Circularly polarized patch antenna with high Tx / Rx-separation

Abstract

In this contribution an antenna is described that allows simultaneous transmission and reception in the same frequency band. Circular polarization is employed for a most reliable communication with radio frequency identification (RFID) transponders that typically have linearly polarized antennas. To achieve high separation between transmitted and received signals, a square patch antenna originally transmitting horizontally and vertically polarized radiation is combined with a 3 dB-hybrid circuit. With this hybrid circuit the antenna can simultaneously radiate a right-hand circularly polarized wave and receive a left-hand circularly polarized wave. Furthermore, the transmit signal that is unintentionally leaking into the receive path can be compensated by tuning the hybrid circuit with two variable capacitance diodes. At 866 MHz a maximum Tx / Rx-separation of 65 dB was achieved in a static scenario. In a time variant indoor scenario with a metal object moving on a conveyor belt a Tx / Rxseparation of more than 52 dB was achieved by continuously tuning the hybrid circuit with a minimum-search algorithm.

Isolation Optimization Method on the Coupler

“…The length (L) of the coupler usually equal to a quarter of wavelength which propagates in material as follow [10,11]:…”

Improvement Branch Line Coupler Isolation in S Band Frequency

“…The length (L) of the coupler usually equal to a quarter of wavelength which propagate in material as follow [10][11]:…”

“…This is to obtain the right signal by means of the sender and receiver in a single carrier frequency. In [11], branch-line coupler was used as a separator between the sender and receiver, branch-line divides the input power coupler on one port into two ports are the same as the phase difference +90 o or -90 o . This can be used to acquire the right side (right hand side) and the left side (left-hand side) circular polarization by using two cross-polarization antenna input.…”

“…This can be used to acquire the right side (right hand side) and the left side (left-hand side) circular polarization by using two cross-polarization antenna input. In [11], it is also used two variable capacitance diodes on a circuit which is useful as tuning to obtain the desired operating frequency. The simulation obtained an isolation improvement of -52 dB [12].…”

Evaluation of passive carrier-suppression techniques for UHF RFID systems

“…Although very high performance can be expected from advanced techniques like active carrier cancellers [1] or specialized active antennas [2], descent carriersuppression can be achieved with a circulator, directional coupler, or-even simpler-separate antennas for transmitting and receiving. In many cases, passive techniques are sufficient whereas in advanced interrogator frontend designs they are usually succeeded by active carrier cancellers and carriersuppression algorithms that are performed in digital baseband hardware.…”