## 2 Antenna Design  
Figure 1 shows the proposed olympeak shape mmwave single element design. The antenna is designed on a 0.254 mm ultra-thin RO5880 substrate with a relative permittivity of 2.3. The proposed antenna comprises of ring structures stacked among each other. The circles assembled in such close assembly resembles the \"Olympic\" logo but with an enhanced number of circles. Hence, making a hybrid form which is named as \"olympeak\". The back side of the design comprises of bent corners on each side and a square slot on the middle section which helps in tuning the antenna response to desired frequencies. The design evolution of the proposed olympeak antenna is shown in Figure 2. At the first initial stage, a feed-line with a small single circular ring was presented. In this initial stage, the frequency bandwidth of almost 4 GHz was achieved with a starting frequency of 32 GHz to 36 GHz. This design was further extended with three circular rings intersecting each other in stage 2. The intersecting rings shifted resonance frequency backwards to 30 GHz with a resonance bandwidth of only 2 GHz. The intersecting rings were increased in the proposed stage, by a total of eight rings. The intersecting rings were placed at each side of the lower circular ring and higher circular ring rows. In the proposed stage, the antenna showed a dual wideband response from 24 to 34 GHz and and 37.5 to 41.5 GHz. The s-parameter of the different stages of design evolution can be seen in Figure 2d.  
The impedance matching of the proposed olympeak antenna in evolution stage is shown in Figure 2e. It is seen on the Smith chart that as the number of intersecting rings were increased, the voltage standing wave ratio improved. In the proposed design, additional resonances were introduced to enhance the operational bandwidth of the single antenna by optimizing the intersecting rings.  
The surface current patterns at two resonances of 28 and 38 GHz are shown in Figure 3. At both resonances it can be observed that the currents are focused on the outer ring edges and ground slot which circulates strong currents. The holes etched for connectors have an almost uniform current distribution.  
Figure 1: Olympick antenna (**a**) front and (**b**) back.  
Figure 2: Design evolution (**a**) stage 1 (**b**) stage 2 (**c**) proposed (**d**) reflection co-efficient (**e**) Smith chart.  
The effect of the ground plane notch etched at the top mid-section of the copper ground plane is given in Figure 4. The ground slot played an important role in shifting the resonance response to desired frequency. At 1.8 mm slot width, the maximum resonance was noted at 27.5 GHz which at 1.9 mm resulted in a very sharp dip, hence giving a reflection co-efficient value of \(-45\) dB at the same frequency. With a further gradual increase of 0.1 mm, the optimum response was noted at 2 mm. Further increasing the value resulted in a diminishing resonance response.  
The radiation and total efficiency of the antenna is \(>\)82% in the operating band. The peak gain is noted to be 4.9 dBi while on 28 GHz its 4.2 dBi with radiation and total efficiency of 94 and 95%, respectively.  
The performance parameters of the antenna mentioned above are shown in Figure 5. The radiation pattern at two principle plane \(xy\) and \(zx\) plane is shown in Figure 6. At the \(zx\) plane, the main lobe direction is at \(0^{0}\) with 3 dB angular width of \(85^{0}\) while for the \(xy\) plane, the main lobe is slightly tilted to \(358^{0}\) with a side lobe level of \(-1.3\).  
Figure 4: Ground slot effect on resonance response of the olmpeak antenna.  
Figure 3: Surface currents of the olmpeak antenna at a resonance frequency of (**a**) 28 GHz (**b**) 38 GHz.