EC6701 RF&MW - VSWR Meter

VSWR Meter

1. Academic Year 2018-19 EC6710 RF & Microwave Engineering - VSWR Meter Mrs.R.Chitra , AP/ECE Ramco Institute of Technology, Rajapalayam .

2. UNIT V MICROWAVE MEASUREMENTS Measuring Instruments : Principle of operation and application of VSWR meter Power meter Spectrum analyzer Network analyzer Measurement of Impedance Frequency, Power, VSWR, Q-factor Dielectric constant Scattering coefficients Attenuation S-parameters.

3. INTRODUCTION At low frequency , it is convenient to measure voltage, current and frequency and use them to calculate the power . However , at microwave frequencies, the amplitude of the voltage and current on a transmission line are function of distance and are not easily measurable . Hence , at microwave frequency it is more desirable and simpler to measure power directly . Therefore, it is necessary to measure power instead of voltage and current.

4. The microwave parameters namely are; S-parameters Power Frequency and wavelength Phase Shift Voltage Standing Wave Ratio (VSWR) Noise Figure Q-factor Attenuation

5. MEASURING INSTRUMENT A test and measurement system consists of one or more measuring instruments and one Device Under Test (DUT ). The microwave parameters are measured by using the following instruments: VSWR meter Power meter Spectrum analyzer Network analyzer

6. Standing - wave ratio  (SWR) is a mathematical expression of the non-uniformity of an electromagnetic field (EM field) on a transmission line such as coaxial cable. Usually , SWR is defined as the  ratio  of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line . In  radio engineering  and  telecommunications ,  standing wave ratio  ( SWR ) is a measure of  impedance matching  of  loads  to the  characteristic impedance  of a  transmission line  or  waveguide . Impedance mismatches result in  standing waves  along the transmission line, and SWR is defined as the ratio of the partial  standing wave 's amplitude at an antinode (maximum) to the amplitude at a  node  (minimum) along the line.

7. Principle of Operation and Application of VSWR Meter VSWR meters are widely used to measure VSWR. VSWR is defined as ratio between the maximum voltage to minimum voltage Also it is noticed that, 1 ≤ VSWR ≤ ∞ VSWR = 1 for a perfectly matched network VSWR = ∞ for short and open circuits and mismatched circuits It is a measure of how well the components of the RF network are matched in impedance . Standing waves occur, when Impedances are not identical Signal loss during transmission Poor reception Larger the impedance mismatch

8. CONSTRUCTION The typical VSWR meter is a high gain, low noise amplifier . It is tuned to the modulating frequency of the microwave signal (1 KHz). The display panel of the meter consists of three scales: Normal Scale Expanded Scale dB Scale Normal Scale: If the reading of VSWR meter is between 1 and 4, then the top normal SWR scale can be used. If the VSWR reading is between 3 and 10, the lower SWR scale is used. Expanded Scale: If the VSWR is smaller than 1.3, a more accurate reading can be taken by selecting the Expanded SWR scale, graduated from 1 to 1.3. Expanded dB Scale: Third scale at the bottom is graduated from in dB.

9. To measure VSWR , the meter needle is initially adjusted to 1 after placing the probe in Vmax position within the guide. The gain control panel is used for making this adjustment. For any input voltage, the amplifier output voltage is measured directly in terms of VSWR. The input of the VSWR meter is the detected output voltage of the tuned detector that is fed by a coaxial cable.

10. PRINCIPLE OF OPERATION VSWR meters are widely used to determine the amount of radio frequency that is being reflected back to the transmitter / source compared to the amount that is being sent out during transmission . If the ratio is high, the source will surely damage . The maximum standard rating for a VSWR meter where a transmission can withstand is 1.5:1 . The ideal rating is 1:1 ( i.e., the power reaches the destination and without reflection ) Types of SWR meters Directional SWR meter SWR bridge circuit

11. Directional SWR meter A directional SWR meter is generally used to measure the magnitude of the transmitted and reflected waves by sensing each one separately, with the help of directional couplers. Here the transmitter and antenna are connected through an internal transmission line . The line is electro-magnetically couple with the two directional couplers. The directional couplers are then connected to the resistors at one end and diode rectifiers at the other end. The resistors help in matching the characteristic impedance of the sensing lines. The diode helps in converting the magnitude of the forward and reverse waves to DC voltages will then be smoothened with the help of capacitors.

12. SWR Bridge Circuit A typical impedance bridge is built for determining the SWR impedance. The test impedance is increased until the bridge circuit is balanced, (i.e.) the test impedance will be equal to the reference impedance. Thus, with the help of a bridge circuit we can know whether a SWR is present or not . PROBLEMS WITH VSWR METERS VSWR meter cannot measure reactance and resistance, instead measures the impedance Reading to be accurate , It must be matched on to the line impednace .

13. Any transmission line will have a certain loss. When this loss occours , the reflected signal will be attenuated when it travels back along the line. Thus, for the SWR meter to be efficient, it must be connected very near to the antenna and further away from the load. The value of SWR will be more when it is nearer to the load. Internal diodes of an SWR meters may generate harmonics while transmitting and produce intermodulation products during reception.

14. APPLICATIONS OF VSWR METER Laboratories Live circuits (Live telecast systems) Rail guides, flanges and tabs Telecommunication racks Microwave based medical applications

15. References: Reinhold Ludwig and Gene Bogdanov , “RF Circuit Design: Theory and Applications”, Pearson Education Inc., 2011 David M. Pozar , “Microwave Engineering”, Wiley India (P) Ltd, New Delhi, 2008. Thomas H Lee, “Planar Microwave Engineering: A Practical Guide to Theory, Measurements and Circuits”, Cambridge University Press, 2004. Mathew M Radmanesh , “RF and Microwave Electronics”, Prentice Hall, 2000. Annapurna Das and Sisir K Das, “Microwave Engineering”, Tata McGraw Hill Publishing Company Ltd, New Delhi, 2005.