## 2 Materials and Methods  
There are many universal antennas that can be used for a very wide range of different applications. For many others, however, it is necessary to design special antennas that meet and guarantee functionality in given conditions. Several professional publications are devoted to the design of antennas [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. As can be seen, the physical properties of the antenna and the reader play a very important role in the design of the RFID system. [PERSON] et al. [11] proposed an antenna structure which achieves free space and an on-metal reading range of better than 8 m and 11 m, respectively, in the US RFID band of 902-928 MHz. Their design topology brings some benefits such as bandwidth enhancement in the single band and the possibility of attaining a tunable dual band coverage in the UHF RFID band. [PERSON] et al. [12] presented an overview of antenna design for passive UHF RFID tags. The various requirements have been discussed. Additionally, [PERSON] [13] presents a homogeneous survey of relevant methodologies for the design of UHF passive tag antennas. [PERSON] et al. [14] discuss the basics of antenna design and theory. Finding the requirements for antenna in a radio frequency system is very difficult task. These requirements or antennas' parameters can be various even for similar RFID applications in the same frequency band. [PERSON] et al. [15] studied the effects of nearby objects on the read range of several types of RFID tags, and the impedance, pattern and radiative efficiency of antennas that closely emulate the tag structures, using measurements and simulations. They present that detuning of the load impedance of antenna plays an important role only when the tag is very close to the object. Other authors [16; 17; 18; 19; 20; 21; 22] also describe procedures for antenna design, describe how to properly tune the antenna for a given type of application, etc. For the antenna, we are mainly interested in the impedance, reflection coefficient, radiating directional characteristic, gain, bandwidth, as well as the polarization of the antenna. Parameters of used antenna are listed in Table 1. The key parameters of the reader include the values of transmission power and sensitivity, which indicates the minimum value of the received signal that the reader can detect. To read the tag correctly, it is necessary to meet two basic conditions. The first condition must ensure sufficient transmitted power to activate the tag. The second necessary condition is that the power reflected by the tag is large enough for the reader to record it. Free-space path loss (FSPL). To calculate the attenuation of the signal (electromagnetic wave) due to the scattering of the wave into free space, we use the following formulas:  
\[FSPL=\left(\frac{4\pi d}{\lambda}\right)^{2}=\left(\frac{4\pi df}{c}\right)^{2}, \tag{1}\]  
Variables are defined in Table 2. To calculate the attenuation of free space in decibels, we use the following relation:  
\[FSPL(dB)=10\ \log_{10}\!\left(\frac{4\pi df}{c}\right)^{2}=20\ \log_{10}\!\left( \frac{4\pi df}{c}\right), \tag{2}\]  
\[FSPL(dB)=20\ \log_{10}d+20\ \log_{10}f+20\ \log_{10}\!\left(\frac{4\pi}{c} \right), \tag{3}\]  
The calculation of the specific free space attenuation at 4 m, where in the ETSI band (European Telecommunications Standards Institute) it is allowed to transmit on the frequency 865-868 MHz, is therefore the following:  
\[FSPL(dB)=20\ \log_{10}4+20\ \log_{10}865\ 000\ 000-147.55, \tag{4}\]  
The following figure, Figure 1, shows an illustrative case for how important the above-mentioned parameters are to be considered. This is an example where we have signal propagation in free space. The main intended application is the use for identification purposes using a drone in an outdoor warehouse, where there is no permanent obstacle between the read object and the drone. In a real installation, other properties that affect the signal strength play a key role in the whole chain.