Candidate: Nazanin Rahmati
Title: Designing a Tightly Coupled Ultra-wideband Millimeter-wave Phased Array Antenna
Date: July 26, 2024
Time: 2:00 PM
Place: EIT 3142
Supervisor(s): Majedi, Hamed - Borji, Amir
All are welcome!
Abstract:
The new emerging wireless network technologies of 5G and 6G have attracted a lot of interest in ultra-wideband phased array antennas covering wide range of unlicensed mm-Wave frequencies specially in the range of 24 GHz-71 GHz. Furthermore, in other multifunctional wireless communication systems such as wideband high-resolution radars or high speed satellite internet, a low profile ultra-wideband (UWB) array antenna realizing multiple functions within one single radiating aperture is in great demand. Tightly Coupled Antenna Arrays (TCAs) have received significant attention in recent years as a viable candidate for ultra-wideband phased array implementation. Interestingly, the strong capacitive coupling among elements in a TCA can compensate for the inductive effect of the ground plane at low frequencies and increase the VSWR bandwidth.
Despite the wideband capability of Tightly Coupled Dipole Arrays (TCDAs), a low profile, wideband balun is required for each antenna element to provide balanced feeding and prevent from the common mode resonance which can disturb their wide bandwidth characteristic. Designing a low profile, wide band balun that can fit in each TCDA’s small unit cell area specially at high mm-Wave frequencies above 20 GHz would be challenging. The wideband Marchand baluns are a good solution; however, their through plane assembly at high frequency would be challenging and will make the planar TCDA, high profile in thickness (depth). To overcome the fabrication challenges, a planar Marchand balun was proposed in literature; however, the antenna thickness was still large (more than 1.3mm) to be used in System on Chip (SoC) devices. In this project, the planar Marchand balun is modified to design a thin (0.715mm thickness), planar, UWB antenna element that is optimized to cover the whole frequency bands required for various 5G standards from 24 GHz to 71 GHz. The antenna element is simulated in an infinite periodic array environment and acceptable VSWR across the frequency range 22.6 GHz-72.6 GHz for scan angles up to ±30𝑜 in E and H planes are obtained.
Additionally, the planar Marchand balun is used to design a wideband bowtie element working from 24 GHz to 43 GHz to cover the two commonly used 5G mm-wave frequency bands (24-29 GHz and 37-43 GHz). The simulated antenna shows a VSWR less than 2 (𝑆11 < −10dB) for the entire frequency range 24.2 GHz-43.4 GHz; Furthermore, VSWR is below 2.6, 2.8 in the whole frequency range 24 GHz-43 GHz when the beam is steered up to 45𝑜 in E and H planes respectively. The maximum gain of the antenna element is varying from -2.9 dBi to 3.34 dBi as the frequency increases from 24 GHz to 43 GHz for broadside radiation. The designed wideband antenna element is used to design and fabricate a 2 × 2 bowtie array using low cost PCB manufacturing method. The return loss of the array is above 10dB from 25GHz to 45GHz at all four ports for broadside radiation. The designed 6 mm×6 mm×1.2 mm tightly coupled array is generating 3-6.7dB co-polarized gain over the entire frequency range. The fabricated antenna has been measured in EmRG Laboratory at University of Waterloo and the measurement results match well with the simulation results.
In this project, in order to provide more space for accommodating the complex feeding network and reduce the cost of the array by reducing the number of required T/R modules, only a subset of array elements are excited. The rest are partly terminated or left as open circuit. The idea is to take advantage of the strong mutual coupling among elements to induce proper currents on terminated elements and still achieve acceptable radiation characteristics comparable with the fully excited array. In order to prove this thinned TCDA concept, the designed wideband antenna element is used to make an array of 8 × 8 tightly coupled bowtie array at frequency range 25 GHz-45 GHz. In this designed thinned array only 22 elements are excited. Twenty of the unexcited elements are loaded with proper impedances and the rest are left as open circuit. The scheme of excitation is obtained by investigating the current distribution among elements and with the help of optimization. All the excitation voltages at excited ports and terminating impedances are optimized using GA algorithm in order to achieve an acceptable radiation pattern close to the pattern of a fully excited array. The promising results show that by having strong mutual coupling among elements in a thinned TCDA and optimizing all excitation voltages and terminating impedances, the antenna radiation characteristics will remain in an acceptable level in a wide frequency range and steering angles while having much fewer number of excitations.
The GA optimization is performed at two frequency bands 25 GHz-29 GHz, 37 GHz-43 GHz with 1 GHz step frequency and for broadside radiation, scanning to 15𝑜, 30𝑜, and 45𝑜 in E-Plane. The optimized terminating impedances are realized by cascade of striplines terminated with a resistance. A modified real frequency technique is used to design a wideband, lossy matching network for each 22 excited ports. The twenty two 50Ω GCPW feeding lines are designed to have 22 excitation ports connected to 2.4mm edge connectors on 4 sides of the board. The thinned 8 × 8 TCA is designed in a 6 layer stackup including the antenna array, terminating impedances and feeding networks and is simulated using Ansys HFSS software.
The simulated input reflection coefficient (𝑆11) is below -8 dB at all 22 excited ports and for the whole frequency range and steering angles. The simulation results show the side lobe level below -13 dB and -8.6 dB at higher frequency band (37 GHz-43 GHz) for broadside radiation and steering up to 45𝑜 in E-Plane respectively. The maximum directivity is dropped between 2 dB to 3.5 dB in compare with the fully excited array at 37 GHz-43 GHz frequency band and for the whole steering angles. At the lower frequency band 25 GHz-29 GHz, the maximum directivity is dropped between 2.5 to 5 dB in compare with the fully excited array and the sidelobe level is increased to-10.5 dB and -6 dB for broadside radiation and when steering up to 45𝑜 in E-Plane respectively.