Antenna design is typically driven by the maximum space and weight available on the platform. The space is typically defined as the dimensions (diameter and height) of a cylinder, and this metric (as well as the weight) is typically based on the platform and application. For example, UAVs and man-portable applications are highly weight sensitive and thus it is advantageous to design an antenna which can provide all capabilities (transmit and receive) and frequencies of interest, in a single antenna.
Antenna design should thus strive to produce a multi-band (ideally broadband/ultrawideband antenna). An ultrawideband antenna design is ideal for military applications, in that it can cover all frequencies of interest in a single antenna, thus reducing the quantity and complexity of components in military systems (maximum cost-and-space efficiency, simplifying integration, maintenance and upgrade).
A key antenna design metric for ultrawideband antennas is Voltage Standing Wave Ratio (VSWR). Ideally, the antenna design will have VSWR < 3 for the widest possible range of frequencies. An antenna design’s VSWR level of 3 means that it has 75% port efficiency, in other words: 25% of the power that the antenna design receives is reflected back to the transmitter, while 75% is accepted and re-radiated. The antenna design can still be used for receive even for frequencies where VSWR is > 3, but the level of gain will be lower.
On transmit however, an antenna design should only be used when VSWR is < 3, to prevent potential damage to the transmitter (although many transmitters have automatic shut-down to prevent this). For an antenna design to be considered ultrawideband, the ratio of the maximum to minimum frequency (at which the antenna design’s VSWR is < 3) should be at least 10:1.
While there are a number of vital metrics for antenna design evaluation, the efficient use of the space and weight available, along with the broadest range of frequencies for which VSWR is < 3, are key factors in producing a successful antenna design.
