Antenna

As used here, an antenna acts as a transmitter and receiver of radio waves. In the 900Mhz passive RFID spectrum, the radio wave is approximately 12" in length the optimal antenna size needs to be this large. This is the primary reasons handheld RFID readers don't function well -- a 12" antenna is just too big to carry around.

Pro-Tips

- Some RFID readers use a single antenna for both send and receive. This is known as monostatic. Other RFID Readers use a separate

- - transmit/receive antenna. This is known as bistatic. Bistatic typically has higher performance than mono-static because they create obtuse angles on the RFID tags and this gives a better RF bounce for better reception. Monostatic is easier because a single antenna does both jobs. With todays RFID Tag performance, there is little reason to select a bistatic solution except on the most complex installations.Bistatic: ThingMagic M4 and M5 RFID Reader
  - Monostatic: ThingMagic Astra RFID Reader

- Wavelength: 900Mhz waves are about 12" -- optimally sized antennas will be this large.
- Polarization: Antennas can be circular or linear polarization (aka horizontal or vertical polarization).
  - Circular: Most mounted 'flat panel' antennas will be circular polarization. These sacrifice read range for ease of tag orientation. Circular antennas can see a tag in nearly any orientation. Read range is typically 30% lower than linear polarization.
  - Linear: Few antennas use linear polarization. When radio waves are in linear fashion, a tag more than 45 degrees from polarization will be completely invisible to that antenna. Linear polarization has much longer read range than circular and it's much easier to pinpoint a target with a linear antenna. Linear antennas are often used in handhelds to get better read range and to help locate tags by limiting the number of tag reads. A yagi antenna is an example of a linear antenna. Horizontal/Vertical polarization are just relative to the 'top' of the antenna.
- Radiated Power (ERP) for passive RFID in the United States, the current mandate is no more than 4 Watts ERP from the antenna. An antenna can be tuned to project higher / lower amounts of RF in a wider or more narrow cone. Most passive RFID systems push this to the maximum allowed. The maximum power a reader can transmit is 1 Watt -- the antenna may focus that energy to 4Watts. Using lower power, lowers the read range of RFID.