## 2 Design of the Printed Patch Antenna
### Substrate Properties of Patch Antennas  
When designing and manufacturing a patch antenna, it is very important to know the properties of the selected dielectric substrate. Currently, many substrates made of nonconductive bendable dielectric materials are available. These substrates are divided into several basic groups: semiconductor, synthetic, ceramic, ferromagnetic, and composite. In the selection of a substrate, its properties play a major role, namely relative permittivity \(\varepsilon_{r}\), relative permeability \(\mu_{r}\), loss factor \(tg(\delta)\), their dependence on temperature and frequency, substrate homogeneity, chemical resistance, thermal expansion, operating temperature range, material aging, flexibility, strength, and processability of the substrate [10,11].  
Figure 1: Description of a patch coplanar dipole: (**a**) view from above; (**b**) side view.  
In the production of microwave antennas, a dielectric substrate with a relative permittivity value ranging from 2.2 to 16 and a loss factor ranging from 0.0001 to 0.06 is mostly used. With a high value of permittivity \(\varepsilon_{r}\), the efficiency usually decreases. The thickness of the dielectric substrate \(h\) tends to be much smaller than the wavelength value [11]. An increase in antenna bandwidth and efficiency can be achieved by using a substrate with a bigger height \(h\). However, if the substrate thickness limit is exceeded, surface waves may be excited. This undesirable phenomenon will cause a reduction in antenna efficiency since a part of the power is consumed to excite these waves [11]. Thin substrates with high permittivity are generally desirable for antennas to minimize parasitic radiation and to make the size of the conducting elements small. However, due to higher losses, they are less efficient and have relatively small bandwidths. On the other hand, substrates with large \(h\)-thickness and small permittivity are more suitable for good antenna performance. Such substrates will provide better efficiency and greater bandwidth [11].  
For the production of the three patch antennas, two traditional substrates (FR4, Arlon 25N) and one 3D LDM printer material were selected. The relative permittivity \(\varepsilon_{r}\) and loss factor \(lg(\delta)\) of the Arlon 25N and FR4 substrate were known. The relative permittivity \(\varepsilon_{r}\) and loss factor \(lg(\delta)\) for the 3D printer dielectric material are described in Table 1. This material (substrate) will hereinafter be referred to as 3D material.  
The selected geometry for the tested patch antenna (radiation patch and ground plane) has an important characteristic: the influence of the dielectric substrate has minimal effect on the radiation characteristics of the antenna itself. For this reason, it was also possible to use FR4 material, which was due to the poor properties of the substrate itself, a completely unsuitable material for other geometrical parameters (radiation patch on one side and the ground plane on the other side). In our case, the FR4 material was deliberately selected for the option of using CNC production technology. This technology is cheap; however, it also has its limits in accuracy. For example, the CNC cutter we used had a production accuracy of 0.1 mm.  
The table clearly shows that the substrate properties of the 3D material are very good and suitable for the production of a patch antenna.