to be tagged and the footprint available to receive the tag

• the surface of the asset to be tagged, such as metal, plastic, cloth, living tissue (nursery stock, farm livestock, lab animal, 

   pets, infants), wet paint, wet chemical coating, chemically treated, abrasive

• the duration of time the asset needs to be tagged—short or long term, permanent or temporary

• the duration of time the item will be exposed to environmental conditions, such as extreme temperatures, moisture, 

  chemicals, abrasion, dust, human contact, pressure, salt water, animal exposure, outdoor ultraviolet exposure and weather conditions, 

  industrial processing, temperature shock (going from hot to cold and back), flash freezing (dry ice), and lab testing

• the asset's temperature at the time of tagging; for example, hot molded products just out of the mold, or frozen consumer vending products

The distance from which a tag can be read is called its read range. Read range depends on a number of factors, including the frequency of the radio waves uses for tag-reader communication, the size of the tag antenna, the power output of the reader, and whether the tags have a battery to broadcast a signal or gather energy from a reader and merely reflect a weak signal back to the reader. Battery-powered tags typically have a read range of 300 feet (100 meters). These are the kinds of tags used in toll collection systems. High-frequency tags, which are often used in smart cards, have a read range of three feet or less. UHF tags-the kind used on pallets and cases of goods in the supply chain-have a read range of 20 to 30 feet under ideal conditions. If the tags are attached to products with water or metal, the read range can be significantly less. If the size of the UHF antenna is reduced, that will also dramatically reduce the read range. Increasing the power output could increase the range, but most governments restrict the output of readers so that they don't interfere with other RF devices, such as cordless phones.

This is a mode of operation that prevents EPC Gen 2-compliant readers from interfering with 

one another when many are used in close proximity to one another. Readers hop between channels 

within a certain frequency spectrum (in the United States, they can hop between 902 MHz and 928 MHz) 

and may be required to listen for a signal before using a channel. If they "hear" another reader using that channel, 

they go to another channel to avoid interfering with the reader on that channel.

Active RFID tags have a battery, which is used to run the microchip's circuitry and to broadcast a signal to a reader 

(the way a cell phone transmits signals to a base station). Passive tags have no battery. Instead, they draw power from the reader, 

which sends out electromagnetic waves that induce a current in the tag's antenna. Semi-passive tags use a battery to run the chip's circuitry, 

but communicate by drawing power from the reader. Active and semi-passive tags are useful for tracking high-value goods 

that need to be scanned over long ranges, such as railway cars on a track, but they cost a dollar or more, 

making them too expensive to put on low-cost items. Companies are focusing on passive UHF tags, 

which cost under a 50 cents today in volumes of 1 million tags or more. Their read range isn't as far -- typcially less than 20 feet vs. 

100 feet or more for active tags -- but they are far less expensive than active tags and can be disposed of with the product packaging

Most countries have assigned the 125 kHz or 134 kHz area of the radio spectrum for low-frequency systems, and 13.56 MHz is used around the world for high-frequency systems. But UHF RFID systems have only been around since the mid-1990s and countries have not agreed on a single area of the UHF spectrum for RFID. Europe uses 868 MHz for UHF and the U.S. uses 915 MHz. Until recently, Japan did not allow any use of the UHF spectrum for RFID, but it is looking to open up the 960MHz area for RFID. Many other devices use the UHF spectrum, so it will take years for all governments to agree on a single UHF band for RFID. The government also regulates the power of the readers to limit interference with other devices. Some groups, such as the Global Commerce Initiative, are trying to encourage governments to agree on frequencies and output. Tag and reader makers are also trying to develop systems that can work at more than one frequency, to get around the problem.

Different frequencies have different characteristics that make them more useful for different applications. For instance, low-frequency tags are cheaper than ultra-high frequency (UHF) tags, use less power, and are better able to penetrate non-metallic substances. They are ideal for scanning objects with high-water content, such as fruit, at close range. UHF frequencies typically offer a better range and can transfer data faster. But they use more power and are less likely to pass through materials. And because they tend to be more "directed," they require a clear path between the tag and reader. UHF tags might be better for scanning boxes of goods as they pass through a bay door into a warehouse. It is probably best to work with a consultant, integrator or vendor that can help you choose the right frequency for your application

RFID uses the low-end of the electromagnetic spectrum. The waves coming from readers are no more dangerous than the waves coming to your car radio

The read range of passive tags (tags without batteries) depends on many factors: the frequency of operation, the power of the reader, interference from metal objects, or other RF devices. In general, low-frequency tags are read from a foot or less. High-frequency tags are read from about three feet and UHF tags are read from 10 to 20 feet. Where longer ranges are needed, such as for tracking railway cars, active tags use batteries to boost read ranges to 300 feet or more

Our UHF-105 and UHF-105 UHF long-range reader, it supports read-write or read-only. With read-write chips, you can add information to the tag or write over existing information when the tag is within range of a reader, or interrogator. Read-write tags usually have a serial number that can't be written over. Additional blocks of data can be used to store additional information about the items the tag is attached to. Some read-only microchips have information stored on them during the manufacturing process. The information on such chips can never be changed. Other tags can have a serial number written to it once and then that information can't be overwritten later.