Mohamed A. Elmansouri

Pulse Distortion and Mitigation Thereof in Spiral Antenna-Based UWB Communication Systems

A dispersion of two-arm planar equiangular and Archimedean spiral antennas is investigated. Frequency domain characterization of spiral antennas-based ultra-wideband link is conducted computationally and experimentally. The time domain performance is obtained by post processing frequency domain data using the inverse fast Fourier transform. Effects of source pulse shape and antennas geometrical parameters on the dispersive properties of the channel are discussed and relevant conclusions regarding the low-distortion spiral antenna are presented. Finally, a theoretical pre-distortion compensation method based on a frequency-dependent delay removal technique is employed to enhance the performance of spiral antennas as pulse radiators.

Wideband Monostatic Simultaneous Transmit and Receive (STAR) Antenna

A monostatic ultra-wideband simultaneous transmit and receive (STAR) antenna subsystem is introduced. An inherent geometrical symmetry of a four-arm spiral antenna and feeding rearrangement are exploited to achieve the simultaneous transmit (TX) and receive (RX) functionalities without any time, polarization, or frequency multiplexing. The antenna is configured such that one arm-pair is used for TX and the other for RX. Thus, even though the two antennas are spatially separated by 90°, they still share the same aperture and the system is considered monostatic. Theoretical and computational studies are conducted to demonstrate the feasibility of the proposed approach under ideal conditions as well as in the presence of feed network non-idealities. The experimental data indicate that isolation levels greater than 39.6-50 dB over multiple octaves are achievable with realistic components. To improve the TX and RX far-field patterns, the planar four-arm spiral aperture is grounded via resistor-loaded quadrifilar helix with the two-arm TX/two-arm RX feed arrangement preserved. Furthermore, to simplify the feed network and reduce the impact of hybrid imbalances, an impedance-transforming microstrip feed is integrated with each arm pair. Isolation > 37 dB and similar with high-quality measured and simulated TX/RX radiation patterns are obtained over the operating bandwidth.

Low-Dispersion Spiral Antennas

A two-arm power spiral antenna with inherently low dispersion is proposed. Analytical, computational, and experimental steps are taken to demonstrate its superior time domain performance. When compared with conventional Archimedean and equiangular spirals, up to 40% improved fidelity factor and pulse compression are achieved. It is, however also shown that the power spirals mid and high band axial ratio is compromised. To enhance the power spirals frequency domain characteristics, a combined Archimedean/power spiral is developed. VSWR below 2:1, clean patterns with wobble on the wave (WoW) and axial ratio <; 3 dB over multi-octave wide bandwidth, all similar to those of well-performed frequency domain spirals, and more than 34% fidelity factor improvement over the wide-field of view are achieved. Herein presented results show that the proposed spiral topology can have simultaneously good frequency and time domain characteristics and no additional hardware or software is needed to enhance either.

Simply-Fed Four-Arm Spiral-Helix Antenna

An ultra-wideband four-arm spiral-helix antenna with simple beamformer is developed to have simultaneously good performance in time and frequency. Two microstrip feeds printed on the opposite side of the spiral with impedance following Klopfenstein taper are employed to eliminate the need for 180 ° hybrids used in conventional beamformers. Ferrite beads placed around the coaxial cables connected between the ground and microstrip are utilized to choke unbalanced currents and prevent shorting the spiral aperture to the ground. Helix is used to improve the low-frequency performance and allow easy attachment of resistive loading to the ground plane. A metallic inset is placed inside the cavity to eliminate the destructive pattern interference and improve the high-frequency gain. To achieve low dispersion, the spiral is loosely wrapped. Simultaneously good time and frequency domain performance from 0.5 GHz to 3 GHz with VSWR<;2, nominal gain and radiation efficiency of 6 dBic and 80%, respectively, broadside axial ratio below 1.8 dB and fidelity factor of 93% over wide field of view are demonstrated in simulations and experiments.

