Antenna
http://www.enjoyrfid.com/ 2016-12-17 11:42:23 From：Enjoyrfid Technical Hit：
Both a reader and a tag have an antenna. To enable data communications each must be able to receive some of the transmitted RF energy from the other so the information can be recovered, from the RF carrier, and used.
A simple whip antenna radiates RF energy in most directions, similar to the way a light bulb radiates light energy in most directions. But…
Some types of antenna focus the radiation into a beam. If you send the energy only toward the tags, and you don’t waste any energy by sending it in directions where there are no tags, you increase the RFID range.
Have you seen the Yagi antennas with the directing cross bars on them? Those direct the energy into a beam. The more directors an antenna has, the narrower, or more focused, is the beam. But the narrower the beam, the more accurately you have to aim it, as the target area is smaller and the tags have to be closer together.
But you can’t just add a highly directional antenna to your RFID reader and focus all your power into a narrow beam to increase the RFID range. Any other radio receiver within the influence of this concentrated beam might also be influenced… adversely.
An intensely focused RF transmission from far away might overpower a weaker local RF signal.
You need to limit the power you transmit from the antenna and this depends on the type of antenna you use.
Regulations in the USA limit the transmit power in the UHF band to 4Watts EIRP.
EIRP is the Effective Isotropic Radiated Power and is the amount of focused power (the beam) transmitted from a directional antenna. This power is simply the amount of energy that you’re legally allowed to radiate from your antenna, every second.
If the antenna radiates in all directions (isoptropically), you could put 4 Watts in to the antenna and the EIRP would be 4 Watts out.
But if the antenna focuses all this energy into a beam that has say 10x the intensity it would otherwise have, then if you put 4 Watts into the antenna you would effectively be radiating 4×10 = 40 Watts EIRP out of it (in the beam) and be violating the regulation in the direction the antenna is pointing.
Antennas don’t just transmit, they also receive.
To recover the ID information from a tag the RF signal has to be stronger than any unwanted background electrical noise. If a signal is weak, you may need a high-gain antenna to capture enough RF energy…
and a sensitive receiver to put this into. This way you may get enough RF energy to enable you to recover the ID information.
A simple whip antenna radiates RF energy in most directions, similar to the way a light bulb radiates light energy in most directions. But…
Some types of antenna focus the radiation into a beam. If you send the energy only toward the tags, and you don’t waste any energy by sending it in directions where there are no tags, you increase the RFID range.
Have you seen the Yagi antennas with the directing cross bars on them? Those direct the energy into a beam. The more directors an antenna has, the narrower, or more focused, is the beam. But the narrower the beam, the more accurately you have to aim it, as the target area is smaller and the tags have to be closer together.
But you can’t just add a highly directional antenna to your RFID reader and focus all your power into a narrow beam to increase the RFID range. Any other radio receiver within the influence of this concentrated beam might also be influenced… adversely.
An intensely focused RF transmission from far away might overpower a weaker local RF signal.
You need to limit the power you transmit from the antenna and this depends on the type of antenna you use.
Regulations in the USA limit the transmit power in the UHF band to 4Watts EIRP.
EIRP is the Effective Isotropic Radiated Power and is the amount of focused power (the beam) transmitted from a directional antenna. This power is simply the amount of energy that you’re legally allowed to radiate from your antenna, every second.
If the antenna radiates in all directions (isoptropically), you could put 4 Watts in to the antenna and the EIRP would be 4 Watts out.
But if the antenna focuses all this energy into a beam that has say 10x the intensity it would otherwise have, then if you put 4 Watts into the antenna you would effectively be radiating 4×10 = 40 Watts EIRP out of it (in the beam) and be violating the regulation in the direction the antenna is pointing.
Antennas don’t just transmit, they also receive.
To recover the ID information from a tag the RF signal has to be stronger than any unwanted background electrical noise. If a signal is weak, you may need a high-gain antenna to capture enough RF energy…
and a sensitive receiver to put this into. This way you may get enough RF energy to enable you to recover the ID information.
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