Download ECE 222A Homework 3: Antenna Design and Array Analysis and more Assignments Spanish Language in PDF only on Docsity! ECE 222A–Antennas and Their System Applications Homework #3 - Due Tuesday October 31, 2006 in Class. Problem 1 (Helical Antennas for TV Communications): You see them all the time, the TV-Vulture vans cruising the freeways ready to photograph and transmit to millions of people the gruesome accidents, etc. They generally use an array of 2 or 4 helical antennas with a backing ground plane. The antenna is placed on the roof of the van. a. Design a single-element helical antenna (radiation is on the z-direction). The antenna should have medium gain (12-13 dB) at f ~ 800 MHz to transmit information to the TV station. Give all dimensions and determine the directivity, half power beamwidth and radiation resistance. Plot the polar pattern on a dB scale. Draw the antenna with all relevant dimensions. b. The antenna does not produce enough gain and an array of 2 x 2 helical antennas is needed. Choose a spacing "d" between the 2 x 2 array to get a good far field pattern. Calculate the directivity of the resulting array, and plot the E-plane pattern (on the same graph as the single helical antenna pattern). You will find that d should be around 0.7 to 1.0 λ. – Why would you not choose d to be 1.2 λ even though it will result in a higher gain? Problem 1 (This is as real as they get. Use Matlab to do this problem and add a lot of vectors): Consider a 13-element dipole array on the z-axis with elements at –6d, -5d, …0, … +5d, +6d. The spacing between the elements, d, is 0.55λ. a. Use a raised cosine taper distribution and get a pattern with a sidelobe of –27+/-1 dB. Plot the pattern with no phase distribution (broadside) and determine its HPBW, FNBW and directivity. b. Now, calculate the phase distribution over the array so that it scans to theta=50 deg. (from the z-axis). Plot the pattern and determine its HPBW, FNBW and directivity. Are the sidelobe levels the same as theta=90 deg. (broadside)? What is the effect of the element pattern? c. Assume that you have a 3-bit phase shifter with available phase shifts of 0, 45, 90, 135, 180, 225, 270 and 315 deg.), with a gain which varies linearly between 0 dB for the 0- deg. phase setting and -3 dB for the 315 deg. bit. For a scan angle, theta=50 deg., calculate: a. The digitized phase setting on the 13 element far-field pattern error due to the 3- bit phase shifter. Make sure that the phase errors on the array are random (sometimes higher, sometimes lower) so that the beam still points at 50 deg. b. The corresponding amplitudes on the elements taking into account the phase shifter loss. c. The far-field pattern error due to the 3-bit phase sampling (assume constant amplitude).
