Test Range
Contents
1. National Centre for Radio Astrophysics oe Mi Internal Technical Report GMRT FEED 2014 2 5 i H a a a Test Range Standard Operating Procedures G Sankar Email sankar ncra tifr res in Revision Date Modification Change Ver 1 2014 Initial Version Test Range Standard Operating Procedures G Sankar GMRT TIFR Aug 2013 1 Introduction This report is a user manual for the NCRA Test Range This is in sequence to the detailed design document and a description of the entire setup as given in a student s project report 2 Chapter 2 of 2 gives an overall view of the Test range and the test equipments associated with that Typical measurement results for three different types of feeds and fre quencies are given in Ch 3 of 2 This report is a compendium of smaller reports bearing relevance to the utilization of the Range and the measurement techniques 2 Measurements classification The Range is utilized for measuring the far field radiation patterns of feeds antennas over the chosen frequency range This can be broadly divided into e Principal plane patterns viz E and H planes e Cross polar patterns amp Co polar patterns e Phase centre measurements The SOPs for these sets of measurements will be referenced to the Fig 1 Fig 1 illustrates the typical geometry of the range The transmitting and the test antenna are mounted at such heights that their line of sights coincide2. Test antenna Orient the same way like the trans antenna the testing channel dipole arm should be parallel to x axis Normally a crossed dipole dual orthogonal channel feed arrangement will be mounted While one channel port is aligned the other channel should be terminated in a 50 Q load Face the test antenna towards the trans antenna on line of sight azimuth 0 Power up the signal generator amp the OF link Do not exceed the signal power beyond 8 0 dBm Higher power RFs can damage the OF system components Choose the correct frequency range Note The Agilent Sig Gen N9301A make sure that the MOD on off button is kept on off mode everytime HP 8108A Spectrum analyzer is used as the power detector Choose a convenient fre quency range commensurate with Sig Gen s freq range Set the default settings of RBW of 300 kHz and VBW of 3 kHz Many filters notch band pass types are available at the range they are identical pieces of the corresponding frequency band of GMRT front end electronics Use the correct ones based one the frequency band a list of available filters is given in Appendix B If any RFI lines are seen closer to the chosen frequency of the Sig Gen move away from that and choose a different frequency The received signal power should be 30 to 40 dB above noise floor choose a combination of the modular amplifiers to realize this Check the received power when the test antenna is facing a
3. The heights of these two are 10 5 m With a simple azimuth directional rotation both the E and H plane patterns can be measured The distance between the trans and test antennas is exactly 134 25 metres Later during the course measurements especially for phase centre an alternate shorter distance was chosen This is at 43 632 metres and the transmitting antenna was relocated a switch over to the longer range is easily doable The trans antenna as shown in Fig 1 is a linearly polarized dipole antenna A log periodic antenna linear pol is conventionally used as the trans antenna to cover wide range of fre quencies Z4 Transmitting gt lt Antenna 0 Test Antenna Azimuth rotating x platform Figure 1 Range Geometry 2 1 Powering up the Trans Antenna The longer range 134 25m is powered by a OF link between the two ends details and specifications are outlined in Appendix C Cables losses and additional amplifiers are avoided by this novel OF link The shorter range is powered by a low loss LMR400 RF cable connecting the trans antenna to the RF signal generator On the receiving end modules of amplifiers wide band ones having gains of 11 20 30 dBs are used A portable battery powered amplifier is also available to place it at the trans antenna base having 10 dB gain The battery inside this portable amplifier is a chargeable one This is especially required at higher frequencies above 1 5 GHz Present test equipments
