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Unformatted text preview: B ’ F U & F L F C (100) B ’ F U F L 3-3.1 Bandwidth % Ratio 5 10 20 30 40 50 60 1.05 : 1 1.11 : 1 1.22 : 1 1.35 : 1 1.50 : 1 1.67 : 1 1.85 : 1 67 100 120 133 150 160 163 2 : 1 3 : 1 4 : 1 5 : 1 7 : 1 9 : 1 10 : 1 RADIATION PATTERNS The radiation pattern is a graphical depiction of the relative field strength transmitted from or received by the antenna. Antenna radiation patterns are taken at one frequency, one polarization, and one plane cut. The patterns are usually presented in polar or rectilinear form with a dB strength scale. Patterns are normalized to the maximum graph value, 0 dB, and a directivity is given for the antenna. This means that if the side lobe level from the radiation pattern were down -13 dB, and the directivity of the antenna was 4 dB, then the sidelobe gain would be -9 dB. Figures 1 to 14 on the pages following depict various antenna types and their associated characteristics. The patterns depicted are those which most closely match the purpose for which the given shape was intended. In other words, the radiation pattern can change dramatically depending upon frequency, and the wavelength to antenna characteristic length ratio. See section 3-4. Antennas are designed for a particular frequency. Usually the characteristic length is a multiple of 8 /2 minus 2-15% depending on specific antenna characteristics. The gain is assumed to mean directional gain of the antenna compared to an isotropic radiator transmitting to or receiving from all directions. The half-power (-3 dB) beamwidth is a measure of the directivity of the antenna. Polarization, which is the direction of the electric (not magnetic) field of an antenna is another important antenna characteristic. This may be a consideration for optimizing reception or jamming. The bandwidth is a measure of how much the frequency can be varied while still obtaining an acceptable VSWR (2:1 or less) and minimizing losses in unwanted directions. See Glossary, Section 10. A 2:1 VSWR corresponds to a 9.5dB (or 10%) return loss - see Section 6-2. Two methods for computing antenna bandwidth are used: Narrowband by %, , where F = Center frequency C Broadband by ratio, An antenna is considered broadband if F / F > 2. The table at the right shows U L the equivalency of the two, however the shaded values are not normally used because of the aforementioned difference in broadband/narrowband. n 8 /2 n 8 /2 n 8 /2 or VERTICAL (Elevation) HORIZONTAL (Azimuth) 3-3.2 For an object that experiences a plane wave, the resonant mode is achieved when the dimension of the object is n 8 /2, where n is an integer. Therefore, one can treat the apertures shown in the figure below as half wave length dipole antennas for receiving and reflecting signals. More details are contained in section 8-4....
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This note was uploaded on 01/11/2010 for the course ECE 454 taught by Professor Prof. during the Spring '09 term at University of Illinois at Urbana–Champaign.
- Spring '09