Colubris Networks. Antenna Guide

Transcription

1 Colubris Networks Antenna Guide Creation Date: February 10, 2006 Revision: 1.0

2 Table of Contents 1. INTRODUCTION ANTENNA TYPES OMNI-DIRECTIONAL ANTENNA DIRECTIONAL ANTENNA ANTENNA DIVERSITY ANTENNA PROPERTIES GAIN AND DIRECTIVITY HORIZONTAL AND VERTICAL POLARIZATION VOLTAGE STANDING WAVE RATIO (VSWR) RETURN LOSS COAXIAL CABLE EIRP (EFFECTIVE ISOTROPICALLY RADIATED POWER) LOCATION AND ORIENTATION OF THE ANTENNA POINT TO POINT WDS MODE FRESNEL ZONE FREE SPACE LOSS DATA RATE VS. DISTANCE TABLE COLUBRIS APPROVED ANTENNAS Colubris Networks, Inc. Proprietary & Confidential Page 2 of 13

3 1. Introduction Antennas play an important role in overall WLAN configuration by directing radio frequencies into areas in specific coverage patterns. Becoming familiar with basic antenna technology and understanding the factors affecting antenna function will help in the selection of the right antennas for specific applications. The correct antenna will provide the coverage pattern best suited to individual site requirements. This guide is intended to provide basic information on how to select the correct antenna for your applications. 2. Antenna Types Two types of antennas are typically used in WLAN applications, omni-directional antennas and directional antennas Omni-directional Antenna Omni-directional antennas provide a 360-degree coverage pattern on the horizontal plane. In other words, the pattern is doughnut-shaped, which is ideal for square or close to square areas (Figure 1). Colubris Networks, Inc. Proprietary & Confidential Page 3 of 13

4 2.2. Directional Antenna Directional antennas concentrate the coverage in one direction by creating a conical pattern (Figure 2). Antenna pattern directionality is specified by the angle of the beam width, which is typically from 15 degrees to 90 degrees. Directional antennas are ideal for elongated areas, corners and outdoor point -to- point applications Antenna Diversity All Colubris access points have two antenna connectors per radio (except MAP-330R which only has one antenna per radio).one of these is used as the primary transmitting and receiving port, while the other is periodically checked (polled) to see if it is receiving a stronger signal than the main antenna. This is called a "diversity" antenna system. Diversity operation helps improve system performance when signals are being reflected along different paths to the antenna. It can help to reduce variations in signal strength as you vary the location of an access point and a client. Colubris Networks, Inc. Proprietary & Confidential Page 4 of 13

5 3. Antenna Properties 3.1. Gain and Directivity Antenna gain is a measure of directivity. Think of an antenna as a light bulb. An isotropic source has radiated energy that is spherical and provides equal intensity radiation in all directions. This is very similar to the light given by a single bulb without any fixture. When the light bulb is placed into a housing that provides a reflector (like a flashlight), the light is emitted in a direction that is controlled by the reflector and focused at some point / direction. This is similar to the concept of adding gain to an antenna. The power consumed by the bulb or antenna is not any greater than the input power to the bulb or antenna, but the signal is directed and shaped to provide a higher intensity of illumination / radiation at a desired location and a lower intensity of illumination / radiation at a point that is not desired. In the case of antennas, passive structures cannot generate power. db is used to describe the ability of these structures to focus energy in a part of space. Power Gain 3 db = 2X power 6 db = 4X power 10 db = 10X power 20 db = 100X power Power Loss -3 db = 1/2 power -6 db = 1/4 power -10 db = 1/10 power -20 db = 1/100 power Since all real antennas will radiate more in some directions than in others, you can say that gain is the amount of power you can reach in one direction at the expense of the power lost in the others. When talking about gain it is always the main lobe that is discussed. Gain is determined by many factors. Type of antenna, electrical length, antenna placement and antenna orientation all contribute to pattern shape and subsequently gain. Gain and directivity can be controlled to a certain extent. For instance, in some applications, it is beneficial to direct radiated power towards the horizon for increased range. Colubris Networks, Inc. Proprietary & Confidential Page 5 of 13

