Professional Communications Antennas, Filters and RF Combiners Design, Development and Production 1. What is an antenna? 2. Antenna Types 2.1. Base Station Antennas 2.2. Mobile Antennas 3. How ”good” is the antenna? 4. Measurements of antennas Page 1 of 12 PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved Professional Communications Antennas, Filters and RF Combiners Design, Development and Production 1. What is an antenna? An antenna is a transducer An antenna is a transducer, i.e. a ”converter” between two media. The antenna converts electromagnetic energy in a cable into electromagnetic radiated power in free space. Various characteristics of the antenna determine ”how efficient” this conversion is performed. P radiated P cable 1. To have as much energy as possible carried on as radiation in the room is of great importance, i.e. the antenna should not be capable of consuming energy itself. 2. It is important that the antenna concentrates its radiated energy as efficiently as possible in the required way. If so, either a wide coverage range or a reduced transmitting power can be achieved, until the range is exactly suiting the one required. Omnidirectional Antenna The antenna converts electromagnetic energy in a cable into electromagnetic radiated power in free space Directional Antenna Consequently, antenna technology is simply a question of economizing on the energy. Page 2 of 12 PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved Professional Communications Antennas, Filters and RF Combiners Design, Development and Production 2. Antenna Types Communications antenna types can be distinguished between: 2.1. Base Station Antennas 2.2. Antennas for movable (”mobile”) units Base Station Antenna ”Mobile” antennas can be split up into: 1. Land Mobile Antennas (vehicle antennas) Land Mobile Antenna 2. Marine Antennas (for ships) Marine Antenna 3. Portable Antennas (antennas for portable communication equipment) Portable Antenna (4. Air Craft Antennas) Portable Antenna Air Craft Antenna Colloquially, the word ”mobile antenna” is usually used as designation for a vehicle antenna. Page 3 of 12 PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved Professional Communications Antennas, Filters and RF Combiners Design, Development and Production 2.1. Base Station Antennas Base station antennas can be omnidirectional, directional or have a radiation being specially constructed, the latter yielding the antenna a quite specific coverage range. Omnidirectional LinkAntennas Connection (Directional) Omnidirectional Omnidirectional antennas are mostly applied for communication with mobile units when the positioning is unpredictable in relation to the base station antenna. Directional antennas Directional antennas are applied at areas or in a limited area where the receiver of the transmitted signal is. Antennas with special custom-made directional characteristics Antennas with special custom-made directional characteristics are applied for coverage of an accurately limited area, possibly an unsymmetrical area, usually with the purpose of not interfering with or being interfered by other radio systems. Delimited coverage Page 4 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 2.2. Antennas for Mobile Units 2.2.1. Land Mobile Antennas Quarter-wave Antennas The most common vehicle antennas utilize the car roof as one half of the antenna system. The antenna is a so-called ”monopoly on ground plane”. The basic type is a so-called quarter-wave (¼ λ) antenna. The fact that the antenna being close to 50 Ω is utilized when the whip has a length of approx. 1/4 wave length at the operating frequency. By lengthening the whip, the antenna gain will be increased. The gain will, however, again be reduced if the whip length is above 5/8 wave length. By lengthening the whip, the antenna gain will be increased. Many antennas utilize the car roof as one half of the antenna system. Colinear Antennas In case higher gain should be achieved, so-called ”colinear” antennas should be applied, for which more radiating elements are stacked and operate together, thus concentrating the radiation in the horizon. The optimum mounting site is in the centre of the car roof, at which the optimum omnidirectional characteristics are achieved. Page 5 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 2.2.2. Marine Antennas Marine antennas are in a way very similar to omnidirectional base station antennas except for the fact that usually intensified requirements as to mechanical sturdiness and corrosionresistance are made. Maritime communication is mainly taking place on medium wave, short wave and the maritime VHF bands. HF Wireantenna HF antenna HF antenna Marine antennas are similar to omnidirectional base station antennas but with intensified requirements of sturdines. Page 6 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 2.2.3. Portable Antennas Quarter-wave Antennas Like mobile antennas portable antennas are mostly ¼ wave ”monopoly” antennas, which utilize the chassis of the portable radio as ground plane, i.e. as one half of the antenna system. This usually results in insufficient ground plane as well as the fact that the radiation from the portable radios is mostly very badly defined owing to the presence of the hand or body, and in general a low efficiency for portable antenna systems should be taken into consideration, both because of disadjustment loss and loss owing to ”overturned” radiation. Portable antennas are mostly ¼ wave ”monopoly” antennas. An improvement of 5 dB can be achieved Half-wave Antennas by applying a ½ wave antenna together with If, however, antenna whips with a an adaption circuit. ½ wave length are applied together with an adaptation circuit the antenna being independent of the cabinet can be achieved. The antenna functions ”in itself” and a considerable improvement of 5 dB as average value can be achieved (compared to a ¼ wave antenna on the same device). 