4. 3 Per simon Kildal Foundations of Antennas A Unified Approach Studentlitteratur AB Lund Swedan 2000 4 IEEE Std 149 1979 IEEE Standard Test Procedures for Antennas John Wiley amp Sons Inc 1979 KKKKKK 11 Appendix A A New location of the Transmitting antenna for phase centre measurements A 1 Introduction The need to relocate the transmitting antenna especially for phase center measurements was poitned out in the main report This appendix illustrates the steps taken and records the theodolite survey procedures and associated computations The most viable locations were examined and any such location on the NCRA main build ing s terrace would be most suitable since powering up the trans antenna would be easier the FO technique as discussed on the main report would be dispensed with thus reducing the hardware A pole mounted above the water tank on the northern segment of the building was the final choice Fig 1 illustrates the plan view of the locations of trans and test antenna A 2 Survey techniques An accurate measurement of the line of sight distance between the trans antenna and the test antenna platform must be done first This quantity will be used in the phase centre measure ments The secondary requirement will be to exactly mark the trans anteena height on the pole such that its lies on the horizontal plane of the test antenna s mounting in other words the centres of both the antennas must coinc
5. data is converted into a spreadsheet string before being inserted into an array lution Bandwidth Auto F Wideo Bandwidth PositiolResolution Bandwidt ie Activitjes Clear Write 0 Sweep Time Auto F ideo Bandwidth Auto PEA d E S SANE OPTICAL Acquisiti Er m A Antenna SOSS E Ofer C amp BR Fig2 Spectrum data logging code control panel File Edit View Project Operate Tools Window Help lei 1 200t Avolication Font H Zorjan a amp 2 z n Sweep Time Auto F Manual Sweep 0 1 o 5 4 D x E 31000 Resolution Bandwidth Auto F Attenuation Auto 8 srol EN Jl Frequency Vs Power plot phi 0 Resolution Bandwidth error in Zog KHz fiz Video Bandwidth Auto F in SE Video Bandwidth ER 43 KHz D E 2 ei cree Level Start freq 3 420 0000 J 277M Log Scale 10 0 Stop 410 0000 P r AaS aa raaa A Peaid J 377M Reference Level Position 8 J d ZA 700M SSS a V P Se Activities Clear Write 0 VISA session Clear Write 18 View y GPIBO 18 INSTR K SH Max Min Hold Antenna test ra S Untitled Paint SCT OF Ce DR We 945m Fig3 Spectrum data logging code front panel In the above LabVIEW code we define the spectrum analyzer visa address resoluti
6. position measurement Support structure Rotating Platform fiber optics cable Optical Receiver Optical Transmitter Fig1 Antenna test range setup Signal Generator Data Acquisition System LabVIEW is a graphical programming language from National Instruments Its GUI based programming is very user friendly it can be used for data acquisition signal processing and for controlling system Component used for data logging system at Antenna test range a GPIB Card b NI 6008 USB DAQ Card c HP 8590L spectrum analyzer d Computer Every feed is designed to operate in particular range of frequencies During the feed test time we transmit a continuous wave signal at single frequency which lie in the respective band of the feed using a transmitting antenna placed at far distance from the feed acts as a receiving antenna Feed is mounted on a rotating platform which is having encoder it will give voltage output according to azimuth angular position of the feed The output of the encoder is given to the USB based DAQ card which will continuously acquired angular data in terms of voltage GPIB card has been used to acquire RF data from the spectrum analyzer HP 8590L spectrum analyzer is having maximum 401 sweep points It means that whatever is the span of spectrum analyzer we set using LabVIEW program it will divide the frequency span by 400 and save the corresponding power at above sample
