1、 TSB-179 November 2010 Terrestrial Land Mobile Radio Antenna Systems - Total “Antenna System” Return Loss Measurements NOTICE TIA Engineering Standards and Publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating
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22、IA WITHOUT SUCH LIMITATIONS. i TABLE OF CONTENTS FOREWORD ii Patent Identification . iii INTRODUCTION . iv 1.0 SCOPE . 1 2.0 BACKGROUND 2 3.0 DEFINITIONS AND ABBREVIATIONS . 3 4.0 SYSTEM VSWR ESTIMATION TOOLS . 4 4.1 Naval Electronic Warfare Handbook . 4 4.2 Early Calculation of System Return Loss 5
23、 4.3 Improved System Return Loss Calculation Method 6 4.4 Latest System Return Loss Spreadsheet Calculator 7 4.5 Tower Mounted Amplifier Example . 10 5.0 CONCLUSION 11 BIBLIOGRAPHY 12 ii FOREWORD (This foreword is not part of this Bulletin) This Bulletin was prepared by TIA Working Group TR-8.11.5 a
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28、ION Return Loss or VSWR sweeps taken at the bottom end of the transmission line are used by systems operators and contractors to determine the condition of the complete RF path including the antenna. For many years, this RF path consisted of the main transmission line, jumper cables and an antenna.
29、Several calculators were developed to estimate what the expected return loss or VSWR ought to be if all components were meeting specification. Since the antenna was by far the component with the highest VSWR, out-of-specification readings at the bottom end often could be traced to an out-of-specific
30、ation antenna. However, with the advent of more complex systems incorporating additional devices such as Tower Mounted Amplifiers and Diplexers, the antenna is no longer the only major VSWR contributor in the RF path. This Bulletin addresses the issues associated with measuring the complete RF path
31、and offer some suggestions about how to make more meaningful measurements. TIA TSB-179 1 Terrestrial Land Mobile Radio - Antenna Systems - Total “Antenna System” Return Loss Measurements 1.0 SCOPE In the land mobile radio service there has historically been no clear relationship between the VSWR spe
32、cifications of the individual components in the RF path and the expected resultant “system” VSWR at the bottom end of the transmission line. This makes it difficult, at best, for system operators to set a realistic set of measurement expectations for installers and technicians to use for checking co
33、mpleted installations Various calculators and nomographs have been developed over the years to try to estimate what the expected system VSWR ought to be for the total RF path, but, at best they treat the combination of the individual component VSWR vectors as if they combine in a random fashion. Thi
34、s might be true most of the time, but there might be those instances where random vector combinations do not happen. This Bulletin presents some of the nomographs and calculators that have been employed over the years and introduces an improved calculator that shows both typical and worst case vecto
35、r summation. It also presents some theoretical reference material detailing how the VSWRs of specific components might combine. 1. The use of manufacturers names is not an endorsement, rather an acknowledgement to the source of the material in this Bulletin. TIA TSB-179 2 2.0 BACKGROUND The terms Vo
36、ltage Standing Wave Ratio (VSWR), Return Loss (RL), Reflection Coefficient () and Mismatch Loss are all terms used to define the amount of mismatch between an RF source and an RF load; hence they are a measure of the efficiency of the RF transmission system. They are all mathematically related and c
37、an be easily converted using the equations in 3.0, 2. Some of the earliest information used for estimating the total system VSWR from the bottom end of the transmission line was presented as a simple nomograph. Later, calculation methods based on compiled measured data were suggested and they includ
38、ed “multiplication factor” based on data taken on thousands of antenna systems. With the advent of electronic spreadsheets, more complex calculators were developed to take into account all of the individual components in the RF path along with more specifications associated with the individual compo
39、nents. This approach also allowed for calculation of not only the typical system value where random vector combinations are employed, but also allows for calculation of the worst-case system value where all vectors add together in phase 1. Over the years, an antenna VSWR specification of 1.5:1 (14 d
40、B RL) has been deemed acceptable for most applications. This value equates to 96% forward power transfer or only 4% reflected power. Many system owners have increased this 14 dB return loss number by a few dB, which improves the VSWR (e.g., 16 dB RL to include a transmission line contribution) and d
41、efine it as the total system return loss goal. As the industry progresses to very high-speed data transmission, there is another phenomenon that may need to be considered. For example, throughput for 64 QAM modulations can be degraded due to distortion. A higher system VSWR translates to reduced mod
42、ulation fidelity, caused by non-optimal loading of the power amplifier (PA). This concept was introduced as non-optimal PA loading in a Network Interface Card (NIC) by its antenna. 7, 8. TIA TSB-179 3 3.0 DEFINITIONS AND ABBREVIATIONS EVM: Error Vector Magnitude loge: The natural logarithm MHA: Mast
43、 Head Amplifier (same as Tower Mounted Amplifier) NIC: Network Interface Card PA: Power Amplifier QAM: Quadrature Amplitude Modulation RF: Radio Frequency RL: Return Loss RF Path: The path an RF signal travels to get from the transmitter output until it is radiated by the antenna. This is not to be
44、confused with the propagation loss of the RF path between antennas. TMA: Tower Mounted Amplifier (same as Mast Head Amplifier) Equations: VSWR = 11= 20201 101 10RLRL= 11RFRFPPPP(1) Return Loss (RL) = 1 0 1 0 1 011 0 o g 2 0 o g 2 0 o g1FRP V S W RL L LP V S W R (2) Voltage Reflection Coefficient 201
45、 ( ) 1 01RLRFV S W R PV S W R P (3) Where: Return Loss(dB) is a positive value 0 TIA TSB-179 4 4.0 SYSTEM VSWR ESTIMATION TOOLS 4.1 Naval Electronic Warfare Handbook Early total system VSWR estimates made use of nomographs like the one shown in Figure 1 below from the Naval Electronic Warfare Handbo
46、ok 2: Figure 1 Reduction of VSWR by Attenuation The results obtained from Figure 1 are somewhat over-simplified in that they do not take into account the VSWR of the attenuator (transmission line). Instead, it simply adds twice the attenuator value (once to get up to the load and a second time to ge
47、t back to the bottom end) to produce the load VSWR. As an example, a load (antenna) with a 10:1 VSWR connected by an attenuator (transmission line) with a loss of 6 dB yields an input VSWR of 1.5:1. Although this is a somewhat optimistic number, it still provides insight into how the combined compon
48、ents react together.TIA TSB-179 5 Early Calculation of System Return Loss An early calculation of System Return Loss 3 was targeted for microwave systems and it first introduced the “multiplication factor” based on data taken on thousands of antenna systems. This calculator included only an antenna,
49、 top jumper, transmission line, surge arrestor and bottom jumper. It did not take into account jumper cable(s) and surge arrestor losses. This method calculates peak or worst case system VSWR. Step 1. Convert VSWR or return loss to reflection coefficients in decimal form for all components in the system. Step 2. Divide the components into 3 groups: Top antenna and jumper(s) Transmission Line Bottom surge arrestor and jumper(s) Step 3. Add the reflection coefficient of all the top components and convert
