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What You Need To Know
The RFID reader antenna transmits a wave that has both electrical and magnetic properties and is known as an electromagnetic wave. There are 3 different types of RFID antennas that we will describe.
The electromagnetic wave propagates entirely in one plane (Vertical or Horizontal) in the direction of the signal propagation. This is the best wave propagation when the tag orientation is known and fixed. The RFID antenna and RFID tag should be matched in polarization to obtain the best read rates.
The electromagnetic wave propagates in two planes creating a circular effect (like a corkscrew) making one complete revolution in a single wavelength time-frame. Since the RFID antenna continuously emits a wavelength the rotational field will eventually cover any tag that is in its path. This is best to use when tag orientation is unknown, but you lose at least 3dB when compared to a linear polarized antenna. Circular polarization can be right or left handed hence the RHCP and LHCP options for circular polarized antenna.
Monostatic is the most common RFID antenna and uses a single common port to transmit and receive signals.
Bistatic uses 2 RFID antennas in the same physical housing and uses one port to transmit and the other port to receive.
Monostatic RFID readers can be 1, 2 or 4 port readers. Sometimes there is also an LBT (Listen Before Talk) port making it a 5 port reader. On the monostatic readers the port transmits first and then receives signals on the same port. Bistatic RFID readers usually have 8 ports - 4 transmit and 4 receive so that each port is always active either transmitting or receiving signals. RFID antennas are tuned to resonate only to a narrow range of carrier frequencies that are centered on the designated RFID system frequency. RFID systems use the decibel (dB) to describe antenna gain, cables losses and power output for all hardware specifications and regulations. The Decibel is a ratio between two signal strength levels and is a 10th of a Bel. Bel is named after Alexander Graham Bell (hence the letter B is capitalized). Incidentally Bell invented the telephone; as well as numerous other inventions. These calculations are logarithmic scale measurements hence they use the logarithm of a physical quantity instead of the quantity itself!
Bel = log (P2/P1)
dB is also a logarithmic measurement and gives simple numbers for large-scale variations in signal strength. This is very useful as you can easily calculate the RFID system gain and losses by adding and subtracting whole numbers.
The dB unit allows big variations in signal strengths/levels to be handled with simple math.
Gain is Positive and Loss is Negative
When you attach a reader to an antenna you use coaxial cable. Depending on the quality and length of the cable you will lose a certain amount of power between the reader and the antenna. This is known as line loss. Higher quality (low noise) cable reduces the amount of line loss and allows for longer cable runs while providing maximum power to the antenna from the reader. The antenna usually provides gain to make up for what the line loss is between the reader and the cable.
A 3 dB gain on an antenna is equal to a 2 times increase in the signal strength that came from the end of the cable attached to the antenna. Likewise a 3dB loss such as using a circular polarized antenna over a linear antenna with the same reader power means a loss of about 50% of the signal strength. The 3 dB loss using circular polarization is usually made up with a gain of at least 6 dBi on the antenna to bring back the 2 times increase.
A 10 dB gain on an antenna is equal to a 10 times increase in the signal strength. A 10 db loss means you lose 90% of the signal strength.
A 20 dB gain is equal to 100 times increase. A 20 db loss is equivalent to losing 99% of your signal strength. If you are not careful when you put together an RFID system you can easily exceed the legal power limits.
The antenna size and gain are also proportional so the higher the gain in dB on the antenna the bigger the antenna is! Likewise small antenna equals lower gain.
The RFID antenna propagates the wave in both vertical and horizontal dimensions. The field coverage of the wave and also its signal strength is partially controlled by the number of degrees that the wave expands as it leaves the antenna. While the higher number of degrees means a bigger wave coverage pattern it also means lower strength of the signal.
Azimuth (AZ) is the horizontal radiation plane of the antenna wave and uses degrees to depict the amount of expansion horizontally from the centerline of the antenna at a maximum variation of 3 dB .(Deviation of this 3 dB is also known as Beamwidth - so you will experience a 50% loss of signal strength when near or at the 3 bB Beamwidth angle) Elevation (EL) is the vertical radiation plane of the antenna and also uses degrees to depict the amount of expansion vertically from centerline of the antenna at a maximum variation of 3 dB. Note the same signal strenght information applies for the Elevation Beamwidth as the Azimuth Beamwidth. Both Linear and circular antenna can have different AZ and EL degrees thus providing different read patterns depending on your requirements. The larger the number of degrees the wider the wave read zone and thus the lower the wave signal strength. The RFID antenna gain and the radiation plane width are mutually dependent and the larger the gain, the narrower the radiation plane. Where the AZ and EL are not identical in degrees the wider side controls the narrower plane. The strength coverage between the AZ and EL planes is called Axial Ratio Level and is expressed in dB and indicates the degradation value between the maximum gain (usually at centerline) and the AZ or EL plane. Once we know the read zone that we wish to create we can now calculate the basic AZ and EL degrees that could form the zone for our tag reading requirements. A typical high quality linear Azimuth and Elevation radiation pattern could look like this:
Whereas a typical high quality circular Azimuth and Elevation radiation pattern could look like this:
VSWR is also an important consideration in antenna selection. VSWR is the Voltage ratio of the amplitude of a partial standing wave at the maximum compared to the adjacent wave at its minimum. For those who are really interested in the full details please email us for the 8 page explanation! Suffice to say the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. Generally speaking the lower the ratio to a 1:1 the better antenna efficiency of transmitted power - theory only!
Axial Ratio is defined as the circular polarization of the RFID antenna - only for circular not linear antenna. This is the ratio between the maximum and minimum linear gain. Low Axial Ratio antennas allow for more tags to be read with the same power output of another antenna with a higher Axial Ratio. So look for low axial ratios when you need to read more tags in the read zone.
Now that you have a basic understanding what the types of antennas are and what those strange numbers and ratios mean, you have the capability of predetermining what you need to obtain a good read rate before you start your implementation. Using the same antenna in different circumstances does not provide the best read rates or the required read zone. What you also need to understand when you implement an RFID system is that any reflection, diffraction or absorption changes the theoretical read zone and any other waves such as a cellphone, power lines, WiFi, motors, engines - virtually anything that propagates a wave can and will cause signal strength degradation.
We hope this information assists you in the understanding of RFID Antennas. We also understand that this information can be very confusing as there is a lot to understand in order to determine your antenna requirements. Simply put all antennas are not created equally and you should have an expert such as Cloud Collected assist you with your antenna selection to maximum the read rate for your specific RFID implementation.