7. add a coupler cable to the trans antenna side and scan again Rotate the test antenna by 5 to 10 intervals and record the data One will see a Gaus sian profile of the phase values From these set of data and knowing the line of sight distance the phase centre at the chosen frequency of measurement can be calculated If p is the phase centre distance from the axis of rotation azimuth then it is given by Ro xo 2p 1 l1 cos 6 4 2 Ro 20 where is the measured phase difference between the peak and chosen value Ro is the line of sight diatance between the trans and test antennas zo is the distance between the rotation axis and the vertical plane of the test antenna sighting during the theodolite survey see Appendix A and is the azimuth angular difference between the peak phase value and the chosen value in other words 2 dn corresponding to 62 dn where du and 6 are the values of measured phase 10 and measured az angle respectively for the peak of the phase pattern 9 Repeat the measurements so that a consistent value of the phase centre location is ar rived 10 Change the frequency and repeat from steps 1 thro 8 References 1 G Sankar The NCRA Test Range for Wide band Feeds Development under XI Plan Int Tech Report AG 01 10 Sep 2010 2 Vivek D Devendra S Radiation Pattern characterization of GMRT Feeds Dept of Avionics IST Thiruvananthapuram April 2012
8. at the range can support up to 3 GHz Source antennas trans one working up to 2 GHz and another up to 3 GHz are also available 2 2 Test Antenna Platform The platform is a rigid structure mounted on a rotating disc of 800 mm diameter powered by a geared motor The motor control and encoder display is part of the system A Grey scale absolute encoder of 13 bits resolution with two angular limits is provided the platform can rotate in both directions and the angular speed is controlled by a multi turn potentiometer The encoder s digital readout is provided on the front panel and it can be tapped for digital data acquisition A brief technical data of the hardware is given below e Azimuth coverage 200 to 200 e Azimuth rotation speed max 6 sec e Encoder resolution 0 014 e Encoder Model Lika make AS58 13 6S 6 E Absolute enc Az Drive motor 0 5HP 3PH Platform Weight 300 kg without feed antenna 3 SOP for Principal plane Radiation Pattern Measurements The digital encoder output and the spectrum analyzer output are connected to a NI data logger NI USB 6009 This is can accept 8 analog inputs and has 14 bit I O The data acqui sition is done by a LabVIEW code whose details are given in Appendix D The Operating Procedures are listed below 3 1 1 E plane patterns Trans antenna Ref Fig 1 Orient the antenna such that the dipole arms are parallel to x axis parallel to xy plane
9. d H 700 mm Hence Rg 24 658 m Step 3 The measured azimuth angle y 84 54 57 So Du 43 632 m this is the line of sight distance between the trans and test antennas This value will be used for finding the phase centre of any feed being mounted Step 4 The measured elevation angle of the test feed s centre gi 5 36 07 and therefore E t 38 248tan 5 36 7 du arctan 51658 and that works out to be da 8 38 54 This elevation angle is set and a punch mark was done on the mounting pole KKKK KK Appendix B B List of Filters amp Amplifiers Q Range B 1 Filters Sl No Description Nos Remarks 327 MHz BPF 500 MHz LPF 550 900 MHz BPF 1600 MHz LPF Mobile Notch Filter 925 950 MHz oP WN rR Ka th Fi Fi Fa Dual port cascade able B 2 Amplifiers Sl No Device Nos Gain Remarks 1 MAV 11 1 11 dB 12V supply 2 HMC 740 1 30 dB 12V supply 3 Serenza 2 20 dB 5V supply 4 Portable Amp Serenza 1 20 dB 5V Battery KKK KKK Data logging system using LabVIEW for Antenna test range G Sankar Sanjeet Rai Objective To have automatic data logging at antenna test range at NCRA campus Introduction The GMRT system is getting upgraded to achieve seamless coverage from 30 MHz to 1600 MHz The feed for the upgrade system is tested in antenna test range To make RF power measurement easier we have developed a program using LabVIEW for RF data logging and angular
10. form As shown in Fig 1 the angle w the included angle between R and tz is measured Hence the line of sight distance be tween the trans antenna and test antenna Rg can easily be computed using the cosine rule Ro yR Ro 2R Rzcos Y 3 A 2 4 Step 4 The final step is to mark the line of sight on the mounting pole so that the trans antenna can be mounted exactly on the colinear plane to the test antenna s centre Refer Fig 2 the lower half The elevation angle du of the test antenna s centre plane is measured From the figure it can be shown that Roatan 2 R tan Q 4 Angle dn can be computed from the above relation as the remaining quantities are known With the computed value for 2 that elevation is set and a line is marked on the mounting pole This marked line must coincide with the trans antenna s centre line Thus the colin earity of both the antennas is achieved A 3 Measurements Carl Zeiss electronic theodolite model ELTA 3 was used for the measurement Following are the measured parameters and the relevant computed dimensions Step 1 b2 6 10 25 KI 5 OI Tt and k 700 mm Test antenna Water tank amp Theodolite platform trans antenna Figure 2 Establishing the horizontal central line Therefore Ry 38 248 metres Step 2 Bo 8 36 10 By 79 0 93 an
