Omni WiFi antennas, broadcast / propagate a full 360 degree radiation pattern, completely surrounding and saturating the radio signal in all directions outwards from the center of the antenna. Antena Omni Hyperlink. Saya ingin buat kampung saya menjadi kampung internet. Antena Omni Wifi 2.4 Dari Hp –Laptop langsung Jarak 1,5 Km. Percobaan membuat antena Omni dengan bahan paralon. Cara membuat sendiri antena untuk wifi - Duration: 8:04. Pratama Record 24,270 views. Wi-Fi Alliance juga memprakarsai standar keamanan pada 802.11i yang disebut Wi-Fi Protected Accses (WPA). Setifikasi Wi-Fi adalah proses untuk memastikan interoperabilitas antar peralatan wireless LAN 802.11, termasuk access point dan kartu-kartu jaringan wireless yang biasanya mempunyai beberapa form factor yang sangat beragam.

Introduction

This document gives basic antenna definitions and discusses antenna concepts with a focus on the pros and cons of omni and directional antennas.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

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Basic Definitions and Antenna Concepts

An antenna gives the wireless system three fundamental properties: gain, direction and polarization. Gain is a measure of increase in power. Gain is the amount of increase in energy that an antenna adds to a radio frequency (RF) signal. Direction is the shape of the transmission pattern. As the gain of a directional antenna increases, the angle of radiation usually decreases. This provides a greater coverage distance, but with a reduced coverage angle. The coverage area or radiation pattern is measured in degrees. These angles are measured in degrees and are called beamwidths.

An antenna is a passive device which does not offer any added power to the signal. Instead, an antenna simply redirects the energy it receives from the transmitter. The redirection of this energy has the effect of providing more energy in one direction, and less energy in all other directions.

Beamwidths are defined in both horizontal and vertical plains. Beamwidth is the angular separation between the half power points (3dB points) in the radiation pattern of the antenna in any plane. Therefore, for an antenna you have horizontal beamwidth and vertical beamwidth.

Figure 1: Beamwidth of Antenna

Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). In other words, a theoretical isotropic antenna has a perfect 360 degree vertical and horizontal beamwidth or a spherical radiation pattern. It is an ideal antenna which radiates in all directions and has a gain of 1 (0 dB), i.e. zero gain and zero loss. It is used to compare the power level of a given antenna to the theoretical isotropic antenna.

Figure 2: Radiation Pattern of an Isotropic Antenna

Antennas can be broadly classified as omnidirectional and directional antennas, which depends on the directionality.

Unlike isotropic antennas, dipole antennas are real antennas. The dipole radiation pattern is 360 degrees in the horizontal plane and approximately 75 degrees in the vertical plane (this assumes the dipole antenna is standing vertically) and resembles a donut in shape. Because the beam is slightly concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi, which is in comparison to an isotropic antenna. The higher the gain of the antennas, the smaller the vertical beamwidth is.

Imagine the radiation pattern of an isotropic antenna as a balloon, which extends from the antenna equally in all directions. Now imagine that you press in on the top and bottom of the balloon. This causes the balloon to expand in an outward direction, which covers more area in the horizontal pattern, but reduces the coverage area above and below the antenna. This yields a higher gain, as the antenna appears to extend to a larger coverage area.

Figure 3: Radiation Pattern of an Omni Antenna

Omnidirectional antennas have a similar radiation pattern. These antennas provide a 360 degree horizontal radiation pattern. These are used when coverage is required in all directions (horizontally) from the antenna with varying degrees of vertical coverage. Polarization is the physical orientation of the element on the antenna that actually emits the RF energy. An omnidirectional antenna, for example, is usually a vertical polarized antenna.

Figure 4: Antenna Polarization

Directional antennas focus the RF energy in a particular direction. As the gain of a directional antenna increases, the coverage distance increases, but the effective coverage angle decreases. For directional antennas, the lobes are pushed in a certain direction and little energy is there on the back side of the antenna.

Figure 5: Radiation Pattern of a Directional Antenna

Another important aspect of the antenna is the front-to-back ratio. It measures the directivity of the antenna. It is a ratio of energy which antenna is directing in a particular direction, which depends on its radiation pattern to the energy which is left behind the antenna or wasted. The higher the gain of the antenna, the higher the front-to-back ratio is. A good antenna front-to-back ratio is normally 20 dB.

Figure 6: Typical Radiation Pattern of a Directional Antenna with Calibrated Lobes

An antenna can have a gain of 21 dBi, a front-to-back ratio of 20 dB or a front-to-side ratio of 15 dB. This means the gain in the backward direction is 1 dBi, and gain off the side is 6 dBi. In order to optimize the overall performance of a wireless LAN, it is important to understand how to maximize radio coverage with the appropriate antenna selection and placement.

Indoor Effects

Wireless propagation can be effected by reflection, refraction or diffraction in a particular environment. Diffraction is the bending of waves around the corners. RF waves can take multipaths between the transmitter and receiver. A multipath is a combination of a primary signal and reflected, refracted or diffracted signal. So on the receiver side, the reflected signals combined with the direct signal can corrupt the signal or increase the amplitude of the signal, which depends on the phases of these signals. Because the distance traveled by the direct signal is shorter than the bounced signal, the time differential causes two signals to be received.

These signals are overlapped and combined into a single one. In real life, the time between the first received signal and the last echoed signal is called the delay spread. Delay spread is the parameter used to signify multipath. The delay of the reflected signals is measured in nano seconds. The amount of delay spread depends upon the amount of obstacles or infrastructure present between the transmitter and receiver. Therefore, delay spread has more value for the manufacturing floor due to lot of metallic structure present as compared to the home environment. Overall, multipath limits the data rate or lowers the performance.

Figure 7: Multipath Effects in Indoor Environment

Indoor RF propagation is not the same as it is outdoors. This is due to the presence of solid obstructions, ceilings, and floors that contribute to attenuation and multipath signal losses. Therefore, multipath or delay spread is more in the indoor environment. If the delay spread is more, the interference is more and will cause lower throughput at a particular data rate.

Indoor environment can also be classified as near line of sight (LOS) and non LOS. In near LOS environments, where you can see access points (APs) such as in the hallways, multipath is usually minor and can be overcome easily. The amplitudes of the echoed signals are much smaller than the primary one. However, in non LOS conditions, the echoed signals can have higher power levels, because the primary signal might be partially or totally obstructed, and generally more multipath is present.

Multipath has been a semi-fixed event. However, other factors such as moving objects can enter into play. The particular multipath condition changes from one sample period to the next. This is called time variation.

Multipath interference can cause the RF energy of an antenna to be very high, but the data is unrecoverable. You should not limit the analysis only to the p

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