EEE161 Lab 10 Antenna

I ObjectiveIn this laboratory exercise you will learn how to use a network analyzer. You will measure and display measurement results for a one-port passive circuit, i.e. an antenna. A Network Analyzer can display input impedance s11. S11 is also called refection coefficient, or return loss.In this lab we will also measure antenna radiation pattern with a scalar network analyzer set up to measure s12 between a source antenna built into the anechoic chamber, and your Antenna Under Test (AUT).

II What is an antenna?According to IEEE Standard Definitions of Terms for Antennas IEEE Std 145-1993, antenna is defined as that part of a transmitting or receiving system that is designed to radiate or to receive electromagnetic waves. Antenna is a shell, made most commonly of metal, that helps the signal in the cable “escape" the cable and continue moving forward in air.In lieu of a Lab final exam we will have a culminating final project where each student does a take home final.The final will be due finals week into SACCT

III General InstructionsThose that finish Lab 9 will make their patch antenna on 11/27 or 11/28 with RK in lab 5019The patch antenna and matching network needs to be no larger than ¼ sheet i.e. 4.5” by 6”Attach the feed connector at the end of the matching network. The other lab groups that do not have a patch design will make monopole antennas. Monopole groups read ¶9.3.3 on page 417 figure 9-15 of Ulaby “Applied Electromagnetics”Each group bring a thumb drive on 12/4 or 12/5 to get the data from the chamber computer “Quiet” or move the output file to your folder on Gaia.

The antenna data file names include the date and time measured.

Antenna measurements will be taken in Santa Clara 1119D on 12/4 or 12/5Each lab group has 1 hour split between impedance and antenna pattern tests.

IV Impedance Measurement instructionsNetwork Analyzer Safety In Power Laboratory we are usually concerned about student safety. In microwave lab, we are concerned about student safety as well, but it will be very difficult for you to get hurt, because the signals that we are using are of the order of several milliwatts (mw) and frequency of operation is usually very high. Therefore, we are mostly concerned about instrument safety in this case. Network analyzer is a very sensitive piece of equipment. This is an experimental part of the lab, where you calibrate the network analyzer, then measure the calibration standards and some other circuits as well. Network Analyzers are very expensive. Please be careful and read the list of the instrument-safety precautions. ● Do not bend semirigid cables, they are very expensive. If the cable is broken, we will not be able to buy new ones. However, flexible cables can be bent.● Do not touch the inside pin of the connectors, and do not force the connectors together. Only male and female connectors of the same type can mate. Study the connector types listed before. Align them carefully, and then use the wrench from the calibration kit.● put a wrist-strap on (if available).

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Inside Network Analyzer Network Analyzer measures and displays the reflection coefficient (also impedance, delay, S-parameters etc). It displays the ratio of the reflected voltage and a known incident voltage. It is different than the spectrum analyzer because the spectrum analyzer displays an unknown signal generated by another piece of equipment or device under test (DUT). Network analyzer generates its own incident signal, sweeps the signal over the frequency range that you specify, then measures the reflected (and transmitted signal) and displays the ratio of reflected and incident signal (or ratio of transmitted and incident signal). The block diagram of a network analyzer is shown in Figure 1. The figure shows the source, signal separation block (directional couplers) to separate incident reflected and transmitted signals, receiver/detector that down converts and detect the signals, and processor/display to calculate the and show the results.

Figure 1) Simplified block diagram of a network analyzer.Reflection (S11) MeasurementsMeasure the reflection coefficient of the antenna around 2.5 GHz. Be sure that the antenna points away from metal objects. Sketch the reflection coefficient magnitude. What are the bandwidths of the antennas? How well are they matched at the design frequencies? what is your center frequency?

1. Set up the network analyzer to measure reflected power.2. Set the frequency sweep for 2 GHz to 3 GHz.3. Connect your antenna to the incident/reflected port.4. Observe and note the center frequency at the lowest reflected power.5. Plot the reflected power if possible or take a screen photo.

Why do we calibrate Network Analyzers? There are three types of errors in any measurementsystem: systematic errors that are predictable; random errors; and drift errors that occur dueto changes in the system after the calibration has been performed, such as temperature change.Network analyzer calibration is performed to remove systematic errors, and set the reference plane for measurements at the Device Under Test (DUT). For Lab 10 the network analyzer will be calibrated each day before the beginning of the lab, and will be sufficient for the duration of the lab.

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Transmitted not used in this part

V Antenna Pattern informationAntenna radiates most power in a certain direction in space. The exact distribution of this power in space is called an antenna pattern. We will measure antenna patterns in the lab. This property of an antenna is usually drawn on x-y plane as power (on y-axis) as a function of degrees on the antenna axis, as shown in Figure 2.

-18

-15

-12

-9

-6

-3

0

3

6

9

12

15

18

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

Gain

, dBi

Azimuth, E-plane degrees

Horn B, E-plane 2.4 GHzSecond pattern, Cable I Horn B Horiz. E plane, j1 -90Σfrom source Horn A, j2 -90Σcable C, range = 91”

Figure 2) Example of Antenna's radiation pattern

Antenna Beam The shape of the radiated power in 3-D space looks like a beam and it is called antenna beam.

Beamwidth Beam width is the measure of antenna beam in degrees. It shows how many degrees away from the axis of the antenna most power is concentrated in. We define the point at which the beam-width in degrees is calculated as the point where the antenna power falls off to ½ of the maximum power, -3 dB is ½ of the power. The E-plane beamwidth of horn B at 2.4 GHz is 43⁰.

