Antenna Engineering Lecture 3: Basic Antenna Parameters

Transcription

1 Antenna Engineering Lecture 3: Basic Antenna Parameters ELC 405a Fall 2011 Department of Electronics and Communications Engineering Faculty of Engineering Cairo University

2 2 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

3 3 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

4 Field and Power Pattern Linear and Decibel Scales Power Pattern z z θ θ 4 Field Pattern 1 Normalized Linear Scale db Decibel Scale The radiation pattern is a graphical representation of the angular variation of the radiated field/power.

5 Field and Power Pattern Main Beam and Side Lobes 5 Main Beam (Lobe) Side Lobe Null First-Null Beamwidth Half-Power or 3-dB Beamwidth Side-Lobe Level (SLL)

6 Field and Power Pattern Back Lobes and Grating Lobes 6 Front-to-Back Ratio Back Lobes Grating Lobe A grating lobe is a lobe having the same amplitude of the main beam. What is the problem of having grating lobes in the radiation pattern?

7 Beam Efficiency Beam Area and Efficieny 7 Ω A U θφ, d Ω U U n max θ, φ dω 1 Ω A Normalized W U θ, φ dω U Ω rad max A Normalized Ω Ω Ω A M L Beam Efficiency ε M Ω ΩM A The beam area is the solid angle through which all the power radiated would stream if the power per unit solid angle equaled the maximum value over the beam area.

8 Cross Polarized Fields Cross-Polar and Co-Polar Fields 8 Co-polar Field Pattern Cross-polar Field Pattern Cross-polar fields are fields radiated in a polarization orthogonal to that desired to be radiated (co-polar). What is the problem of having cross-polar fields?

9 9 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

10 Linear and Circular Polarization Examples on Linearly and Circularly Polarized Antennas 10 Linearly Polarized Antenna Circularly Polarized Antenna Antennas are either linearly or circularly polarized.

11 Linear and Circular Polarization Experiments Demonstrating Polarization of Antennas 11

12 Linear and Circular Polarization Example 12 Example Determine an expression for a unit vector along the direction of the electric field radiated by a straight wire antenna in terms of the unit vectors along the dipole axis and along the direction of observation. Solution:

13 13 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

14 Gain and Directivity The Relation between the Directivity (Gain) and the Beam Area 14 U θ, φ U θ, φ U θ, φ D θ, φ 4π 4π U U U θ, φ dω U maxωa iso avg D max 4 π Ω A G max η 4π Ω A The directivity (gain) of any antenna is inversely proportional to the beam area. The maximum directivity (gain) is used to determine the E and H planes. (What is their definition?)

15 15 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

16 Resonance and Matching Input Impedance of a Dipole vs. Frequency 16 Z in f in in R f jx f What is the difference between resonance and matching?

17 17 Outline 1 Radiation Pattern 2 Polarization 3 Directivity and Gain 4 Input Impedance 5 Bandwidth

18 (db) Radiation Pattern Polarization Directivity and Gain Input Impedance Bandwidth Impedance Bandwidth Definition of the Impedance Bandwidth 18 0 Γ BW = 47% Feeding Structure RL Antenna 20log Γ Return Loss f Frequency (GHz) 1 2 f f f f f BW 2 f f f For antennas, the impedance (matching) bandwidth is usually the range of frequencies over which the return loss is more than 10 db. It is usually expressed in percentage (of the center frequency).

19 Radiation Pattern and Polarization Bandwidth Definition of the Overall Antenna Bandwidth 19 Radiation Pattern of a 50 cm Dipole at 300 MHz Radiation Pattern of a 50 cm Dipole at 900 MHz The overall antenna bandwidth is the intersection between the matching bandwidth, the radiation pattern bandwidth and the polarization bandwidth. The latter two bandwidths are defined as the range of frequencies over which they do not change significantly from the design frequency.

20 20 Conclusion Different parameters are associated with the radiation pattern, namely the beamwidth, the side-lobe level, the null and maxima directions etc. Determining the antenna polarization is essential to be able to receive the signal efficiently. The antenna gain is inversely proportional to the beam area. A resonant antenna has a resistive input impedance. A matched antenna has an input impedance equal to the characteristic impedance of the feed line. The antenna bandwidth is related to the matching, the radiation pattern and the polarization bandwidth. There are other important antenna parameters related to the receiving mode of the antenna such as the effective length, the effective aperture, the polarization mismatch factor etc.