Antennas - parameters and definitions.
First, Some Definitions
An antenna is any device which converts electronic signals (i.e., signals in cables) to electromagnetic waves (i.e., signals out in the "ether waves") - or vice versa. They come in a huge range of sizes and designs, depending on the frequency of the signals they handle and their operating parameters. Functionally, any antenna can either transmit or receive signals. However antennas designed for high-power transmission must be capable of handling large amounts of power. Common antenna performance parameters are shown in Table 1.
The Antenna Beam
One of the most important (and misstated) areas in the whole EW field has to do with the various parameters defining an antenna beam. Several antenna beam definitions can be described from Figure 1, which is the amplitude pattern (in one plane) of an antenna. This can be either the horizontal pattern or the vertical pattern. It can also be the pattern in any other plane which includes the antenna. This type of pattern is made in an anechoic chamber designed to prevent signals from reflecting off its walls. The subject antenna is rotated in one plane while receiving signals from a fixed test antenna, and the received power is recorded as a function of the antenna's orientation relative to the test antenna.
Boresight: The boresight is the direction the antenna is designed to point. This is usually the direction of maximum gain, and the other angular parameters are typically defined relative to the boresight.
Main Lobe: The primary or maximum gain beam of the antenna. The shape of this beam is defined in terms of its gain vs. angle from boresight.
Beamwidth: This is the width of the beam (usually in degrees). It is defined in terms of the angle from boresight that the gain is reduced by some amount. If no other information is given, "beamwidth" usually refers to the 3-dB beamwidth.
3-dB Beamwidth: The two-sided angle (in one plane) between the angles at which the antenna gain is reduced to half of the gain at the boresight (i.e., 3-dB gain reduction). Note that all beamwidths are "two-sided" values. For example, in an antenna with a 3-dB beamwidth of 10 [degrees] the gain is 3 dB down 5 [degrees] from the boresight, so the two 3-dB points are 10 [degrees] apart.
"n" dB Beamwidth: The beamwidth can be defined for any level of gain reduction. The 10-dB beamwidth is shown in the figure.
Side Lobes: Antennas have other than intended beams as shown in the figure. The back lobe is in the opposite direction from the main beam, and the side lobes are at other angles.
Angle to the First Side Lobe: This is the angle from the boresight of the main beam to the maximum gain direction of the first side lobe. Note that this is a single-sided value. (It makes people crazy the first time they see a table in which the angle to the first side lobe is less than the main beam beamwidth - before they realize that beam width is two sided and the angle to the side lobe is single sided).
Angle to the First Null: This is the angle from the boresight to the minimum-gain point between the main beam and the first side lobe. It is also a single-sided value.
Side-Lobe Gain: This is usually given in terms of the gain relative to the mare-beam boresight gain (a large negative number of dB). Antennas are not designed for some specific side-lobe level - the side lobes are considered bad, and thus certified by the manufacturer to be below some specified level. However, from an EW or reconnaissance point of view, it is important to know the side-lobe level of the transmitting antennas for signals you want to intercept. EW receiving systems are often designed to receive "0-dB sidelobes" - which is to say that the side lobes are down from the main lobe gain by the amount of that gain. For example, "0 dB" side lobes from a 40-dB gain antenna would transmit with 40 dB less power than observed if the antenna boresight is pointed directly at your receiving antenna.
More about Antenna Gain
In order to just add the antenna gain to a received signal's strength, we need to state signal strength out in the "ether waves" in dBm - which is not really true. As discussed in the September 1995 "EW 101" column, dBm is really a logarithmic representation of power in milliwatts - which only occur in a circuit. The strength of a transmitted signal is more accurately stated in micro-volts per meter (mv/m) of field strength, and the sensitivity of receivers with integral antennas are often stated in mv/m. That same column gives convenient formulas for the conversion between dBm and mv/m.
From an EW point of view, the most important effect of polarization is that the power received in an antenna is reduced if it does not match the polarization of the received signal. In general (but not always), linearly polarized antennas have geometry which is linear in the polarization orientation (e.g., vertically polarized antennas tend to be vertical). Circularly polarized antennas tend to be round or crossed, and they can be either right-hand or left-hand circular (LHC or RHC). The gain reduction from various polarization matches is shown in Figure 2.
Commonly Used Antenna Performance Parameters Term Definition Gain The increase in signal strength (commonly stated in dB) as the signal is processed by the antenna. (Note that the gain can be either positive or negative and that an isotropic antenna) has unity gain, which is also stated as 0-dB gain. Frequency The frequency range over which the antenna can Coverage transmit or receive signals and provide the appropriate parametric performance. Bandwidth The frequency range of the antenna in units of frequency. This is often stated in terms of the percentage bandwidth [100% x (maximum frequency - minimum frequency)/average frequency]. Polarization The orientation of the E and H waves transmitted or received. Mainly vertical, horizontal or right- or left-hand circular - can also be slant linear (any angle) or elliptical. Beamwidth The angular coverage of the antenna, usually in degrees (defined below). Efficiency The percentage of signal power transmitted or received compared to the theoretical power from the proportion of a sphere covered by the antenna's beam.
An important EW polarization trick is to use a circularly polarized antenna to receive a linearly polarized signal of unknown orientation. You always lose 3 dB but avoid the 25-dB loss that would occur if you were cross polarized. When the received signal can have any polarization (i.e., any linear or either circular), it is common practice to make quick measurements with LHC and RHC antennas and choose the stronger signal. The value of 25 dB for cross-polarized antennas is normal for the types of antennas common to EW systems (usually covering wide frequency ranges). Narrow-frequency-band antennas (for example, in communication satellite links) can be carefully designed for cross polarization isolation of greater then 30 dB.
Next month, we'll cover a wide range of types of antennas important to EW and discuss the selection criteria. For your comments and suggestions, Dave Adamy is at Internet: firstname.lastname@example.org.
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|Title Annotation:||antennas for electronic warfare applications|
|Publication:||Journal of Electronic Defense|
|Date:||Sep 1, 1997|
|Previous Article:||A sampling of EW expendables.|
|Next Article:||DES is dead. long live DES!|