2.2.4. Air Craft Antennas Formerly, air craft antennas were always placed out of the fuselage, but nowadays the technology being necessary for building-in the antennas as an integrated part of the fuselage is mastered, thus still obtaining a satisfactory radiation characteristic. Nowadays air craft antennas are an integrated part of the fuselage. Page 7 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 3. How to describe how good an antenna is? Four ”figures” describe how good an antenna is compared to the required performance: SWR = Standing wave ratio D = Directivity G = Gain BW = Bandwidth (We will not deal with the term ”polarisation” in this respect). 3.1. SWR If the impedance of the antenna is different from the impedance of the cable, the antenna will reflect back some of the induced energy through the feeder cable to the transmitter, which naturally is undesirable. Normally, the impedance of the cable is 50 Ω. If Ra indicates the impedance of the antenna, the standing wave ratio is defined as: SWR = Ra/50 Ω (if Ra is more than 50 Ω) SWR = 50/Ra Ω (if Ra is less than 50 Ω) Examples: If Ra = If Ra = If Ra = 50 Ω 100 Ω 25 Ω SWR = SWR = SWR = 1.0 2.0 2.0 Consequently, it is of importance that the SWR is as close to 1.0 as possible thus obtaining the highest power being transmitted from the cable to the antenna. Page 8 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 3.2. Directivity D The directivity D is an indication of the capability of the antenna to conduct the radiated power ”to a certain site”. Normally, omnidirectional or directional antennas are mentioned. Omnidirectional An omnidirectional antenna with high directivity has a radiation being similar to a pancake. Directional A directional antenna with high directivity has a radiation being similar to the cone of light from a projector. Omnidirectional Omnidirectional Directional Directional 3.3. Gain G Omnidirectional The gain of an antenna is defined as G = η ·D, where η indicates the efficiency of the antenna. Consequently, in the gain value possible loss in the antenna is comprised. The η -figure is always than the directivity. For most antenna types the own loss is so low that G = D can be considered. 3.4. Bandwidth The bandwidth of the antenna is the frequency range, in which it operates properly, i.e. both gain and SWR are within the more specified limits. Page 9 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 4. How to measure how good an antenna is? 4.1. Measuring SWR Coaxial cable Standing Wave Ratio The standing wave ratio is normally measured by inserting a device in the feeder cable of the antenna. How much power reflected back from the antenna compared to the power transmitted forward is measured. Such a device is called a directional coupler. Forward power Reflected power Directional couplers Directional couplers or SWR meters for frequencies lower than 150 MHz are relatively cheap and quite accurate. For frequencies above 400 MHz exact meters are expensive. The most wellknown universal instrument is a BIRD-wattmeter. SWRmeter Transmitter Measuring the standing wave ratio. 4.2. Measuring gain The gain of an antenna is measured in relation to the gain of another further specified antenna through a comparison measurement. The latter antenna is called a reference antenna and the reference antenna is different, irrespective of wether base station antennas, mobile antennas or portable antennas are considered. Direct radiation Reflected radiation Measurement setup/procedure Page 10 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved PROCOM A/S EIA RS-329-1 Standard for Base Station and Mobile Antennas EIA RS-329-1 is an internationally valid standard for measuring gain for base station and mobile antennas. According to this standard the gain of base station antennas is specified compared to a ½ wave dipole and the gain of mobile antennas is specified compared to a ¼ wave whip in the centre of the car roof. EIA RS-329-1 standard is used for base station and land mobile antennas. No international standard for portable antennas No international standard for measuring gain of portable antennas is available. Normal practice is, however, to apply a ” ¼ wave antenna on same device” as a reference antenna. Owing to the indefinable ground plane, which a hand-held radio constitutes, it is necessary to perform statistical average value calculations for a large number of measurements in order to be able to state the gain of a portable antenna rather precisely. CCIR 368.3 Standard for Medium and short Wave Antennas Medium wave and partly short wave maritime antennas are measured in accordance with the international standard CCIR 368.3. The reference antenna is in this respect a ¼ wave monopoly antenna above water, as for which calculated field strength is determined as function of the distance from the antenna when applied with a certain power. Measuring and Calculating By measuring the gain of an antenna, a signal of a known size is applied and the field strength is measured at a certain distance. By calculating back it can be figured out to which degree the antenna has (generally) a poorer performance compared to the reference antenna. Gain will in such cases be specified as an efficiency. Antennas for maritime VHF are measured like base station antennas. Direct radiation Reflected radiation Page 11 of 12 Professional Communications Antennas, Filters and RF Combiners Design, Development and Production PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved 4.3. Measuring BW The bandwidth is generally specified as the area of the lowest part of the frequency ranges, in which SWR and gain observe the specifications. ¼ wave and collinear at the car roof centre A 1/4 wave on roof B Collinear at the roof centre A Collinear on rear wing Antennas placed onBthe and on Antennas placed on car the roof car roof. Collinear on the rear wing. roof Page 12 of 12 PROCOM A/S Smedetoften 12, DK-3600 Frederikssund, Tel.: 48 27 84 84, Fax: 48 27 85 48 ©PROCOM A/S, All rights reserved
© Copyright 2024