An antenna is a device that is made to efficiently radiate and receive radiated electromagnetic waves. There are several important antenna characteristics that should be considered when choosing an antenna for your application as follows:

• Antenna radiation patterns
• Polarization
• Directivity
• Power Gain

Antenna Radiation Patterns
An antenna radiation pattern is a 3-D plot of its radiation far from the source. Antenna radiation patterns usually take two forms, the elevation pattern and the azimuth pattern. The elevation pattern is a graph of the energy radiated from the antenna looking at it from the side as can be seen in Figure 1a.

The azimuth pattern is a graph of the energy radiated from the antenna as if you were looking at it from directly above the antenna as illustrated in Figure 1b. When you combine the two graphs you have a 3-D representation of how energy is radiated from the antenna in Figure 1c.





















Polarization
Polarization is the orientation of electromagnetic waves far from the source. There are several types of polarization that apply to antennas. They are Linear, which comprises, Vertical, Horizontal and Oblique, and circular, which comprises, Circular Right Hand (RHCP); Circular Left Hand (LHCP), Elliptical Right Hand and Elliptical Left Hand.

Polarization is most important if you are trying to get the maximum performance from the antennas. For best performance you will need to match up the polarization of the transmitting antenna and the receiving antenna.

Directivity
The directive gain of an antenna is a measure of the concentration of the radiated power in a particular direction. It may be regarded as the ability of the antenna to direct radiated power in a given direction. It is usually a ratio of radiation intensity in a given direction to the average radiation intensity.

Power Gain
The power gain of an antenna is a ratio of the power input to the antenna to the power output from the antenna. This gain is most often referred to with the units of dBi, which is logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect spherical radiation pattern and a linear gain of one.

Yagi antennas consist of an array of independent antenna elements, with only one of the elements driven to transmit electromagnetic waves. The number of elements (specifically, the number of director elements) determines the gain and directivity. Yagi antennas are not as directional as parabolic dish antennas, but more directional than flat panel antennas.

Parabolic dish antennas use physical features as well as multiple element antennas to achieve extremely high gain and sharp directivity. These antennas use a reflective dish in the shape of a parabola to focus all received electromagnetic waves on the antenna to a single point.


























The parabolic dish also works to catch all the radiated energy from the antenna and focus it in a narrow beam when transmitting. The parabolic dish antenna is very directional. By harnessing all of the antenna's power and sending it in the same direction, this type of antenna is capable of providing high gain.

Microstrip antennas can be made to emulate many of the different styles of antennas explained above. Microstrip antennas offer several tradeoffs that need to be considered. Because they are manufactured with PCB traces on actual PCB boards, they can be very small and lightweight.

















This comes at the cost of not being able to handle as much output power as other antennas, and they are made for very specific frequency ranges. In many cases, limiting the frequencies that can be received is actually beneficial to the performance of a radio. Due to this characteristic, microstrip antennas are not well suited for wideband communications systems.

Fractal is a design of the complex geometry of which is composed of the geometry of the same size just in a different scale different, this is usually referred to as the 'self-similar' or 'self-symetric'. In addition to the form of geometry, the design could also form patterns that are always repeated, this is usually called the 'initiator' or 'generator'.










There are many mathematical structures that is a form of fractal are Sierpienski's gasket, Cantor's comb, von Korch's snowflake, the Mandelbrot set, the Lorentz attractor, and others. Fractal also described the objects in nature is like a cloud, mountain, coastal beaches and others, where they are not identical with the form of a simple wake geometry.

Can be defined so that the fractal is an antenna that utilizes principles of geometry or the repetition of patterns in a particular the size of the scale is different, so can a radiation electro magnetic energy to the air or vice versa can also catch the electro magnetic energy in the air.

Fractal antenna can work multiband, where the key factor is the ability of because the pattern is repeated several times with a different size scales - different. Because of this reason, the fractal antenna is very concise and suitable for the purpose multiband or wideband and very useful in microwave communications and mobile phone.