There are several antenna options available in RFID systems that use 435 MHz, 2.45 GHz, and 5.8 GHz frequencies. The gain of such a small antenna is limited, and the size of the gain depends on the type of radiation pattern. The omnidirectional antenna has a peak gain of 0 to 2dBi; the gain of a directional antenna can reach 6dBi. The size of the gain affects the range of the antenna. The first three types of antennas in the following table are linearly polarized, but microstrip antennas can be circularly polarized, and logarithmic spiral antennas are only circularly polarized. Since the directionality of the RFID tag is uncontrollable, the reader must be circularly polarized. A circularly polarized tag antenna can generate a signal with a strength of 3dB.
For maximum power transmission, the input impedance of the chip behind the antenna must match the output impedance of the RF antenna.
For decades, antennas have been designed to match 50 or 70-ohm impedance, but it is possible to design antennas with other characteristic impedances. For example, a slot antenna can be designed to have an impedance of several hundred ohms. The impedance of a folded dipole can be 20 times the impedance of a standard half-wave dipole. The lead-out point of the printed patch antenna can provide a wide range of impedance (usually 40 to 100 ohms).
It is very important to choose the type of antenna so that its impedance can match the input impedance of the tag chip. Another problem is that other objects close to the antenna can reduce the return loss of the antenna. For omnidirectional antennas, such as dual dipole antennas, this effect is significant. Changing the distance between the double dipole antenna and a can of ketchup made some actual measurements, showing some changes.
Other objects have similar effects. In addition, it is the dielectric constant of the object, not the metal, that changes the resonance frequency. A plastic bottle of water reduces the minimum return loss frequency by 16%. When the distance between the object and the antenna is less than 62.5mm, the return loss will result in an insertion loss of 3.0 dB, while the free space insertion loss of the antenna is only 0.2dB. The antenna can be designed to match the situation close to the object, but the behavior of the antenna is different for different objects and different object distances. It is not feasible for omnidirectional antennas, so design antennas with strong directional characteristics that are not affected by this problem.
The antenna model was tested in a non-reflective environment, including various objects that need to be labeled, and the performance was severely degraded when using an omnidirectional antenna. The performance degradation caused by the cylindrical metal can be the most serious. When the distance between it and the antenna is 50mm, the return signal drops by more than 20dB. When the distance between the center of the antenna and the object is 100-150mm, the return signal drops by about 10 to 12dB. At a distance of 100mm from the antenna, several bottles of water (plastic and glass) were measured, and the return signal decreased by more than 10dB. Experiments on the liquid in the wax carton, and even on the apple, got similar results.
When using a handheld instrument, a large number of other nearby objects seriously distort the radiation pattern of the reader antenna and tag antenna. This can be calculated for the operating frequency of 2.45GHz, assuming a representative geometric shape, compared with free space, the display return signal is reduced by 10dB, and when dual antennas are used at the same time, the reduction is more than expected. In the warehouse environment, it is a problem to have one label for a box of goods. Several labels are attached to a box to ensure that one label is visible at all times. The use of portable systems has several antenna problems. Two antennas per box are sufficient for detection by the access control device, so the influence of the local structure becomes no longer important because the card reader antenna of the access control device is fixed at the entrance and exit of the warehouse and points directly to the labeled object.
The gain of the RFID antenna and whether to use an active tag chip will affect the distance of the system. Optimistically, when the radiation intensity of the electromagnetic field meets the relevant UK standards, full-wave rectification under the passive condition of 2.45GHz, the driving voltage is not more than 3V, the optimized RFID antenna impedance environment (impedance 200 or 300 ohms), the operating distance It is about 1 meter . If the WHO restriction  is used, it is more suitable for global use, but the range is reduced by half. These limit the electromagnetic field power from the reader to the tag. The working distance decreases as the frequency increases. If you use active chips, the range can reach 5 to 10 meters.
Omni-directional antennas should be avoided in tags, but directional antennas can be used, which have less radiation pattern and return loss interference. The choice of antenna type must match its impedance with free space and ASIC. It seems infeasible to use an antenna in a warehouse unless an active tag is used, but in any case, the antenna radiation pattern in the warehouse will be severely distorted. The use of an access control system would be a good choice. Passive tags with short-range can be used. Of course, the access control system is more expensive than the hand-held instrument, but the hand-held instrument staff needs to use it to search for items in the warehouse, and the personnel cost is also expensive. In the access control system, each item box requires only 2 instead of 4 or 6 RFID tags.
The above information is provided by the RF antenna Factory.