11. ide on the horizontal line of sight A 2 1 Step 1 A convenient point on the terrace of NCRA building was chosen first designated as Um Fig 1 as the theodolite station point from where clear visibility of both the trans ans test antennas is ensured The theodolite is positioned exactly over this point a punch mark is placed and the test antenna is viewed With azimuth rotation locked on the theodolite the elevation angles of the bottom and top Water tank Theodolite E Station T Te Trans antenna B NCRA Building A 5 Test Antenna Platform A on the terrace of R No 301 KA Figure 1 Plan view centre lines of the square frame of the feed mounting are measured as angles 01 92 respec tively With the known dimension of the square frame k is known ref Fig 2 the horizontal distance R is given by k D rem tan From the measured values of 01 02 and k is known R can be computed A 2 2 Step 2 Similarly the horizontal distance Rg can be computed by an exactly similar survey of 2 points marked on the mounting pole whose vertical distance Kl is known If 31 32 are the elevation angles of the lower and higher points on the mounting pole then R is given by H P rent S A 2 3 Step 3 Theodolite being at the same station next measure the azimuth angle between the trans antenna mounting pole and the test antenna plat
12. ipole reflector types of feeds orient only the dipoles the reflector wont be required to re orient in the case of aperture type of feeds horns waveguides etc the whole feed should be re oriented Make sure that the orthogonal polarization channel is terminated in 50 Q load Follow steps 3 to 7 of the above procedure for E plane to get the H plane pattern Repeat steps 1 amp 2 for another frequency set at the Sig Gen 4 SOP for Co polar patterns aly Orient the trans antenna inclined 45 in the xz plane Place the test antenna too inclined by 45 in the xz plane Ensure the orthogonal polarization port is terminated by 50 Q load A complete 360 azimuth rotation yields the co planar pattern Follow steps 3 to 7 given in Sec 3 Note For circularly symmetric radiation patterns this is average of E OG H planes patterns 3 Repeat steps 1 to 4 for another frequency set in the Sig Gen 5 SOP for Cross polar patterns 1 Place the trans antenna inclined 45 in the xz plane 2 Orient the test antenna inclined 135 in the xz plane i e orthogonal to the trans antenna s E vector plane 3 The orthogonal polarization port of the test antenna must be terminated by a 50 Q load 4 If a co polar pattern has been measured earlier the following step can be skipped Oth erwise repeat step 2 of Sec 4 i e orient the test antenna inclined by 45 to the xz plane 5 Repeat steps 3 to 7 of Sec 3 Azimu
13. measured at either ends should be same within first decimal accuracy Figure 2 Phase centre measurement set up 1 The trans and the test antennas are mounted as per E plane pattern measurements vide Sec 3 2 First connect the spectrum analyzer and examine the RFI scenario choose the relatively RFI free band of frequencies Note down the azimuth angle when the test antenna receives maximum RF power make a table of these peak power for the chosen frequencies Connect the test antenna output to the Vector Voltmeter Note HP 8508A VVM will depict out of range if the input Power is lt 25 dB Choose a higher power at the Sig Gen or add more amplifier modules Move the test antenna by 60 away from the peak Similar to the Cross polar pattern SOP the data logger won t be useful for these mea surements Manually record the data in a spreadsheet form Record the az angle and the phase of the received signal with respect to the input phase of the Sig Gen Make a quick scan of the az angles from 60 to 60 If the measured phase is well within 175 to 175 it is worth recording If not one will see phase reversal occur ring within the scan which would be detrimental to the phase centre measurements and computation Change the frequency of the Sig Gen by a few to tens of MHz and do another scan the new frequency should be free from RFI Check up with Step 2 If problem persists