Directivity Directivity is the measure of the narrowness of the antenna beams. The higher the directivity the narrower the beam is. The formula to estimate directivity is

D = 4π/ (φ θ) θ, φ in radians (2)The angle φ is the beamwidth in the azimuth (xy plane) and θ is the beamwidth in the elevation plane.

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Elevation top to bottom

Azimuth around equator

The standard spherical coordinate system for antenna systems from IEEE Standard Test Procedures for Antennas, ANSI/IEEE Std 149-1979 is shown in figure 3.

Figure 3) Standard Coordinate System for Antenna Measurements

The antenna measured in figure 2 is aligned with the x axis, that is θ = 90⁰ and φ = 0⁰.Both θ and φ angles are converted to radians by multiplying the degree reading by π/ 180 . Directivity is usually quoted in decibels. To find directivity in decibels, use the formula below:

DdB = 10log (D) (3)Gain is measured in dB isotropic (dBi) by comparing the AUT to an antenna with known gain. Gain and directivity are related by equation 4 where η is the efficiency.

G = D η (4)Isotropic is a theoretical antenna that radiates equally in all directions.The gain of horn B at 2.5 GHz (not shown) is 5 dBi.

Laboratory setupThe antenna laboratory measurement setup is shown in Figure 4.

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WindowsDesktop Computer

USB

HP8757C Scalar Network Analyzer (SNA)_16&17

ABCRHPIB (8757 System Interface)POS Z BlankSweep in 0-10vStop Sweep

HPIB#16

r

Cable 1

Cable 2

HP 8340B Sweeper_19

HPIB#19

POS Z Blank-Sweep out/in-Stop Sweep-

Po

Agilent 82357B USB/GPIB Interfacefor Windows

HPIB

USB

.

AUTSource Antenna

LLNA

HP85025 detector

SA 4131 Positioner Controler

HPIB #1

Figure 4a) Antenna Measurement Block Diagram

HP 8757C Scalar Network Analyzer measures S21 amplitude only

Scientific Atlanta SA4131 Positioner Controller

HP8304B Signal Generator

Figure 4b) Antenna Measurement Equipment Rack

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Start Arrow (running)

VI Antenna Pattern Measurement instructions1. To measure antenna pattern first rotate the

positioner to 0⁰ with the SA4131 in manual mode set on its front panel.

2. Secure the AUT in the anechoic chamber as shown in figure 5 and align the antenna with the 0⁰ position.

3. Rotate the angle in the reverse direction past -180⁰ to ~+175⁰ with the SA4131.

4. Open the antenna control software in \\gaia\ruckerd\Anechoic chamber\Antenna Lab 2017, a LabVIEW instrument type file.

Figure 5) AUT set up

Figure 6) Source Antenna

5. Plug in the Preamplifier power supply6. Set the source antenna to Vertical polarization for an

H-plane test as shown in figure 6The LabVIEW main window will appear briefly then the antenna control window shown in figure 7 will appear.

7. Set the Start Frequency to 2.58. Set the Stop Frequency to 2.59. Set the Num Frequency Points to 110. Leave the default Start Angle at -18011. Leave the default Stop Angle at 18012. Leave the default Step Angle at 1013. Leave the default Source Power (dBm) at 1014. Click the run arrow at upper left; it will fill in to black while the antenna pattern is

running15. The current angle, last angle measured, and distance travelled in degrees will display

during the test for observing test progress.16. After each measurement the value measured will not appear in the graph when only

one frequency is selected

17. While the test is running make a note of the current settings being run such as the time, polarization, your antenna name, the set up alignment, and any other pertinent

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information so you can manually add it to your data in post processing. The time will help you find the data file name to save the correct identification.

Figure 7) Antenna Measurement Front Panel

18. At the end of the test a data dialog box will appear as shown in Figure 8. Click OK to save the data.

19. Then a rewind dialog box will appear as shown in Figure 9. Click REWIND.

Figure 8) Data dialog box figure 9) Rewind dialog box20. The antenna will be rewound and be ready for the next test.

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21. The data will be saved in an .xls text file as “antenna data_##_## (date & time)”. The frequencies and angles measured will automatically be stored as column and row headings.

22. Leave the front panel settings as they are, and change the source polarization to Horizontal. 23. Click the start arrow and repeat the process for an E plane test. This will check the AUT

polarization completing the Azimuth patterns. 24. Re-orient your antenna inside the anechoic chamber to the orthogonal polarization 25. repeat the E plane and H plane test. This will provide Elevation patterns. Refer back to figure 3,

Azimuth is the XY plane. Elevation is from +Z to -Z at any Azimuth.26. When opening your data file you need to select yes to have it open as a spread sheet. You may

want to save it as an .xlsx file for convenient data manipulation, or print it for manual data manipulation, or paste the data into another graphic application.

Take home Final, Data Post-ProcessingCopy and paste all the data in Excel .xlsx. Select Graph x-y plot. Plot each pattern. Which pattern is in the matched antenna polarization, and which is cross polarized? Hint: you can tell by which amplitude is highest. Calculate the beamwidth in matched polarization both E & H planes. Calculate antenna directivity.Your final report must be individual (1 per student) and include:

1. A summary of your observations from Lab 7 “Lumped Element Impedance Matching” 2. A description of your patch & matching network as far as you got on Labs 8 & 9 3. A photo of your group’s antenna4. The center frequency and reflection coefficient from impedance (s11) test in part IV5. Plots of azimuth & elevation patterns (s12) from the .xls text files generated in the anechoic

chamber computer and brought out by you6. Calculation of directivity

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