JED - January 2013 - (Page 31)
A SAMPLING OF EW, SIGINT AND DF ANTENNAS
By Ollie Holt
his month’s technology survey examines antennas for use in electronic warfare (EW), signals intelligence (SIGINT) and direction finding (DF). Let’s jump right to the two major issues driving EW antenna design – gain and installation/fit. There is a third (radar cross section or reflectivity), but that is just a side effect of gain and it will be discussed with gain. High gain is the main goal of good antenna design. The more gain the antenna provides, the less amplification that is required by the electrical circuits further down the signal path and the greater the operational range of the EW system. The problem is that higher gain usually means a larger radar cross section (RCS) and therefore a greater reflectivity, which often compromises a weapons system’s stealth characteristics. There are two types of reflectivity that impact the antenna’s RCS; one is called Structural Mode Scattering and the other is called Antenna Mode Scattering. Structural scattering is energy reflected back into space by the antenna structure. This can be reduced by adding radar-absorbing material (RAM) to the antenna and/or by pointing the antenna away from the desired Field of View (FOV) and causing reflections to occur outside the desired FOV. In both cases, it reduces the gain of the antenna in the desired direction. Antenna Mode Scattering is caused by antenna-to-electronics mismatch and can be reduced by controlling the impedance match between the antenna and electronics. The better the match, the lower the amount of antenna mode scattering. Antenna installation tends to drive the design engineer to make compromises between gain/performance and size/ weight. Typically a weapons system is not designed around an EW antenna, so the EW system designer has to find ways to work within the size and weight constraints available on the platform. This typically impacts the field of view and the performance of the antenna. Also, not all signals the EW system desires to detect are radiated in the same polarization (horizontal, vertical, circular, etc.), so additional performance impacts have to be accounted for in the installation. Antenna installation on the platform drives another set of design considerations, such as aerodynamic drag (on aircraft); obscuration caused by engines, wings or other platform structures; covert visual signature, etc.
All of these issues drive the design and selection of the antenna for your particular application.
The Journal of Electronic Defense | January 2013 The Journal of Electronic Defense | January 2013
For this survey of EW, SIGINT and DF antennas, we asked antenna manufacturers to describe antenna type and operating frequency range (EW antennas typically have a relative wide frequency range compared with radars or radios, for example). In most EW applications the antenna will be a single element antenna of some type but it could also be a simple array of elements. The Voltage Standing Wave Ratio (VSWR), also referred to as SWR, defines the ability of the antenna to deliver energy to the antenna feed line or the feed line to deliver energy to the antenna in the case of a transmitter. It is the ratio of the power at the antenna to the energy delivered to the feed line. An ideal SWR would be 1.0:1 or SWR=1. That would mean that all the power received by the antenna is passed on to the receiver. If only 75% of the energy gets to the cable, then the SWR would be 3:1 or 3 and the reflectivity would be 0.5. Any SWR value higher than 1 means some of the received power is being reflected back towards the antenna and into space – Antenna Mode Scattering. This is also very important if the antenna is to be used for transmitting (jamming) because it defines the power reflected back to the power amplifier from the antenna. A poorly matched system could damage the power amplifier. You will probably never see an ideal SWR of 1, but the design should get a close as possible to that number. The antenna pattern defines the shape of the circumference over space of a constant gain value. Depending on the type of antenna this pattern can be very broad (i.e. a 360-degree circle) to a very narrow 5-degree rabbit ear. This is usually defined both in azimuth and elevation. If the antenna can also be used in an EW transmitter application, the maximum power handling value is listed in the table. This is the maximum power before the antenna may suffer breakdown or arcing, a value you typically don’t want to exceed. JED’s next survey, in the March issue, will cover radar warning receivers (RWRs) and electronic support measures (ESM) systems.
Table of Contents for the Digital Edition of JED - January 2013
The View From Here
From the President
Program Profi le: SEWIP
Index of Advertisers
JED Quick Look
JED - January 2013