Journal of the Royal Naval Scientific Service. Volume 29, Number 6 ...

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Wrap-Round Antenna: Wyatt 253 7 _ KITHOW I501ATION DISC ISOLATION BETWKKN TWO WRAP ROUND ANTENNAS 6.5ft APART ON loin DIAMFTHR MAST „ — / \ so 40 - dH / *.--«• ^~^3r * -' ^*C "•* j*^ KITH ISOLATION DISC ' \ * 30 Ml" i . I . 1 . 1 . 1 I 1 . 1 I 1 i i ! 240 260 280 300 320 340 360 380 400 FIG. 2. Antenna isolation. ohms should be of the order of 0-12A. If we fix the diameter of the reflector at 12*5 inch to allow for cable clearance (n), the pitch circle diameter of the radiators at mid-band should be around 21-5 inch which is a convenient size to accommodate four radiators and feeds spaced a little more than a half-wavelength at the highest frequency. This parameter meets the circular pattern requirement (d), These considerations give rise to the form of antenna depicted in Fig. 1. Prototype Antenna Impedance Based on the above considerations a laboratory model was constructed in such a way that various parameters could be readily adjusted. By the application of shunt stubs across the halves of each pair of radiators at a distance of approximately A/4 from the feed point at midband frequency (Fig. I), the extremities of the terminal impedance plot were brought in to obtain a V.S.W.R. of better than 2 : 1 over the band (Fig. 4). Radiation Patterns It was found that the circularity of the azimuth patterns was affected by the setting of the tuning stubs and the best pattern circularity did not quite coincide with the best impedance plot, so that the final choice of stub length was a compromise giving a circularity of better than ±1 dB over the band. By itself, the antenna gave smooth dipoletype patterns in the vertical plane and, as might be expected, the vertical patterns were to some extent distorted when a mast was inserted within the reflector (Figs. 5, 6, and 7). These distortions are due to residual currents on the mast surface resulting from the restricted length of the reflector necessitated by weight considerations. FEED-POINT RESISTANCE OP A \/2 DIPOLfc AS A FUNCTION 01 [[> SPACING FROM A\ INFINITE CONDUCTING PLANT FIG. 3. Resistance of a dipole with reflector. 0 0.
254 J.R.N.S.S.,Vol.29,No.6 SMITH CHART SMALL RATIO 50 OHMS j. .11 •. 11 FIG. 4. Impedance plot complete antenna. Isolation Within the confines of the polemast—a navigation radar at the lower end and a HFd.f. at the top —the spacing of the wrap-round antennas on the mast was restricted to a maximum of 6-5 feet centre to centre. This resulted in the isolation curve shown dotted in Fig, 2 Experiments with various size discs placed centrally between the two antennas, showed that a minimum diameter disc .of 30-5 inch was required to give the 40 dB isolation necessary (full line curve in Fig. 2). Transformer The four 50 ohm dipoles when connected in parallel give an impedance of 12-5 ohms. As a 50 ohm feeder is used to connect to the antenna, a step-up transformer has to be incorporated. To cover the 1 -8 : 1 band requirement the transformation was carried out in three stages and, using Tchebycheff techniques, this should give a theoretical V.S.W.R. of better than 1-04 over the band. The feed end of the transformer was fitted with a standard E I A flange and' the other end with a four-way /
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Page 27 and 28: FIG. 1a. Distribution of R. fOO tio
Page 29 and 30: Reliability Assessment: Lukis 275 R
Page 31 and 32: P. and fi.E. Inchterj: McLanuclian
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Page 35 and 36: P. & E.E. lnchterj: McLanachtm 281
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Page 39 and 40: 285 Vickers Oceanics take over most
Page 41 and 42: 287 The Controller of the Navy. Adm
Page 43 and 44: Notes and News 289 First Sea Lord a
Page 45 and 46: 291 BOOK REVIEWS Theoretical Nuclea
Page 47 and 48: 293 INDEX TO VOLUME 29 1974 Persona
Page 49 and 50: Index 295 Lakey, J. R. A. The Third
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