6 3.2. Horizontal and Vertical Polarization Antenna polarization is a very important consideration when choosing and installing an antenna. Knowing the difference between polarizations and how to maximize their benefit is very important to the antenna user. An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The electric field or "E" plane determines the polarization or orientation of the radio wave. A linear polarized antenna radiates only in one plane containing the direction of propagation. An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the ground. Horizontally polarized (linear) antennas have their electric field parallel to the ground. The polarization of each antenna in a system should be properly aligned. Maximum signal strength between access point and client station occurs when both are using identical polarization. In general, horizontally polarized antennas generally propagate better within a building, probably due to reflections from the floor and ceiling. When the WLAN signal hits an object, such as a metal cabinet or pole, it is reflected, and its polarization is scattered. Inside any work area there will be a mixture of vertically and horizontally polarized signals. On line-of-sight (LOS) paths, it is most important that the polarization of the antennas at both ends of the path use the same polarization. In a linearly polarized system, a misalignment of polarization of 45 degrees will degrade the signal up to 3 db and if misaligned 90 degrees the attenuation can be 20 db or more Colubris Networks, Inc. Proprietary & Confidential Page 6 of 13

7 3.3. Voltage Standing Wave Ratio (VSWR) VSWR is a measure of the amount of energy absorbed and radiated by an antenna compared to the amount it reflects back to the transmitter. A VSWR value of 1:1 is perfect (no reflected energy), while a WLAN antenna should have an SWR less than 1.5:1. A 2:1 VSWR implies a reflected voltage twice that of the forward voltage, the actual loss in radiation is 10% or 0.5 db. Therefore, it is very important to attain the best impedance match possible for maximum efficiency in the antenna system i.e. an antenna with 50 ohm impedance should be used with 50 ohm cable Return Loss This is basically the same thing as VSWR. If 50 % of the signal is absorbed by the antenna and 50 % is reflected back, we say that the Return Loss is -3dB. A very good antenna might have a value of -10dB (90 % absorbed & 10 % reflected). 4. Coaxial Cable It is critical to use high quality coaxial cable with SMA or N-type connectors to minimize cable loss. For example, RG-174 thin coax has attenuation (loss) of about 3dB for every 5 2.4GHz (3 5GHz) i.e. it will loose half the signal power every 3 or 5 feet of cable length. On the other hand, LMR-400 (Times Microwave) cable only has a loss of 3dB for every 45 feet of cable at 2.4GHz (30 5GHz). Typical attenuation for common cable types: Cable Type Cable Length (feet) Frequency GHz) Attenuation (db) RG LMR-100A LMR LMR SMA Connector N-type Connector Colubris Networks, Inc. Proprietary & Confidential Page 7 of 13

8 5. EIRP (Effective Isotropically Radiated Power) Effective isotropically radiated power (EIRP) is the amount of power that would have to be emitted by an isotropic antenna (that evenly distributes power in all directions) to produce the peak power density observed in the direction of maximum antenna gain. EIRP takes into account the losses in antenna cable and connectors and the gain of the antenna. The EIRP is often stated in terms of db (decibels) over a reference power level that would be the power emitted by an isotropic radiator with equivalent signal strength. EIRP (dbm) = (power of transmitter (dbm)) (losses in cable/connector (db)) + (antenna gain (db)) A good rule of thumb to remember here is that a change of -3dB is equal to a 50 percent power drop. And conversely an increase in power of 3dB is equivalent to doubling the power level. This can be seen in Table below. dbm mw Note: dbm uses a reference of 1mW dbm = 10 log(power out mw/ 1mW) Colubris Networks, Inc. Proprietary & Confidential Page 8 of 13

9 6. Location and Orientation of the Antenna Another important factor when designing an antenna system is the location and orientation of the antenna on the wireless device. Propagation within buildings is governed by the following factors: attenuation due to walls, reflection from walls, ceiling, floor, etc. and diffraction from obstacles within a building. With this in mind, it is best to mount the antenna higher than any obstacle between the transmitter and receiver and to orient it in the same electrical direction (polarity) as the receiver antenna. Unfortunately, this is usually not possible in most portable applications. Two important things to consider when placing the antenna are: 1. Any metal surfaces that surround or partially surround the antenna will distort the radiation pattern. This distortion will impair the quality of transmission. Therefore, the antenna must be placed outside of the shielded housing of the device. Also, it should be placed as far away horizontally from any surface that would block the line of sight of the receiver antenna. 2. Another consideration while locating an antenna on the appliance is the user effect. It is best to locate the antenna away from the user s body. The interaction between user and the antenna can cause a de-tuning effect on the antenna and also the user can absorb energy that was to be transmitted into radio signals. Proper antenna positioning and installation will ensure the best coverage and service. For indoor applications, antennas should be mounted as high and clear of obstructions as possible. If possible, mount on or close to the ceiling. When mounting antennas on the ceiling, it is important to try to keep at least two feet from sprinkler heads and metal lighting fixtures. For both indoor and outdoor applications, try to keep RF cables as short as possible to minimize RF loss. Try to place the access point as close to the antenna as possible by bringing the Ethernet cable to the access point. Colubris Networks, Inc. Proprietary & Confidential Page 9 of 13

