# 3D Printed High Gain Complementary Dipole/Slot Antenna Array
## 1 Introduction  
Fifth generation (5G) wireless communications standard in the US was announced in July, 2016 by the Federal Communications Commission (FCC), which would provide a higher data rate than today's Fourth Generation (4G) wireless networks. A particular spectrum that is in the higher bands for the new Upper Microwave Flexible Use Service is in 28 GHz (from 27.5 GHz to 28.35 GHz). This millimeter-wave band for 5G cellular network was demonstrated by [PERSON] and his research team as early as in 2013 [1]. To ensure a sufficient link budget at millimeter-wave band, high gain antennas are required. The waveguide slot antenna is normally used as the basic element to realize a high gain antenna array. The slot element possesses advantages of low cross polarization, high radiation efficiency and planar features, whereas it mainly suffers from the drawback of a narrow impedance bandwidth. Applications for using slot-waveguides for optical devices, such as demultiplexers and power splitters, have been proposed [2; 3; 4]. These are more convenient for combining the device to the antenna input. The fractional impedance bandwidth of most slot antenna arrays is in the order of a few percent. Some improved designs have achieved more than 10% [5; 6; 7; 8]. In Reference [9], a planar waveguide slot antenna array in the Ku-band was developed with 256 slot elements, where the basic radiating unit is a cavity-backed 2 \(\times\) 4 slot subarray. Therefore, 32 subarrays were grouped together and connected by an equal amplitude and equal phase parallel waveguide feed network. Through the use of the 2 \(\times\) 4 slot subarray, the design of the feeding circuitry was simplified significantly, as the subarray with 8 slot radiators were concurrently excited by a common backing cavity. This design is also helpful for achieving higher aperture efficiency as the single subarray showed an 18 dBi gain performance, much higher than that of a single slot element. The measured efficiency of the array was around 85% which is higher than the printed microstrip array technique [10; 11]. In addition, the measured impedance bandwidth was 12% (Standing Wave Ratio (SWR) <2), which is relatively wide for the waveguide slot antenna. For the 5G waveguide slot antenna array operating from 40.5 GHz to 43.5 GHz [6], it has 24 \(\times\) 24 elements with a 33.8 dBi maximum gain, and a gain variation of 2.1 dB. The obtained impedance bandwidth was 5.5% (SWR <2), where each subarray consisted of 6 \(\times\) 6 radiating slots. Another design by [PERSON] and his research team was demonstrated at 60 GHz with a 32 dBi gain over a 4.8 GHz bandwidth [12]. A full-corporate-feed network was integrated to the antenna array to achieve wideband performance, in comparison to the series-feed design. The measured fractional impedance bandwidth is 8.3% for reflection coefficient <\(-\)14 dB, which is acceptable at such a high frequency band. A wide slot radiator was used as the basic radiating unit with an aspect ratio of 0.64. By applying the concept of the complementary antenna [13; 14; 15; 16; 17; 18; 19] for each radiating element, an electric dipole, consisting of two metallic cuboids, was introduced to each slot of the waveguide slot array in Reference [20] for Ku-band applications. The whole structure was constructed using aluminum. The element can be considered a simplified version of the Clavin element [13]. In this paper, the complementary dipole antenna array is enhanced for 28-GHz applications and ease of fabrication using 3D printing and electroplating. Metallization of 3D printed W-band bandpass filters has been demonstrated in Reference [21]. The metallization process consisted of electroless plating followed by electroplating. In this paper, the electroless plating is replaced by simple coating of a thin copper film on the 3D printed surface using a spraying gun. This process has the advantages of low cost, light weight, and stable gain within the operating band.