A novel filter prototype composed of a slotline resonator and back-to-back microstrip-slotline transition is an eligible candidate for radio-frequency identification (RFID) reader applications. Two microstrip feedlines are placed on the opposite plane, perpendicular to the slotline resonator, which is further twisted to an M shape to reduce the overall filter size, thus assigning two transmission zeros towards the out-of-band signal, thereby suppressing the spurious passband. The filter has been fabricated in microstrip line and simulated by CST Microwave Studio software. The dimensions of the module can be controlled within 16 × 15 mm, and the results exhibit a passband from 5.3 to 5.9 GHz with mid-band insertion loss of 0.5 dB and group delay variation ,0.5 ns.Introduction: Recently, wireless communication and personal area network technology have been evolving rapidly and wide markets have been growing and developing, RFID specially. RFID uses RF signals for automatic identification of objects, through readers and tags, which can provide effective and exact data for tracking, supply chain management, security access to controlled areas and so on in commercial systems. RFID has been emerging over the past decade as a substitute for the barcode, and offers numerous outstanding advantages such as: rewritable data; greater storage capacity of data; security; can still be read in a harsh or dirty environment; long lifetime; greater reading range; high range resolution; low cost; and compact size [1]. So it is rapidly becoming the preferred technology for a whole range of applications [2,3]. Globally, each country has its own frequency allocation for RFID. The existing RFID bands are: 135 kHz, 6.78 MHz, 869 MHz, 915 MHz, 2.45 GHz, 5.7 GHz etc. [4].RFID is a short-range radio technology used to communicate mainly digital information between a stationary location and a movable object or between movable objects. RFID is generally characterised by use of tags or transponders on one end of the link and readers or interrogators on the other end of the link. Tags can be powered by a battery (active tag) or by rectification of the radio signal sent by the reader (passive tag) and reflecting it. A tag is composed of a transponder chip and a matched antenna enabling it to either capture electromagnetic energy from an interrogating field or communicate with an RFID reader; the two operating frequencies should be matched. Longer-range systems often use high transmitted power, higher frequencies, and active or semi-passive transponders for range enhancement [5]. Using subcarrier load modulation, there are two sidebands including useful signals at the transmission frequency of the reader, which when filtered and demodulated, reproduce the fundamental frequency signal of the modulated data stream. So they should have a bandpass filter to ensure that the radio signals match with the transponder.Reference [6] discussed the transmission zeros that can be properly generated at the frequency points where the transmission coefficients ...
