## 2 Quasi-optical Design of the MMW Channel, Calculations and Alignment  
The whole quasi optical design for the realization of MMW communication PTP link is shown in Figure 1. This unique design is composed of two standard TV satellite dishes, 100 GHz MMW source, 2 Arduino Uno microcontroller, GDD detector circuit shown in Figure 2 and an oscilloscope. The 100 GHz source (TX272 VDI) used here was manufactured by Virginia Diodes Inc (VDI). This MMW source in the W band is based on three GaAs frequencies and has a multiplication Factor of 8 to its local oscillator frequency range of 12.625-13.625 GHz [24]. The maximum output power of that source is 600 mW at 100 GHz. The encoded message bits were generated by the user and were transmitted from the Arduino microcontroller Tx pin to the TTL modulation input of the MMW source (0 to 5 V). The maximum TTL modulation rate of this MMW source is 100 MHz. Figure 1: Block diagram of the experimental setup of MMW projection system. The GDD circuit contains an amplifier with voltage gain of 5, a dc block capacitor and a resistor \(R_{L}\). The modulated radiation generated by the MMW source is coupled out to free space using a Virginia Diodes Conical horn antenna and a satellite dish antenna coated with aluminum. The horn antenna specifications are as follows: horn length L\({}_{\text{Gh}}\) = 35.5 mm and aperture diameter \(D_{h}\) = 16.3 mm. The total cost of the two dish antennae and the aluminum tape is less than 50 $. Figure 3 shows photos of those coated antennas. The receiver side of the MMW channel is based on a similar dish antenna. Calculations of the expected beam spot on the transmitter dish, receiver dish, and the GDD is shown next. In those calculations only the fundamental Gaussian mode (TEM\({}_{00}\)) is being considered. This is since, according to VDI specifications for typical conical horn antenna that was used in this demonstration, 87% of total power is transmitted to free space in the fundamental Gaussian mode [25; 26; 27; 28; 29]. The initial alignment of the quasi-optical set up shown in Figure 3 was carried out using a laser source that was placed instead of the MMW source. The laser beam is assumed to be Gaussian with an irradiance profile that follows an ideal Gaussian distribution. Further alignments were performed by adjusting the position of quasi-optical dishes and the GDD in order to receive a stronger MMW signal. The MMW source horn antenna was located at the focal point of the transmitted dish (\(F_{disl}\) = 600 mm). The Gaussian  
Figure 3: The dish antennas setup for the receiver (**a**) and transmission side (**b**) in the experiment. (**c**) Demonstration of the alignment process. Figure 2: The glow discharge detectors (GDD) electrical circuit. beam approximation was used to calculate the beam waist (Equation (1)), and the total power collected by the receiving dish is calculated in Equation (2). \[\omega(z)=\omega_{0}\sqrt{1+\left(\frac{z}{z_{R}}\right)}\ z_{R}=\frac{\pi a \omega_{0}}{\lambda} \tag{1}\]  
\[P=P_{0}(1-e^{-\frac{D^{2}}{2\omega(z)^{2}}}) \tag{2}\]  
where \(\omega(z)\) is beam waist, \(z\) is the distance of propagation, \(\lambda\) is the MMW wavelength, \(Z_{R}\) is the Rayleigh range of a Gaussian beam, \(P_{0}\) is the total beam power emitted from the MMW source, and \(D\) is the diameter of the receiving element. The diameter of the transmitting and receiving dishes is 800 mm. Equation (2) can be used to calculate the power collected by the transmitting and receiving dishes and also by the detector (assuming a circular detecting aperture). To use Equation (2), the MMW beam waist has to be calculated in those positions. According to VDI's nominal horn specifications the MMW beam waist radius refers to the input of the conical horn antenna and is fixed by that geometry to \(\omega_{0}=6.2\) mm [25; 26; 27; 28; 29]. The MMW source was placed in the focal point of the transmitter dish and the GDD was placed at the focal point of the receiving dish (see Figure 1).