# Design of a Double-Matched Cross-Polar Single Antenna Harmonic Tag
## 1 Introduction
[17] proposed a regression model of the impact of external factors acting on the readability of the RFID transponder, and ref. [18] focuses on the impact of the temperature of the environment on the readability of RFID tag, proposing a suitable mathematical model. To reduce the mismatch effect due to the tag placement on objects, it would be appropriate to provide the tag with an adequate shield (ground plane) in order to counteract the electromagnetic coupling between the tag and the object, while, to reduce the camouflage effects due to scattering from the environment, polarization diversity techniques can be used. In challenging application scenarios, harmonic tags present a valid alternative to traditional RFID systems. Simple harmonic transponders have already found utility in diverse applications, including the study of bee behavior [19] and avalanche victim rescue operations [20]. This suitability arises from the inherent robustness of harmonic systems in cluttered environments and their reliance on simple circuit architectures. Nevertheless, one limitation of harmonic systems in comparison to standard UHF RFID transponders is the necessity for transmitting and receiving antennas operating at distinct frequencies on the tag. This requirement can lead to the creation of somewhat unwieldy transponders [21]. Some research efforts have explored solutions based on dual-frequency single antennas [22,23]. These solutions predominantly rely on dipole-like antennas and, more broadly, on omni-directional radiators. When these antennas interact with the materials of the items to which the tag is affixed, they can become detuned, potentially compromising the reliability of the entire system. To generate harmonics from an RF signal, a nonlinear device is used [24]. In the existing literature, various types of active or passive frequency multipliers, such as diodes [25; 26], nonlinear transmission lines (NLTL) [27; 28], transistors [29], or delayed lock loops (DLL) [30], have been suggested to achieve frequency multiplication ranging from 2\(\times\) to 8\(\times\). Schottky and varactor diode-based harmonic generators are ideal from a power consumption perspective due to very minimal or almost no power consumption. This paper presents a novel harmonic tag model based on microstrip antenna technology having dual-band dual polarization with the doubler circuit included inside the antenna. Figure 1 provides a schematic representation of the proposed design. The main novelties are as follows:  
* A new concept of a one-port harmonic transponder matched at both frequencies;
* A new exploitation of the modal behavior of annular ring antenna that allows dual polarization using only one compact antenna;
* The insensitivity of the tag to the object on which it will be placed (e.g., it can be wearable) thanks to the use of microstrip technology. Compared to a dipole-like antenna, an annular ring antenna has the advantage of having a higher gain and being insensitive to where it is placed. The inclusion of dual polarization allows for better distinguishing the tag response from cross-talk phenomena between the receiving and transmitting channels of the reader. In this work, we report a comprehensive study of the design, realization, and measurements of the harmonic tag. The tag incorporates a dual-band harmonic circuit (i.e., a single-port doubled-matched doubler) connected to an annular patch antenna specially modified to achieve both matching and orthogonal linear polarized radiation at two harmonic bands. The orthogonal polarization is a requirement used to mitigate the effect of environmental clutter. Thanks to the ground plane of the antenna, the tag has good flexibility and adaptability for a wide range of wireless sensing applications from wearable sensing to IoT applications. We design the tag to operate at 1.7 GHz and 3.4 GHz, as those bands are allocated for mobile and radiolocation in all European countries participating in the CEPT ECC (Electronic Communications Committee) [31].