Ampere's Law illustrates that a current-carrying element or antenna creates a time-varying magnetic field which then creates a time-varying electric field and so forth to generate a free-space electromagnetic wave.

When the antenna is attached to a load, it radiates the load's information in an energy-storing electromagnetic wave. The reciprocity of antennas dictates that an antenna can equally translate a free-space electromagnetic wave into guided electrical wave.

Antennas parameters such as gain and impedance are determined by the antenna's shape and size. We will now describe antenna characteristics that antenna designers use to maximize antenna efficiency.

The electric field components of a linearly-polarized wave project a line onto a plane where the electric field components of a circularly polarized wave project a circle.

When the load impedance of the microchip Z_l is open or shorted, the reflection coefficient Gamma equals 1 and all the energy is reflected back into the antenna. If however, the impedance of the microchip Z_l and the impedance of the antenna Z_a are equal, Gamma equals 0 and all the energy is absorbed by the microchip. With power levels so low in passive RFID, impedance matching between the antenna and the microchip is extremely important to minimize unnecessary losses.

A VSWR of 1 is desirable because no energy is reflected or "lost" from the load back into the antenna.

In antenna design, bandwidth is more often described in terms of the VSWR or the

Specifically, the length of the radiating patch determines the resonant frequency as follows:

where L is the length of the patch, lambda is the resonant frequency, and h is the thickness of the dielectric. The width W of the patch affects the frequency only slightly, but greatly impacts the impedance of the antenna. The microstrip patch antenna is used in many wireless devices such as cellphones and GPS receivers because of the following characteristics:

- lightweight
- low-profile
*planar*configuration - can be designed to be linearly or circularly polarized
- capable of multiple frequency operations
- mechanically robust

- modified shape patches-- specifically slots
- planar multiresonator configurations
- multilayer configurations
- stacked multiresonator MSAs
- impedance-matching networks
- log-period configurations
- ferrite substrate-based broadband MSAs

- locates global extrema
- largely independent of initial conditions
- places few constraints on solution domain
- can handle discontinuous and non-differentiable functions
- good for constrained-optimization problems

The parameters of a broadband patch antenna include:

- length and width of radiating patch
- position of feed probe
- height of patch above ground plane
- thickness of dielectric material
- dielectric constant of dielectric material

A GA-based optimizer was used to create feedline networks for different shapes of patch antennas. The first image shows a rectangular patch and its corresponding feedline. The second shows a twin-patch antenna and its corresponding feedline. The third image shows a self-complementary rectangular microstrip antenna (SCRMA) and its GA-designed feedline.

These GA-optimized patch antenna designs demonstrate a fourfold bandwidth improvement from standard square microstrip antennas.