10 7. Point to Point WDS Mode 7.1. Fresnel Zone d r The Fresnel zone for a radio beam is an elliptical area immediately surrounding the visual path (see above diagram). It varies in thickness depending on the length of the signal path and the frequency of the signal. One would want a clear line of sight to maintain signal strength, especially for 2.4 GHz wireless systems. This is because 2.4 GHz waves are absorbed by water, like the water found in trees. Any obstructions that enter into the Fresnel zone will reduce the communication range; including buildings, vegetation, the ground, etc. As the antennas get further apart and the diameter of the Fresnel zone increases, the ground can begin to obstruct the Fresnel zone. In order to keep the entire Fresnel zone free of obstructions it is necessary to raise the antennas. To keep the Fresnel zone off the ground the heights of the antennas added together must total more than the diameter of the Fresnel zone at the specific distance. The necessary clearance for the Fresnel zone can be calculated, and it must be taken into account when designing a wireless links. r = x d / 4f Where r = radius of the center line of the link in feet d = distance between the two antennas in miles f = radio frequency in GHz Note the above formula does not take the earth curvature into consideration. Colubris Networks, Inc. Proprietary & Confidential Page 10 of 13

11 Typically, 20% Fresnel Zone blockage introduces little signal loss to the link. Beyond 40% blockage, signal loss will become significant Free Space Loss Free space loss is the power loss of a radio signal as it travels from the transmitter to the receiver. As the name implies, free space loss assumes the transmitter and receiver are both located in free space and does not consider other sources of loss such as reflections, cable, connectors etc. Similarly it does not take account of gains from particular antennas. A particularly convenient way to express free space loss is in terms of db. The loss can be expressed as: FSL = log f + 20 log d Where: f = radio frequency in MHz d = distance between antennas in miles In order for a wireless link to work, the available system operating margin (SOM) must exceed the Free Space loss and all other losses in the system. Receive Signal Level = Tx Power + Tx Antenna Gain Tx Cable Loss + Rx Antenna Gain Rx Cable Loss Rx Sensitivity SOM = Receive Signal Level Free Space Loss => this should be greater than 10dB Free Space Loss and Fresnel Zone Table: Distance d ( miles) 2.4 GHz 5.8 GHz Fresnel Free Fresnel Zone r Space Zone r (feet) Loss (feet) Free Space Loss (db) (db) For example, with our outdoor access point (MAP-320R or MAP-330R) using a standard omni-directional antenna with 5.5dBi gain at 2.4GHz (1Mbps data rate) with Rx Colubris Networks, Inc. Proprietary & Confidential Page 11 of 13

12 sensitivity of -92 dbm and Tx power of 19 dbm. It will be able to cover up 2 miles distance with 10 db SOM. 8. Data Rate vs. Distance Table Distance (meters) data rate Receive Sensitivity(dBm) Antenna Gain(dBi) TX Power(dBm) Outdoors Indoors Mode 2.4GHz b 1Mbps Mbps Mbps Mbps g 6Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps GHz a 6Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps GHz a 6Mbps Mbps Mbps Mbps Mbps Mbps Mbps Mbps Note: 1. Values are based on Colubris standard radio and antenna 2. Outdoor distance provides are based on a clear fresnel zone and line of sight between access point. 3. Indoor distances are based on clear office space Colubris Networks, Inc. Proprietary & Confidential Page 12 of 13

13 9. Colubris Approved Antennas Manufacturer Part # 2.4 GHz Gain 5.3 GHz Gain 5.7 GHz Gain Nearson T614AH-2.4/5.X-S 4 dbi 5 dbi 4.5 dbi Cushcraft S5153WBPX36RSM N/A 6 dbi 6dBi Mini-Box Outdoor Omni 5.5 dbi N/A N/a Centurion WTS2450-RPSMA 2.5 dbi 3.0 dbi 3.4 dbi Colubris Networks, Inc. Proprietary & Confidential Page 13 of 13