Performance Enhancement of 5G Mobile Handset Antennas with Substrate Air-hole

Abstract

As a next-generation trend in wireless communication technology,

5G communication is being widely studied. However, despite intrinsic

advantage of 5G communication such as high data rate, some bottleneck

points regarding antenna hardware remain as tasks in order to enable stable

communication.

5G communication band spectrum can be generally divided in to

two ranges which are sub-6 GHz band under 6 GHz, and millimeter wave

band above 28 GHz. For a thorough mobile handset for 5G communication,

it would be logical for antennas operating at sub-6 GHz and millimeter

wave to have the type of wide band multiple input multiple output (MIMO)

and phase array, respectively.

The main issues in sub-6 GHz MIMO antenna technology are

bandwidth, isolation and gain level in lowest band near 3 GHz. For

overcoming such problem, a wide band MIMO antenna operating at 5G sub-

6 GHz on a mobile handset is designed in Chapter 3. The MIMO antennas

operate in the frequency band from 3 GHz to 7 GHz as a UWB antenna

having wide band performance, good isolation characteristics and relatively

high gain. In order to overcome poor isolation performance in the lower

band near 3 GHz, a loop antenna and a planar inverted F antenna (PIFA)

were used as MIMO antenna elements having orthogonal polarization. For

the loop antenna, gain enhancement at 3.5 GHz was achieved through

matching the resonant frequency of the antenna and the ground. On the

other hand, the PIFA was applied with a window slot in the ground in order

to generate quasi-in-phase superposition of the H fields for achieving

improved gain. Finally, for wide band performance, the loop antenna was

composed of multiple loops with interdigital capacitor. The interdigital

capacitors bring series type resonances to the lower band broadening the

bandwidth of the loop antenna. In addition, the substrate was perforated

with air-holes at the gaps of the interdigital fingers and multi-loop antennas

in order to provide more margin to the reflection coefficient level at certain

frequencies. As a result, the MIMO antenna has -10-dB reflection

coefficient bandwidth of 4 GHz from approximately 3 GHz to 7 GHz with

worst-case isolation level under 20 dB. In addition, the MIMO antenna

shows average gain of approximately 4 dBi in such frequency band with

gain of worst-case 2.9 dBi even in the lowest band near 3.5 GHz.

One of the main issues in 28 GHz array antenna technology is halfpower

beam width (HPBW) in the elevation plane of the antenna for

constructing stable hemispheric beam coverage. To achieve such

characteristics in the 28 GHz band, a 1 × 8 phase array antenna using

dipole elements with air-hole slots was designed in Chapter 4. The antenna

consists of eight folded-dipole elements with modified ground and air-hole

slots between each antenna element. The folding of the dipole enhances

front-to-back ratio (FBR) and isolation level between radiating elements

while the modified ground improves impedance matching characteristics.

The slots in the ground generates complementary dipole radiator of which

the radiation pattern is summed with the original pattern of the dipole array

forming wider HPBW in the elevation plane. In addition, by overlapping the

air-holes to the ground slots, the current intensity around the edge of the slot

is reinforced so that the radiation from the complementary dipole element is

even more strengthened. The 1 × 8 phase array antenna has -10-dB

reflection coefficient bandwidth of 4.2 GHz from 25.8 GHz to 30 GHz with

worst-case isolation level between elements under 20 dB. The antenna has

stable scan angle of ±45° with worst-case peak gain and side lobe level

(SLL) of -10 dBi and 23 dB, respectively. Finally, the antenna shows HPBW

of 219 degree in the elevation plane.

By summing up the performance criteria of the designed sub-6 GHz

MIMO antenna and 28 GHz phase array antenna, such two components can

be expected to construct a stable antenna hardware composition for

thorough 5G mobile handset device.