Frequency- and Time-Domain Performance of Four-Arm Mode-2 Spiral Antennas

The performance of simply-fed four-arm Archimedean and equiangular spiral antennas operating in mode-2 (conical beam) and their suitability for pulsed communication applications are investigated. The frequency-domain transfer function of spiral antennas-based ultra-wideband (UWB) link is synthesized using complex far field information measured in a spherical near-field chamber from 2 to 12 GHz. The time-domain performance is obtained by post-processing frequency-domain data using the inverse fast Fourier transform. The synthesized approach is validated computationally and using direct link measurements. The quality of radiated pulses is evaluated in terms of the fidelity factor over the full field of view. The study demonstrates that the mode-2 spirals exhibit good time- and frequency-domain performance over a multioctave bandwidth which allows multiple communication services including UWB and time-frequency RF surveillance applications to be integrated into a single aperture system.

Wideband simultaneous transmit and receive (STAR) bi-layer circular array

The main challenge with the full-duplex simultaneous transmit and receive (STAR) systems is achieving high isolation over a wide bandwidth while preserving identical (or similar) radiation patterns in both, the transmitting and receiving modes. In this paper, a novel low profile wideband bi-layer STAR circular array configuration is presented. The two layers, one for transmit and the other for receive, are identical and composed of four antenna elements. The proposed approach is based on exciting TX/RX layers with the circular mode 1 phase progression while applying a phase difference between the two layers. Virtually identical radiation patterns and TX/RX isolation >50dB over the wide bandwidth is demonstrated.

Time and frequency domain analysis and design of circularly-polarized spiral antenna arrays

A circularly polarized metal-backed hexagonal array operating over multi-octave bandwidth is presented. A four-arm spiral antenna designed to have simultaneously good time (TD)-and frequency-domain (FD) performances and a simple mode-former is used as a unit cell. The antenna arms are resistively loaded to improve its VSWR and reduce interaction with the ground plane. Experiments verify excellent TD/FD performances with fidelity factor > 90%, VSWR<; 2:1, axial ratio <; 1dB, side lobe level (SLL) <; -10dB, and high efficiency.

Wideband dual-polarized bi-static simultaneous transmit and receive antenna system

A wideband dual-polarized bi-static simultaneous transmit and receive (STAR) antenna system utilizing quad-ridge horns is introduced. To enable dual-polarized capability, orthomode transducer (OMT) is designed and integrated with both antennas. Over the 6.5-18.5GHz range, the fabricated STAR antenna system achieves return loss >10dB, isolation >45dB in free space and >52dB when antennas are flush mounted on a finite size ground plane.

Miniaturization of TEM Horn Using Spherical Modes Engineering

The spherical mode expansion is exploited to miniaturize a transverse electromagnetic (TEM) horn antenna and achieve good frequency and time domain performances. To adjust the radiated powers of fundamental transverse electric and transverse magnetic modes, the combined loop and TEM horn antenna is engineered. The constructive combination of the two modes below the horns turn-on frequency miniaturizes the antenna and improves its gain. The magnitude and phase tuning of the desired modes is achieved by modifying the loops geometry. The spherical modal spectrum at higher frequencies is also studied and the antennas geometry is further modified to maintain constructive interference between the higher order spherical modes. A prototype with electrical size of 0.188 λ x 0.145 λ x 0.087 λ (length x width x height) at the turn-on frequency is fabricated over a ground plane. Over three times lower turn-on frequency is obtained compared to a conventional TEM horn with up to 9 dB improvement in the realized gain at the low end and better radiation characteristics over more than 5:1 bandwidth (from 0.87 to 4.5 GHz). Unidirectional operation with front to back ratio 9 dB and excellent time-domain performance with fidelity factor 95% over wide field of view are also demonstrated.

On wideband simultaneous transmit and receive (STAR) with a single aperture

Multi-arm spiral antennas have not only ability to radiate multiple frequency independent modes [1,2], but also inherent ability to act as monostatic co-channel simultaneous transmit and receive (STAR) apertures. Four-arm spirals are herein used for demonstrating this capability.