Lab 7: Radiation and Antennas

Transcription

1 Lab 7: Radiation and Antennas NAME NAME NAME Introduction: In wireless communication systems the actual transmission of signals over air is carried out by antenna systems. An antenna can be defined as a transducer between a guided wave propagating in a transmission line and an electromagnetic wave propagating in free space (or an unbounded medium). The impedance of the antenna needs to be matched with the transmission line characteristic impedance in order to avoid reflection. The dimension of an antenna is usually measured in units of the wavelength of the wave it is launching or receiving. Hence, the antenna dimension is usually referred to wavelength unit. Most antennas are reciprocal devices, exhibiting the same radiation pattern for transmission as for reception. The directional function characterizing the relative distribution of power radiated by an antenna is known as the antenna radiation pattern. The radiation pattern can be affected by many factors such as mounting height, position angle, foreign conductors, air humidity, etc. When several antennas are connected together, the combination is called an antenna array and the arrays as a whole behaves as if it were a single antenna. By controlling the magnitude and phase of the signal feeding each antenna individually, it is possible to shape the radiation pattern or to steer the direction of the radiation beam. Scattering matrix analysis is a useful tool for the examination of the radiation pattern of any system at its individual site. The measurement of the S-parameters can be performed using a network analyzer. (Please review the scattering matrix theory for a 2-port system before participating in this lab). In this lab, we will use a vertical antenna with a ground place as the transmitting antenna and a magnetic coil as a receiving antenna. The vertical antenna has a capacitive cap on top and an inductive coil in the middle to make the antenna have a real 50-Ω impedance to match the 50-Ω transmission line at the antenna resonant frequency. Signals will be sent by the network analyzer to the transmitting vertical antenna. Then the vertical antenna generates radiation in space, which will be received by the receiving coil as shown in the setup diagram. 1

2 Vertical Antenna Receiving Coil Ground Plane Network Analyzer Vertical antennas have cylindrical symmetry, thus a cylindrical symmetric (omnidirectional) radiation pattern, which is very useful in situations that equal power is desired for transmitting and receiving no matter which direction it is facing. (e.g. car radio receiving antenna). However, for many other applications, an antenna with directional gain is more desirable. A typical application for an antenna with good directivity is the satellite TV dish. For many applications the choice of the best directional gain antenna usually falls on Yagi-Uda arrays as shown below. Yagi arrays usually consist of a radiating dipole, a reflector element and a director element. The distance between the elements is such that through coupling, currents are induced into the elements by the dipole phased in a way as to create a radiating or receiving pattern that favors a particular direction. We will examine such a system in the second part of this lab. A typical Yagi antenna 2

3 Note that in this experiment, measurements will be very approximate, because of stray reflections from the objects surrounding the antenna in the room. Procedure: A. Radiation from a Vertical Antenna 1. Network analyzer calibration Calibration Procedure: Turn on the network analyzer, and set measure S11, format SWR. Press start 800 MHz and stop 900 MHz. So the frequency span is MHz for this lab. Press the Cal on the front panel. Then choose softkey Calibrate Menu full 2-port Reflection. Use the calibration kit to perform S11 open, short, load and S22 open, short, load from the softkey menu. Then press Reflection Done. Now choose Transmission and connect port 1 and port 2 using the coax that will be used for the receiving magnetic loop. Then calibrate the transmission through the two ports by pressing all softkeys in sequence. Then press Transmission Done. Now select Isolation, then press Omit Isolation, Isolation Done and Done 2-port. Finally press SaveReg 1 to save the calibration in register Antenna resonance frequency Connect the coaxial cable hooked to the vertical antenna to port 1, and the coaxial cable for the receiving coil to port 2. Use the marker to find the resonant frequency f r of the vertical antenna and the SWR at this frequency. f r = MHz f r = Now change measurement to S22. Is the receiving coil also matched at the antenna resonance frequency? What is the SWR for the receiving coil at f r? 3. Reciprocity Press Display on the front panel, and choose Dual channel On by pressing the first softkey. The top window is the one in the previous measurement for S11. Now press Channel 2 under the active channel menu on the front panel. The light for channel 2 should be on when it is active. Now set up the measurement for the bottom window, which shows the results of channel 2. 3

4 Press Measure button and choose Trans: FWD S21, [B/R]. Press Scale and then auto-scale softkey. Hold the receiving coil high and close to the antenna. The loop surface should be perpendicular to the ground plane (as shown in the setup diagram). Now the bottom window displays the frequency response of this transmitting-receiving system. The marker reading indicates the received power at the resonance frequency. Observe the signal strength change when you move around the receiving coil. Keep the receiving coil at the same condition and change the measurement to S12, do you see any change on the response? Why? 4. Radiation pattern Keep the plane of the receiving loop perpendicular to the ground plane at all times, so that the magnetic field vector goes through the coil loop along the surface-normal direction, which will increase the detected signal strength. Move the coil in space following the angle reference provided. Sketch the radiation pattern on the graph provided below. Please label the radius in db. B. Directional Antenna Attach the 3-element Yagi antenna to the output of the signal generator. Enter the frequency of MHz and amplitude of +0.0 dbm, with zero modulation. Attach the another coil receiver to the HP spectrum analyzer. Set the frequency span from 140 MHz to 150 MHz, what do you see on the spectrum analyzer? 4

5 Press marker then peak search, so that the marker is on top of the signal. On the top right hand corner you should see the signal strength from the antenna in dbm (decibels referenced to 1 mw). Rotate the antenna and mark the position that shows the strongest signal. The antenna front-to-back ratio is the difference between its best gain and the gain 180 degrees away from its best gain. Measure the antenna front-to-back ratio. Front-to-back ratio = max signal (dbm) - signal at 180 o off (dbm) = db 5