5. Experimental Results

Transcription

1 5. xperimental Results Prttype mdels f the duble spherical helix the hemispherical helix studied in Sectins were cnstructed measured. Fabricatin f these antennas measurement f their radiatin patterns axial-ratis are discussed in this chapter. 5.1 Fabricatin f Prttype Spherical Helices The duble spherical helix was cnstructed using a styrfam ball with a diameter (D) f 0.05 meter, while the 4.5-turn hemispherical helix is cnstructed using a ball f 0.04 meter in diameter. These balls nly prvide mechanical supprt fr the antennas have negligible effects n their radiatin prperties. A steel wire, meter in radius, is used t cnstruct the spher-helical windings. Als, a circular pan f 0.25 meters in diameter is chsen as a grund plane. With this grund plane, a minimum width f at least 0.5 wavelength at the lwest measurement frequency is maintained. The cnstructin f the duble spherical helix the 4.5-turn hemispherical helix began by first marking tw directly ppsite ples n each ball. The equatr fur quadrature lngitudinal lines in the directins f, 90, 180, 270 were drawn n the sphere t ease defining the psitins where the wire will pass thrugh. 5. xperimental Results 57

2 D Since the spacing between the turns ( S = ) f the duble spherical helix the 4.5- N turn hemispherical helix is cnstant, we can easily calculate the psitin alng the z-axis crrespnding t thse lngitudinal lines by using (3.2) fr the duble spherical helix (3.5) fr the 4.5-turn hemispherical helix. The psitins alng the z-axis f each lngitudinal line were marked n a ruler that has the same length as the sphere s dimensin then prjected nt its wn lngitudinal line. When the pints were marked n the sphere, the steel wire was wund ver the sphere by placing the wire adjacent t demarcated pints n the lngitudinal lines. Spray adhesive was applied t secure the wire t the sphere. ach cnstructed antenna was then sldered t a high perfrmance lw RF leakage cnnectr. Finally, the antenna assembly was munted n a grund plane. 5.2 Measurement Results Fabricated antennas were measured in the Virginia Tech Antenna Labratry. The antenna range instrumentatin is designed t perate ver a wide frequency range, 1 GHz up t 30 GHz. Far-field pattern measurements fr cmpnents were carried ut. Tw sets f additinal measurements in perfrmed in rder t btain axial ratis planes were als T achieve the desired plane cut fr the radiatin pattern, the surce antenna was munted n a psitiner that can be rtated, thus allwing signals t radiate in varius planes. The surce antenna is cnnected t a generatr whse frequency range pwer level can be adjusted. The test antenna is cnnected t a receiver a pattern recrder by means f a caxial cable. 5. xperimental Results 58

3 The measured radiatin pattern data include amplitudes f their phases. Unfrtunately, nly the amplitudes f are useable. This is due t the fact that, first, phases f are very sensitive t distance. Secndly, these phases were btained frm the measurements at the end f the cable terminated int the receiver rather than at the test antenna terminals. Fr these reasns, the measured phase data are nt reliable nt can be used in accurately determining the axial rati Measurement f the Duble Spherical Helix The duble spherical helix was measured in the frequency range 1.6 GHz f 2.1 GHz. With regard t the circular plarizatin perfrmance f this antenna, as discussed in Sectin 4.5.1, the frequency f = GHz was used t cmpare the measured calculated radiatin patterns. Figures 5.1 (a) (b) illustrate these patterns. It is emphasized that the pattern in Figure 5.1 (b) is nrmalized t 0 db, while the pattern is nrmalized relative t the pattern. The cmparisn f cmputed measured results shws gd agreement between radiatin patterns. The magnitude f is abut 2 db larger than that f ver the main beam. Several reasns may be stated fr the difference between measured calculated patterns, including deviatin frm precise spherical winding, envirnmental effects such as wind, reflectins frm surrunding bjects walls, misalignment f surce test antennas Measurement f the 4.5-Turn Hemispherical Helix The hemispherical helix was measured ver the frequency range 2.6 GHz f 3.5 GHz. Figures 5.2 (a) (b) shw the cmputed measured far-field patterns at a frequency f 2. 8 GHz. It is seen frm these figures that far-field patterns f the 4.5-turn hemispherical helix exhibit lw backlbes gd symmetry. 5. xperimental Results 59

4 (a) G G (b) Figure 5.1 (a) Cmputed (b) measured radiatin patterns fr the duble spherical helix at f = 1.85 GHz. 5. xperimental Results 60

5 (a) G G (b) Figure 5.2 (a) Cmputed (b) measured far-field patterns fr the hemispherical helix at f = 2.8 GHz. 5. xperimental Results 61

6 The backlbe level is generally 20 db belw the maximum peak. Mrever, this antenna prvides a brad beamwidth ver the entire frequency range. The 3-dB beamwidth f the 4.5-turn hemispherical helix ver test frequencies is mre than 60 degrees, while the 10- db beamwidth is abut 120 degrees. The calculated measured patterns are in relatively gd agreement. Radiatin patterns at ther frequencies are prvided in Appendix D. Bth numerical experimental results signify that hemispherical helix is a prmising bradbeam, circularly plarized, narrw-b antenna. 5.3 Measurement f Axial Rati Since the measured phases f cmpnents f fields are nt reliable, the axial rati is determined using a technique based n multiple amplitude cmpnents [19]. T use this technique, additinal measurements f cmpnents in directins are required. The axial ratis calculated frm this methd are shwn in Figure 5.3. Cmparisn f Figures indicates that predicted measured axial ratis are in reasnably gd agreement. The lwest measured axial rati ccurs at f = 2.85 GHz is abut 2 db. 5. xperimental Results 62

7 Axial Rati (db) = = 20 = 30 = Frequency (MHz) Figure 5.3 Cmputed axial rati frm measured pattern data fr the 4.5-turn hemispherical helix. 5. xperimental Results 63