Basic characteristics of antenna systems for mobile satellite applications

St. D. Ilcev – Mangosuthu University of Technology (MUT), South Africa

 

Note:  This article is a shorter and improved version of an article first published with the title “Antenna Systems for Mobile Satellite Applications” by the Microwave & Telecommunication Technology, 2009. CriMiCo 2009. 19th International Crimean Conference in Sevastopol and published by the IEEE.

 

Over a century ago, radio started to be used on-board ships for communication and safety purposes. Nowadays, Mobile Satellite Communication (MSC) is used to support all sorts of applications via mobile antenna initially from ships, and subsequently from road vehicles, trains and aircraft. The antenna systems for MSC, where the propagation characteristics are greatly affected by movement and the changeable local environment, differ greatly from those used in fixed satellite systems.  In order to meet the mechanical and transmission requirements of antenna hardware for MSC, many factors need to be considered and optimised.  This article describes evolution and development of mobile antenna systems, their classification and types, and the characteristics of antennas for ships, vehicles and aircraft.

Evolution of antenna systems for mobile radio communications

The Russian professor of physics, Popov, designed the world’s first radio receiver in 1895 with antenna in the shape of wire mounted on a balloon in the air and transmitter with a lightning conductor as an antenna.  Shortly after, Marconi started the commercial deployment of radio and antenna equipment on board merchant ships, establishing his own company for the production of maritime radio and antenna equipment.  The requirements then was for long distance wireless communications at low frequencies.  The first shipboard antenna were little more than haphazard lengths of wire strung as high as possible above the ship’s topside the thinking being that, the longer and higher the wire, the better the results would be. Following that, many kinds of antenna were developed leading more recently to antenna systems for maritime, land and aeronautical MSCs [1, 2].

Development of antennas for MSC

The antenna systems for MSC, where the propagation characteristics are greatly affected by movement and the changeable local environment, differ significantly from those used in fixed satellite systems.  In order to meet the mechanical and transmission requirements, many factors need to be considered and optimised in the antenna design.  A direct line-of-sight between antenna and satellite with total hemispherical coverage is required.  This is achieved through the use of tracking rotatable, high-gain antennas often installed in pairs on board ship to attain full coverage, irrespective of blockage in the form of the funnel, masts and other objects. The ship platform itself imposes even more stringent requirements in that, despite constant vibrating, pitching, rolling and yawing during bad weather conditions, the antenna’s narrow beam must be directed accurately to the satellite.  In that regard, the situation regarding land vehicle or aircraft antenna is less complicated [3, 4].

As the 1970s dawned, optimism and enthusiasm about satellite communications was so great with ideas to virtually replace High Frequency (HF) radio in the US Navy with the new Fleet Satellite Communications System military mobile system.  Several types of VHF and UHF transceiving antennas were developed for US naval applications.  These included a crossed-dipole array antenna, its improved version and the so-called wash-tube similar to the Short Backfire[1] antenna and one type of Super High Frequency parabolic dish antenna.  However, the first real global MSC system was the US Marisat military system which employed ship earth stations and L-band antenna systems similar to the obsolete Inmarsat-A and current Inmarsat-B terminals [5, 6].

Classification of Mobile Satellite Antennas (MSA)

MSAs can be classified into four major types according to the service offered: communications, broadband (multimedia), navigation, and military.  However, at the more fundamental level of gain values and technical characteristics, MSAs fall into three main groups: low-gain omnidirectional, medium-gain directional, and high-gain directional.              

In many respects the antennas currently available for MSC applications constitute the weakest link of the system.  If the antenna has a high gain, synchronising with and tracking the satellite is complex taking into account the motion of the antenna and the orbital motion of the satellite.  If the antenna has low gain, tracking the satellite is not such an issue but the capacity of the communications link is limited.  There are three types of MSA according to the transmission direction: transmitting and receiving or so-called transceiving, receive-only (part of the special Inmarsat Enhanced Group Call), and transmit-only satellite beacon antennas.

Research has been conducted by the various mobile satellite providers into the development and improvement of all network segments, including different types of MSA, for the transmission of voice, video and different data rates between ships, land vehicles and aircraft.  Experiments have included low-speed data transmission with briefcase-sized transportable equipment onto which small printed antennas were mounted.  [1, 7]. 

Vehicleborne MSA

Vehicleborne antennas have been developed for use on trucks, trailers, buses, cars, rail locomotives and wagons.  Antennas for data transmission on-board road and rail vehicles are illustrated in Figure 3 and 4 respectively. Figure 3 shows an automatic tracking high gain antenna for all types of vehicles and Figure 4 shows a phased array-type antenna (described later).  Antennas for the transceiving Inmarsat-C, D+, M, Mini-M and other systems such as Iridium, Globalstar and Orbcomm, are also used on board land vehicles.  Similar to the maritime application, the Cospas-Sarsat system supports a hand-held satellite emergency personal locator beacon with small built-in UHF transmitting antennas [7].


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Figure 3:  2-Axis stabilised antenna for vehicleborne applications

Image courtesy of A&PST Co., Ltd, Korea (see www.anpst.com/product_1/745)


                                     Figure 4:  Phased-array antenna for vehicleborne applications

Image courtesy of A&PST Co., Ltd, Korea (see www.anpst.com/product_1/898)