A broadband antenna is defined by its frequency range, i.e. the ratio of its maximum to minimum operating frequency, which should be at least 2:1 and as high as < 10:1 (antennas with a ratio of 10:1 and better are categorized as ultrawideband antennas).
An antenna’s frequency range is defined by its voltage standing wave ratio (VSWR), which is a measure of the antenna efficiency. Specifically, VSWR is a measure of antenna “impedance match” to the feed line (the ideal VSWR value is 1, the closer to 1 the better). A common antenna requirement of VSWR < 3 corresponds to 75% antenna port efficiency, meaning that 75% of the power delivered to the antenna is actually accepted by the antenna and radiated outward (implicitly assuming that the antenna’s internal losses are small, which should be the case for properly designed antennas).
A broadband antenna can be directional or omnidirectional in its radiation pattern. Directivity refers to the level of power that the antenna transmits in a particular direction when compared to a hypothetical isotropic antenna which radiates equally well in all dimensions. Gain is a measure of the wideband antenna’s directivity, multiplied by its port efficiency. Directional broadband antennas have high gain in one direction or sector, while omnidirectional broadband antennas have equal radiation in all directions in the horizontal (azimuth) plane and typically have decreasing gain in the vertical or elevation plane, above/below the horizon.
A broadband antenna is typically vertically polarized. Vertical polarization can be understood as follows: picture an electric field vector perpendicular to the surface of the earth; a vertically polarized antenna can handle (transmit and/or receive) vertically polarized signals. An analogy for this is: when looking through a vertical viewfinder, one can fully view objects which are oriented vertically (e.g. such as a tall lamp).
Finally, a broadband antenna can also have low radar cross section. Radar cross section is defined as follows: Large objects partially reflect/scatter and partially absorb incident electromagnetic wave energy. If one replaces the particular object with a perfect reflector of a certain area, that equivalent perfect reflector’s area is the object’s radar cross section for the given directions of incidence and observation (in general, the RCS varies with the incidence/observation angles)
Low RCS (in-band and out-of-band, i.e. within and outside of the antenna’s intended operating frequency range) is very important for aircraft and for surface and subsurface ship mast antennas, to reduce the probability of an adversary detecting and tracking the platform
Further key decisions include the space available for the broadband antenna on your platform, and any weight restrictions that may apply. The desired frequency range (fmin, fmax) should also be specified, along with whether the antenna must transmit, receive, or perform both functions.
A thorough understanding of these antenna metrics (directivity, gain, RCS, space, weight, frequency range) is a good starting point for selecting the right broadband antenna for your particular application and platform.
