A Inside Look at Electronic Warfare Types of Antenna’s

Contributor:  Ron Milione
Posted:  11/17/2011  12:00:00 AM EST
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INSIDE ELECTRONIC WARFARE ANTENNAS

ANTENNA GAIN

 The most important characteristic of any type of EW antenna is antenna gain. Antenna gain is a measure of the ability of an antenna to concentrate energy in the desired direction. Antenna gain should not be confused with receiver gain, which is designed to control the sensitivity of the receiver section of a radar system. There are two types of antenna gain: directive and power.

 a. The directive gain of a transmitting antenna is the measure of signal intensity radiated in a particular direction. Directive gain is dependent on the shape of the radiation pattern of a specific radar antenna. The directive gain does not take into account the dissipative losses of the antenna. Directive gain is computed using Equation 1-1.

 

Figure 1 – Equation 1-1 - Directive Gain

 

b. The power gain does include the antenna dissipative losses and is computed using Equation 1-2.

Figure 2 – Equation 1-2 - Power Gain

c. The term isotropic antenna describes a theoretical spherical antenna that radiates with equal intensity in all directions. This results in a spherical radiation pattern. The power density for any point on an isotropic antenna is the radiation intensity and can be calculated by dividing the total power transmitted (PT) by the total surface area of the sphere, as shown in Equation 1-3.

 

Figure 3 – Equation 1-3 - Power Density for an Isotropic Antenna

d. The radiation pattern of an isotropic, or spherical, antenna would provide neither azimuth or elevation resolution and would be unusable for radar applications. To provide azimuth and elevation resolution, a practical antenna must focus the radar energy. The power density of a practical antenna differs from the isotropic antenna only in terms of antenna gain (G). Solving Equation 1-3 for the power density of a practical antenna yields Equation 1-4.

 

Figure 4 – Equation 1-4 - Power Density for a Practical Antenna

e. The actual power gain (G) of a practical antenna can be calculated by using Equation 1-5.

 

Figure 5 – Equation 1-5 - Power Gain of a Practical Antenna

 

POWER DENSITY

The power density and gain of an antenna are a function of the antenna pattern of a radar system. Figures 6 and 7 illustrate the antenna pattern of a typical parabolic antenna. Most of the power density of the radar is concentrated in the main beam. However, since the radar is not a perfect reflector, some radar energy is transmitted in the sidelobes. In addition, there is spillover radiation due to the energy radiated by the feed that is not intercepted by the reflector. Finally, the radar has a back lobe caused by diffraction effects of the reflector and direct signal leakage. Sidelobes and backlobes are all undesirable radiations that adversely affect the maximum radar range and increases the vulnerability of the radar to certain jamming techniques.

Figure 6 – Radiation Pattern for a Parabolic Antenna

 

 

 

Figure 7 – Radar Antenna Pattern

a. All radars have a primary main beam, which is where the radar has the most power and where target detection usually occurs. The dimensions of this main beam are highly dependent on the design of the antenna.

b. Besides the main beam, all radars have what is called a backlobe. This lobe is directly opposite to the location of the main beam. The sensitivity and signal strength associated with the backlobe is significantly less than that in the main beam.

c. Sidelobes add another dimension to the radar pattern. As with the backlobe, sidelobes do not have the signal strength or sensitivity associated with the main beam. Normally, the sensitivity associated with the sidelobes is 40-50 decibels (dBs) less than the main beam. The radar signal weakness in the backlobe and sidelobes of the main beam make these areas of the radar signal

vulnerable to jamming. It is much easier to introduce jamming into these areas because of the reduced jamming-to-signal ratio needed to be effective. It is difficult for jamming to be effective in the main beam because the radar signal is very powerful in that region.

 

CIRCULAR SCAN

A circular scanning radar uses an antenna system that continuously scans 360° in azimuth (Figure 8). The time required for the antenna to sweep one complete 360° cycle is called the scan rate. Scan duration is the number of “hits per scan,”or the number of pulses, reflected by a target as the radar beam crosses it during one full scan. Most pulse radars require 15 to 20 hits per scan to obtain sufficient information to display a target. The factors that determine the number of hits per scan the radar receives include pulse repetition frequency (PRF), antenna beamwidth, and scan duration.

 

 

Figure 8 – Circular Scan Radar

Circular scan radars provide accurate target range and azimuth information. This makes these radars ideal for the roles of early warning and initial target acquisition. To accomplish these missions, the antenna generates a fan beam that has a large vertical beamwidth and a small horizontal beamwidth. Since elevation information will normally be provided by height finder radars, the size of the vertical beamwidth is not a limitation. This antenna scan allows the radar to scan large volumes of airspace for early target detection. Since early detection is the primary goal of early warning radars, accurate altitude and azimuth resolution are secondary considerations.

Circular scan radars designed for early warning transmit a radar signal with a low PRF. A low PRF allows sufficient time for the radar pulse to travel long distances, and return, before another pulse is transmitted. This gives the radar system a long, unambiguous range capability. Circular scan radars with low PRFs generally use long pulse widths in order to increase their average power and long-range detection capability. The scan durations of early warning radars are relatively long to provide the required “hits per scan” for long-range targetdetection. The plan position indicator (PPI) scope display is normally used with circular scan radar (Figure 9).

 

 

Figure 9 – PPI Scope Display

In order to provide coverage for a large volume of airspace, the beamwidth associated with a circular scan radar is relatively wide. This wide beamwidth, coupled with the long pulse width and low PRF, gives the circular scan radar a large resolution cell, especially at long ranges (Figure 10). This limitation can be exploited to mask force size and composition. However, as range decreases, the dimensions of the resolution cell decrease, and a circular scan radar will begin to break out target formations.

 

Figure 10 – Resolution Cell

Circular scan radars provide range and azimuth information for both early warning and acquisition roles. Modified circular scan radars that can also provide elevation information may be used for ground control intercept (GCI) roles. Two modified circular scan radars that determine range, azimuth, and elevation are the V-beam and the stacked beam.

(1) The V-beam radar transmits two fan-shaped beams that are swept together (Figure 11). A vertical beam provides range and azimuth information. A second beam, rotated at some convenient angle, provides a measure of the altitude of the target.

 

 

Figure 11 – V-Beam Radar

(2) A stacked beam radar (Figure 12) employs a vertical stack of fixed elevation “pencil” beams which rotate 360°. Elevation information is obtained by noting which beam contains the target return. Range and azimuth information is determined in the same manner as in an early warning radar.

 

 

Figure 12 – Stacked Beam Radar

Next Article (Electronic Warfare 113) we will continue with  Electronic Warfare regarding “Electronic Warfare Scanning Methods.”

Ron Milione Contributor:   Ron Milione


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