A Vivaldi-antenna is a co-planar broadband-antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

, which is made from a on both sides metalized dielectric plate.
The feeding line excites a circular space via a microstrip line, terminated with a sector

Circular sector

A circular sector or circle sector, is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the...

-shaped area.
From the circular resonant area the energy reaches an exponential pattern via a symmetric slot line.

Vivaldi antennas can be made for linear polarized waves or - using two devices arranged in orthogonal direction - for transmitting / receiving both polarization orientations.

If fed with 90deg phase shifted signals orthogonal devices can transmit / receive circular

Circular polarization

In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....

 oriented electromagnetic waves.

Vivaldi antennas are useful for microwave

Microwave

Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 frequencies exceeding 3GHz.

Advantages of Vivaldi antennas are their broadband characteristics (suitable for ultra-wideband

Ultra-wideband

Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...

 signals), their easy manufacturing process using common methods for PCB

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

 production, and their easy impedance matching

Impedance matching

In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

 to the feeding line using microstrip line modeling methods.

The MWEE collection of EM simulation

Computer simulation

A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

benchmarks includes a Vivaldi antenna http://www.cst.com/Content/Applications/Article/15 http://www.mwee.com/magazine/2000/cad_benchmark.html
http://www.mwrf.com/Article/ArticleID/19382/19382.html
http://www.ieeexplore.ieee.org/iel2/3094/8735/00385130.pdf
http://www.digbib.ubka.uni-karlsruhe.de/volltexte/documents/688417
http://www.freepatentsonline.com/7088300.html.