Bode 100 - Application Note Evaluation of Broadcast and Ham Radio Antennas Page 1 of 19 Evaluation of Broadcast and Ham Radio Antennas with the Bode 100 By Stephan Synkule © 2011 Omicron Lab – V1.0 Visit www.omicron-lab.com for more information. Contact support@omicron-lab.com for technical support. Smart Measurement Solutions Bode 100 - Application Note Evaluation of Broadcast and Ham Radio Antennas Page 2 of 19 Table of Contents 1 Abstract .3 2 Measurement Tasks .3 3 Theory, Measurement Setup & Results .4 3.1 VSWR, Impedance and r.4 3.2 Direct Measurement of a Broadcast Antenna .5 3.2.1 Description of the Measured Antenna .5 3.2.2 Reflection Loss & Impedance Measurement .5 3.2.3 Measurement of VSWR and r .8 3.2.4 Potential Problems with Direct Measurements .9 3.3 Broadcast Antenna Measurements with External Coupler .10 3.3.1 Measurement setup .10 3.3.2 Impedance / VSWR and r Measurement in the External Coupler Mode .11 3.4 Measurement of a Ham Radio Antenna .14 3.4.1 Description of the Measured Antennas .14 3.4.2 Impedance / VSWR and r Measurement.15 4 Conclusion .18 Note: Basic procedures such as setting-up, adjusting and calibrating the Bode 100 are described in the Bode 100 user manual. Note: All measurements in this application note have been performed with the Bode Analyzer Suite V2.31. Use this version or a higher version to perform the measurements detailed in this application note. You can download the latest version at http://www.omicron-lab.com/downloads.html . Smart Measurement Solutions Bode 100 - Application Note Evaluation of Broadcast and Ham Radio Antennas Page 3 of 19 1 Abstract This application note explains how to measure the impedance, reflection factor and VSWR of broadcast antennas as well as Ham Radio Anten nas. These antenna parameters are measured with the following two measurement methods supported by the Bode 100: ? ? Direct Impedance / Reflection / VSWR measurement in the frequency sweep mode Impedance / Reflection / VSWR Measurement with increased output power using an external power amplifier and an external directional coupler. In the appendix of this document you will find a summary on things to consider when you use external directional couplers and amplifiers in combination with the Bode 100. 2 Measurement Tasks To avoid reflections and to ensure that the maximum broadcast power is achieved, the impedance of a broadcast or Ham Radio antenna needs to match the impedance of the connected signal source. The Voltage Standing Wave Ratio (VSWR) and the reflection factor (r) are two internationally used parameters to qualify how well an antenna matches its reference resistance. By analyzing a medium wave broadcast antenna as well as a two different Ham radio antennas the following topics are covered: ? Some theory about the relationship between impedance, VSWR and r ? Direct Impedance / VSWR / r measurement with the Bode 100 ? W hat problems can arise when antennas are measured? ? The advantages of Impedance / VSWR / r measurements using the external coupler mode of the Bode 100 ? Appendix: Using amplifiers with the Bode 100 Smart Measurement Solutions Bode 100 - Application Note Evaluation of Broadcast and Ham Radio Antennas Page 4 of 19 3 Theory, Measurement Setup & Results 3.1 VSWR, Impedance and r The Bode 100 directly measures the complex impedance Z by analyzing the forward and reflected wave. The reflection coefficient r and the resulting VSWR are than calculated from the measured Z and the reference resistance R0 using the following relations: …voltage at the reference plane …current at the reference plane …Impedance V I Z r …reflection coefficient VSWR …Voltage Standing Wave Ratio R0 …reference resistance Note: The reference resistance R0 can be set in the Measurement area. The reference resistance is used for the calculation of the reflection coefficient and therefore influences the VSWR result as well. The Bode Analyzer Suite enables you to change the reference resistance in the range from 1 ?.1 k?. This allows calculating the VSWR and r also for systems with an R0 ? 50 ?. Example: The center of the Smith chart indicates the point with ideal matching ( r =0). Reflection: R0= 50 ? 40 Reflection: R0= 75 ? 50 60 75 30 75 112.5 45 150 100 30 20 225 150 200 250 10 300 375 15 750 500 10 20 30 40 50 1,000 2,500 75 100 15020050 500 2,500 2 1,000 -2,500 -1,000 15 30 45 60 75 1,500 3,750 112.5150 22530075 750 3,750 3 1,500 -3,750 -1,500 -750 -500 -10 -250 -200 -15 -375 -300 -225 -150 -30 -20 -150 -100 -30 -40 -45 -75 -60 -50 TR2: Reflection -112.5 -75 TR2: Reflection The charts above show a measurement of the same filter terminated with 50 ?. The left chart was calculated for R0= 50 ? while the right one shows the results for R0= 75 ?. Smart Measurement Solutions Bode 100 - Application Note Evaluation of Broadcast and Ham Radio Antennas Page 5 of 19 It is important to minimize reflected signals to maximize the power radiated by the antenna. The optimum efficiency of a broadcast antenna is reached when the impedance of the antenna exactly matches the source resistance (= reference resistance R0). The reflection coefficient r is defined in the range from 1 to -1 and the VSWR parameter in the range from 1 (for ideal matching with r =0) to infinite. 3.2 Direct Measurement of a Broadcast Antenna 3.2.1 Description of the Measured Antenna All measurements described in this chapter where performed on a medium wave transmission site which was not in use at the time this document was written. The measured broadcast antenna has the following characteristics: ? ? ? ? The antenna aerial has a triangular cross section and an omni directional, vertically polarized antenna pattern The antenna height (length) is 50 meters The electrical length of the antenna is defined by its roof capacitance and a coil which is connected in series to the antenna. The electrical antenna length was optimized for an AM medium wave signal with a carrier frequency of 1.026 MHz. 3.2.2 Reflection Loss & Impedance Measurement Before we started our measurements we used a spectrum analyzer to check the signal received by the antenna to exclude possible problems for the input channels of the Bode 100. Smart Measurement Solutions