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Frequency Tunable Antennas and Novel Phased Array Feeding Networks for Next Generation Communication Systems

  • Author(s): Avser, Bilgehan
  • Advisor(s): Rebeiz, Gabriel M
  • et al.
Abstract

The thesis presents three dual-band frequency tunable antennas for carrier aggregation systems and two new feeding networks for reducing the number of phase shifters in limited-scan arrays. First, single- and dual-feed, dual-frequency, low-profile antennas with independent frequency tuning using varactor diodes are presented. The dual-feed planar inverted F-antenna (PIFA) has two operating frequencies which are independently tuned at 0.7--1.1 GHz and at 1.7--2.3 GHz with better than −10 dB impedance match. The isolation between the high-band and the low-band ports is $>$ 13 dB; hence, one resonant frequency can be tuned without affecting the other. The single-feed contiguous-dual-band antenna has two resonant frequencies, which are independently tuned at 1.2--1.6 GHz at 1.6--2.3 GHz with better than −10 dB impedance match for most of the tuning range. And the single-feed dual-band antenna has two resonant frequencies, which are independently tuned at 0.7--1.0 GHz at 1.7--2.3 GHz with better than −10 dB impedance match for most of the tuning range. The tuning is done using varactor diodes with a capacitance range from 0.8 to 3.8 pF, which is compatible with RF MEMS devices. The antenna volumes are $63 \times 100 \times 3.15$ $mm^3$ on $\epsilon_r = 3.55$ substrates and the measured antenna efficiencies vary between 25\% and 50\% over the tuning range. The application areas are in carrier aggregation systems for fourth generation (4G) wireless systems.

Next, a new phased array feeding network that employs random sequences of non-uniform sub-arrays (and a single phase shifter for each sub-array) is presented. When these sequences are optimized, the resulting phased arrays can scan over a wide region with low sidelobe levels. Equations for analyzing the random arrays and an algorithm for optimizing the array sequences are presented. Multiple random-solutions with different number of phase shifters and different set of sub-array groups are analyzed and design guidelines are presented. The performance of the random array feeding scheme is compared to the conventional uniform sub-arraying for multiple cases. It is shown that with the random feeding networks, the number of phase shifters can be reduced up to 30\% while preserving the system performance. This results in more affordable and more reliable systems. The proposed feeding network is demonstrated for a 30 element array of slot- fed patch antennas at 7.9 GHz. The fabricated array uses 12 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 14^o$ with sidelobe levels less than -15 dB.

Another phase shifter reducing method, the interwoven feeding networks, is investigated. These passive feeding networks are composed of power dividers, couplers and resistive attenuators. In this configuration, each phase shifter feeds all of the antennas and creates a sinc-like current distribution over the array elements which results in a boxcar function-like element pattern. This element pattern is used to cancel the grating lobes. By changing the inter-element spacing and the coupling and attenuation coefficients of the feed network, it is possible to adjust the width of the scan region. Different network configurations along with theoretical limitations are investigated to determine the scanable region, side-lobe level and power loss. For the demonstration, two prototype linear arrays with 28 elements are fabricated. Both of the arrays operate at 7.9 GHz. The first array employs 14 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 24^o$ with sidelobe levels less than -15 dB. The second array uses 7 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 11^o$ with sidelobe levels less than -15 dB. Both of these arrays show state-of-the-art performance in terms of reducing the number of phase shifters while still keeping a low sidelobe level and reducing the effect of the grating lobes.

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