Christian Volmer

An Eigen-Analysis of Compact Antenna Arrays and Its Application to Port Decoupling

Placing the radiators of antenna arrays closer than aggravates the problem of power mismatch. Based on efficiency considerations, a general analysis of this effect is presented, putting forward a simple tool to quantify, compare, and optimize the performance of antenna arrays. This analysis is not restricted with respect to the number of radiators or the degree of compactness. In order to improve power matching, a systematic approach for the design of lossless decoupling and matching networks based on 180 directional couplers is suggested for up to eight radiators. Implications of network losses, which have not yet received appropriate attention by researchers in the past, will be analyzed and discussed by means of a manufactured three-element prototype array.

Broadband Decoupling and Matching of a Superdirective Two-Port Antenna Array

Decoupling and matching networks may be used to improve the performance of compact antenna arrays where mutual radiator coupling has caused a degradation of the diversity capabilities. A popular network consists of a 180deg rat-race directional coupler, which decouples the antenna ports, followed by impedance matching networks at each port. Researchers, however, usually neglect the presence of losses both within the antenna array and the decoupling and matching network. For this reason, we have built various narrowband and broadband matching networks and compare their performances with the help of calibrated far-field measurement data.

Eigenmode decoupling of miniaturised diversity antennas using compact quasi-lumped networks

This paper investigates a three-port antenna array with an element separation of lambda/10. A compact decoupling and matching network, realised by quasi-lumped elements, compensates for the problem of mutual coupling between the radiators. One special feature of this network is the excitation of the eigenmodes of the antenna array, which results in a broad decoupling bandwidth. The physical properties of such a system as well as practical realisation strategies are discussed. Measured data are presented for the power matching and the efficiency of the system.

A Descriptive Model for Analyzing the Diversity Performance of Compact Antenna Arrays

Mutual coupling has negative impact on the radiation properties of compact antenna arrays when the spacing of individual radiators is considerably closer than half a wavelength. We have previously put forward a description of the radiation efficiency of an arbitrary antenna array in terms of orthogonal radiating eigenmodes. The present publication elaborates on this concept and presents a descriptive equivalent circuit model for a coupled antenna array. We show that eigenmodes undergo independent Rayleigh fading under appropriate conditions. Also, we demonstrate how practical closed-form expressions in terms of the arrays scattering matrix can be derived, which characterize the diversity performance and therefore are convenient measures for the radiation quality of a compact antenna array.

Tracking antenna for mobile bi-directional satellite communications in Ka-band

A satellite tracking antenna for mobile satellite communications in Ka-band was developed, employing a conical scan produced with a tilted and fast rotating sub-reflector. This tracking technique facilitates a simple and low-cost dual-frequency feed structure suitable for bidirectional operation. Two separate tracking voltages are generated by the system — one for elevation and one for azimuth. The antenna is able to detect misalignments up to 2° from the satellite direction with a bandwidth of 20 Hz.

Implementation of a miniaturised antenna array with predefined orthogonal radiation patterns

This paper describes the realisation of an antenna array system consisting of three, quarter-wavelength spaced, monopoles and a passive network. This compact planar network decouples and matches the antenna array, in order to prevent gain reduction. At the same time, the network forms orthogonal radiation patterns at the system ports, which divide the space into three different sections. The described system promises switched beam applications in small mobile platforms, where lack of space and processing power prevents the use of conventional antenna arrays.

Miniaturized Antenna Arrays with an Element Separation down to λ/10

Antenna arrays play an integral role for the exploitation of spatial diversity and in MIMO systems. In small mobile platforms where lack of space prevents the use of conventional half-wavelength spaced antenna arrays, the radiator separation has to be reduced. This aggravates the problem of mutual coupling between radiators. In previous papers, we have demonstrated a method to decouple and match a miniaturized antenna array. This new contribution investigates and compares manufactured antenna arrays with element separations of lambda/4 and lambda/10, respectively. Passive decoupling and matching networks (DMN) compensate for the problem of mutual coupling between the radiators. The resulting system port patterns devide the space in three sections, which makes switched-beam applications possible. The physical properties of such systems as well as the realization strategies are discussed. Important aspects are the antenna analyses in terms of their eigenmodes and the simplification of the decoupling networks. Comparative data are presented for the bandwidth, the efficiency and the correlation of the system port patterns.

Decoupling and Matching Network for Miniaturised 3-Port Antenna Arrays Based on 180° Couplers

In ordinary antenna arrays the spacing between radiators is usually chosen lambda/2. In order to use antenna arrays in small mobile platforms, the overall dimensions and hence the radiator separation must be reduced, which aggravates the problem of mutual coupling between radiators. Results are highly distorted beam patterns and greatly reduced radiation efficiency. We propose a simple method to design a decoupling and matching network for a three element antenna array with a radiator separation of lambda/4, which excites the orthogonal eigenmodes of radiation of the antenna array.

Noise characterization of a multi-channel receiver using a small antenna array with full diversity for robust satellite navigation

The employment of a DMN in small antenna arrays results in an increased antenna noise temperature due to increased ohmic losses. On the other hand, it minimizes the amplifier noise contribution considerably, thus reducing the equivalent system noise temperature. Therefore, the use of a DMN for small antenna arrays displaying full diversity is not only beneficial but necessary for optimizing receiver performance. The analysis sketched here will be extended to null-steering or interferer-cancellation scenarios.

Performance measurements of a two-branch diversity receiver with superdirective antenna array and decoupling and matching network

The paper presents a two-branch diversity receiver connected to two monopole antennas over a common ground plane. The influence of radiator coupling on the diversity performance of the receiver is investigated by measurement for different element separations down to lambda/20. Decoupling and matching networks (DMN) are known to mitigate the adverse effects of radiator coupling; however, practical network realisations have been shown to possess a low power efficiency. We demonstrate that despite this drawback, a practical DMN is able to deliver almost full diversity out of a highly superdirective two-element lambda/20 array. This also built a two-channel diversity QPSK (quadrature phase shift keying) mobile communications system and gathered statistics about the signal-to-noise ratio (SNR) for more than 90 minutes of received baseband data and for ten different two-port antenna arrays.