# Antennas (Part 2)

This is the second part of antennas. Will be shown other parameters and some types of antennas.

#### Other parameters

• Front-back rate (FBR): Is the rate between radiated energy in the main direction of antenna propagation $P_{m}$ and the energy in the opposite direction $P_{op}$.

$FBR=\frac{P_{m}}{P_{op}}$

$FBR(dB)=10log(\frac{P_{m}}{P_{op}})$

The bigger lobule in the radiation diagram always stays in main direction of propagation. • Aperture efficiency: is the relation between the effective area $A_{eff}$ and the physical area $A_{p}$. Effective area is the rate between received power by the antenna $P_{T}$ and density of incident power $W_{i}$.

$\varepsilon _{A}=\frac{A_{eff}}{A_{p}}$

$A_{eff}=\frac{P_{T}}{W_{i}}$

Space around the antenna is divided in three fields:

• Near field reactive: Is the region dominated by non-radiating or reactive field. The electric and magnetic fields are out of phase 90º from each other due to antenna’s inductive and capacitive effects. $R$ is the distance from antenna, $D$ is the antenna’s diameter or length and $\lambda$ is the wavelength.

$R<0,62\sqrt{\frac{D^{3}}{\lambda }}$

To small antennas in relation to wavelength, the approximation of near field reactive boundary is showed in the equation below.

$R<\frac{\lambda }{2\pi }$

• Fresnel or near radiative field: Is the transition between reactive and radiative fields. In this region, the radiation pattern changes with distance. This is the distance band.

$0,62\sqrt{\frac{D^{3}}{\lambda }}

• Far field: Dominated by radiative field, electric and magnetic fields are in phase.

$R>2\frac{D^{2}}{\lambda }$ #### Cloverleaf and helix antennas

Exist many types of antennas, in this post I will explain these two types. This is the Cloverleaf antenna, can have 3 or 4 petals or wire loops. It is an omnidirectional antenna with circular polarization and it radiation pattern is similar to the dipole. Has low gain and it is better to transmission than reception. This antenna`s diameter is half of wavelength. The Cloverleaf antennas are used in MIMO systems and communication with drones. Below is the helix antenna. Is a directional antenna, the spiral wire produces a circularly polarized wave.

[WPGP gif_id=”9663″ width=”600″]

Has wide frequency band and it is easy to build, however the efficiency decreases with the increase number of turns. The radiation pattern depends on antenna’s dimensions. When the antenna’s diameter is small comparing with the wavelength, the pattern is normal (a). When the dimensions are bigger than necessary to pattern (b), occurs the mode (c). Are used in HF and VHF and satellite communication, like this array of helix antennas. The pitch angle determines this antenna’s dimensions and it is calculated this way.

$\alpha =tan^{-1}\frac{S}{\pi D}$ In the next post of telecommunications, I will talk about the other types of antennas. 