Gain
Gain is how much energy an antenna concentrates in its best direction compared to a perfect sphere. A 30 dBi antenna is 1,000 times more sensitive in its main beam than an isotropic radiator. High gain means long range; it also means narrow beam, so you must steer carefully.
Beam width
The angle within which the antenna's gain drops by 3 dB (half power). A 1° beam at 100 km covers about 1.75 km — that's your azimuth resolution. Larger dishes give narrower beams.
Polarisation
Horizontal, vertical, or circular. Rain returns are strongly polarised; aircraft returns less so. Weather radars use both polarisations to tell rain from hail. Stealth coatings are sometimes polarisation-selective.
Side-lobes
No antenna is perfect. Energy leaks out at angles other than the main beam — these are side-lobes. A high target lit by a side-lobe can look like a low target in the main beam, confusing the system. Good antenna design pushes side-lobes 30 dB or more below the main beam.
Phase Center and Bandwidth
The phase center is the theoretical point from which the antenna's electromagnetic radiation appears to originate. In a simple dipole, this is centrally located, but in complex arrays or horn antennas, the phase center can shift based on frequency or orientation. This becomes critical in high-precision GPS and surveying, where a shift of merely a few millimeters introduces range errors. For broadband antennas like the log-periodic, the phase center literally moves along the structure as the frequency changes, meaning the antenna effectively 'shortens' or 'lengthens' itself to maintain resonance with the incoming wave.
Bandwidth defines the frequency range over which the antenna maintains acceptable performance, usually measured by a Voltage Standing Wave Ratio (VSWR) of less than 2:1. Narrowband antennas, such as those used in early 1940s cavity magnetron radars, were highly efficient but easily jammed. Modern systems utilize Ultra-Wideband (UWB) designs that spread energy across several gigahertz. While wide bandwidth reduces interference sensitivity, it complicates the matching network design because the impedance must remain stable across the entire spectrum to prevent energy reflecting back into the transmitter and damaging components.
Effective Aperture and Reciprocity
Effective aperture, or 'capture area,' represents how much power an antenna can extract from a passing electromagnetic wave. It is not always the same as the physical area of the dish; for a parabolic reflector, the aperture efficiency is typically between 50% and 70% due to spillover and feed-horn blockage. The relationship between gain and effective aperture is fixed by the square of the wavelength. This explains why a small 10 GHz radar antenna can have the same gain as a massive 100 MHz array; the shorter wavelength allows for more efficient energy capture within a smaller physical footprint.
The Principle of Reciprocity states that an antenna's properties—Gain, Beam width, and Polarisation—are identical whether it is transmitting or receiving. This allows engineers to characterize a complex radar array by measuring its passive reception patterns. However, reciprocity fails in non-linear systems or when using ferrites, such as in circulators that protect receiver electronics from high-power pulses. In active electronically scanned arrays (AESAs), while the passive elements are reciprocal, the individual transmit/receive modules apply different gains and phase shifts depending on the signal direction, enabling simultaneous multi-target tracking.
Front-to-Back Ratio
While gain measures the focus of the main lobe, the front-to-back (F/B) ratio quantifies how effectively an antenna suppresses energy radiated in the opposite direction. For directional systems like Yagi-Uda arrays or parabolic dishes, a high F/B ratio is essential to prevent interference from rearward sources. In radar applications, a poor F/B ratio can lead to 'ghost' targets or self-induced noise. Achieving a ratio of 20 dB or higher typically requires precise placement of parasitic elements or optimized dish geometry to ensure that the back-radiated energy is orders of magnitude weaker than the primary signal.
The historical development of this metric traces back strictly to the 1930s with the work of Hidetsugu Yagi and Shintaro Uda. By using reflector elements slightly longer than the driven element, they forced electromagnetic waves to cancel behind the antenna while reinforcing them toward the front. Modern phased arrays manage this ratio electronically rather than physically, using phase shifting to create 'nulls' in the radiation pattern. This allows a stationary system to ignore a specific interfering transmitter at its rear while maintaining high sensitivity for the intended signal path in the forward sector.
Voltage Standing Wave Ratio (VSWR)
VSWR measures how efficiently energy is transferred from the transmission line into the antenna. In a perfect system (1.0:1 ratio), all energy radiates out; in reality, impedance mismatches cause some power to reflect back toward the transmitter. This reflected energy is not only wasted but can also damage high-power RF amplifiers by generating excessive heat. A VSWR of 2.0:1 indicates that about 11% of your power is being reflected. Most professional radar systems are tuned to maintain a VSWR of 1.5:1 or better, ensuring the hardware remains stable during continuous high-power pulse operation.
The concept of impedance matching is the technical hurdle behind 'broadband' antennas. Because an antenna's resonance depends on its physical length relative to the wavelength, it will only have a low VSWR over a specific frequency range. To expand this, engineers use techniques like tapering or adding resistive loads, though these often come at the cost of overall efficiency. Historically, the Smith Chart, developed by Phillip Smith at Bell Labs in 1939, became the standard graphical tool for calculating these reflections, allowing engineers to visualize complex impedance and design matching networks without modern computer simulation.