14. on bandwidth video bandwidth sweep time start and stop frequency acquisition delay time etc
15. points Suppose if we define the frequency range from 200 MHz to 600 M Hz i e span is 400 MHz the processing unit within the spectrum analyzer will divide this 400 M Hz span by 400 hence we will get 1 MHz This means that the RF power at 200 MHz 201M HZ and 202 MHz etc up to 600 MHz will get logged during every acquisition cycle During testing time we do E and H plane measurement The feed placed on rotating platform will scan the transmitting antenna in azimuth plane to get E and H plane data During scanning our program will continuously record the RF power received by the feed and encoder voltage The setting for the spectrum analyzer are RBW 300 KHz VBW 3 KHz and Sweep time 500ms Below is the LabVIEW code that we have used for data logging Eby Acquisition vi Block Diagram File Edit View Project Operate Tools Window Help lei GL P L eal o 240t Avplication Font f ia p b E g b Enter the file name for the third loop File Dialog selected pati peno par frequency int i 0 float res freq 401 res stopf startf 400 freq 0 startf for i 1 i lt 401 i The file position is set to end so that the Write to Text File vi will append the new data to the opened spreadsheet file startf startf res freq i startf The
16. th rotation of 70 to 70 will be sufficient to get the cross polar pattern 6 The cross polar pattern s power level should be scaled down with respect to the co polar maximum i e at azimuth 0 being 0 dB 7 Repeat steps 1 to 4 for a different frequency value 8 The NI data logger wont be needed here manually record the data preferably in a spreadsheet software and plot it later To summarize the orientations of the trans and test antennas are given again in the following Table The angles in the Table are with respect to x axis in the xz plane Pattern Type Angle of Trans Angle of Test Remarks eee eee ed E plane Az scan 180 to 180 H plane Co polar D Cross polar Az scan 70 to 70 6 SOP for Phase centre measurements Elaborate details of the phase centre measurements and techniques are given in 4 The method we adopted is outlined in 2 During the initial measurement attempts by the stu dents the widely varying results as well as not a stable phase values forced us to locate a closer range of the trans antenna Appendix A outlines the method employed and here the range distance was reduced to 43 632 m Fig 2 shows the block diagram of the measurement set up Ro Transmitting Antenna gt Test Antenna 3 dB power s divider Signal Generator Vector Voltmeter Ch A Ch B Note The RF cables connecting to Channels A amp B should be of equal electrical length phase
17. way from the trans antenna azimuth 180 here if the power is a few dB above noise floor it should be okay for continuing the measurements Suitable adapters couplers are available at Range a multi output power supply is also available to power these amplifiers Always check the dc voltage at the amp power ports with a DMM When all the above conditions are met with a slow speed set on the rotating platform few deg sec start the data acquisition from the laptop LabVIEW code 10 11 Stop the rotation and data acquiring once a complete 360 deg rotation is done The acquired data will be a 2 column vector containing the encoder output voltage and the marker power of the Spectrum analyzer Re converting them to angles a liner calibration curve for the encoder is stored in the laptop a linear fit eqn is used to compute the angles from the de output voltage use any graphics software to plot the measured power against the angle of rotation this is the E plane pattern of the feed under test Repeat steps 1 thro 9 for another frequency set in the Sig Gen choose intervals of tens of MHz for the frequencies spanning the band of interest 3 2 1 H plane patterns Orient the trans antenna such that the dipole arms are parallel to y axis parallel to the yz plane Orient the test antenna the polarization channel being tested horizontal vertical parallel to y axis Note Most of the d
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