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High-Gain, Very Low Side-Lobe Antenna with Capability for Beam Slewing

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High-Gain, Very Low Side-Lobe Antenna with Capability for Beam Slewing JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 64D, No. 5, September- October 1960 [:Iigh-Gain, Very Low Side-Lobe Antenna With Capability for Beam Slewingl,2 Alvin C. Wilson (October 8, 1959; revised March 21 , 1960) A corner-reflector antenna having refl ecting surfaces ten wavelengths wide a nd t wo wavelengths long was constructed, adjusted, and test ed, The driven element was a collinear array of ten half-wave dipoles, Dolph-Chebyshev curren t distribution designed for sid e-lobe suppressions to - 45 decibels was computed. The currents in the dipoles were adjusted as nearly as possible to t his distribution. The phase of the dipole currents was graded so as to slew the main beam 10° off the forward direction, The radiation patterns were measured and found to be quite close to t he computed. Check of pattern stability wit h time and wit h changes in temperature and weather co nd itions showed it to be quite stable. Measurements of radiation pattern at freq uencies departing from design frequency showed t he operating bandwidth (determined by t he preservation of the pattern) to be adequate for t he applications likely to be consid ered for such a ntennas, The half-power beam widths of t he main lobe were 9.8° in t he E-plane a nd 32° in t he ll-plane. On the basis of the m easured beamwidths, t he gain was calculated to be approx­ imately 21.2 decibels relat ive to a n isotropic radiator. The gain was experimen t.ally measured t.o be 21.2 decibels. 1. Introduction current distribution across Lhe array is based on the properties of . Chebyshev pol ynomials. When a Operation of VHF ionospheric scatter communi­ Dolph-Chebyshev cllrrent distribution is used Lilc ~ ation services at frequen cies below the F2 MUF main beamwidth will be as narrow as po sible for a 'equires, especially for hi gh speed binary data predetermined side-lobe level ; or, if the main !ransmission, special means to a void long range beamwidth is specified, the side-lobe level will be fl nterference, including self-interference from back­ minimum. ,r catter llV The use of narrow beam antennas When a collinear array of dipoles is used in a lesigned to suppress the radiation outside the main c<;>1'ner reflector, the resultant pattern is lUOl'e obe by at least 40 db has been indicated as one d~r ectlv e than the array pattern for the collinear n easure for reducing such interference, In special dIpoles alone, and further reduction in levels of ·iting situations, severe requirements occa ion ally side lobes is achieved. .rise for suppression of radiation outside the main lobe Computation were made to obtain the Dolph­ 1,8 a means of avoiding interference to neighboring Chebyshev current distribuLions to limit the side adio services. Under some circumstances, there lobes 40 db below the main beam level and the t. ould be an advantage in having an antenna in resultant half-power beamwidth for collinear arrays hich the main beam could be slewed to either side of 6, 8, 10, 12, 14, and 16 half-wave dipoles paced or taking advantage of meteor reflections. 0.5 and 0.8 wavelength apart. Table 1 shows the The performance of corner-reflector antennas relative current distribution and resultant main )ecame of interest to the National Bureau of Stand­ beam half-power beamwidth. ,rds because they appeared to be well suited to iommunication via ionospheric scatter. Previous TABLE 1. Computed dipole current distribution and Tesultant half-power beamwidth of main lob e f or Dolph-Chebyshev neaSlU'ements have shown that this antenna can be current distribution to limit all side lob es 40 db below main rr esigned to have a high gain with very low secondary beam level be-levels l2 , 3]. A collinear array of dipoles may t;e used in a corner-reflector antenna to obtain Half-power beam­ CW'rent distribution, decibels of atttlnuatioll Number width in degrees relative to ceoter elements igher gain and more desirable directive character­ of dipoles atics than are obtainable by the use of a single in O.5A O.SA Dipole right aod left from array center lipoIc in a corner reflector, collinear array t~~~~;'l ~~~~;n l-'---'---'---'--~---- - -- ? Amethod of determining the optimum currentdistri­ dipole dipole 2 4 6 )ution in a collinear array of isotropic radiators has centers centers ------ - -------- ~een described by C. L. Dolph l4]- The resultant ---- - - 6 23.6 15. 0 0 4.2 14.0 ------ ------ - ---- --- --- --- - -- S 18.0 1l. 4 0 2.4 7.6 16.7 ------ - - ---- 1 Contribution from Central Radio Propagation Laborator y, National Bureau ------ ------ 10 14.5 9.2 0 1. 5 4.7 10.0 .1 8. 1 ------ ------ ------ If Standards, Boulder, Colo. 12 12.1 7.6 0 1. 05 3.2 6.7 11. 8 18.6 ------ ---- -- , The work report~d herei n was carried out on behalf of t he U.S. Air Force, 14 10.3 6.4 0 0.76 2.3 4.8 8.3 13.1 18. 9 £~lde r support extended by U.S. Air Force Ground Electronics E ngi ueering and 16 9.0 5. 6 0 .58 1. 8 3.6 6.2 9.6 14.2 18.9 y ~ s t aU atio n Age ncy, Home, N.Y. " Figures in brackets indicate the literature references at t he end of this paper. l 5~ In order to slew the main beam of a collinear 0 0 ---- -------- ---- -------- ----------- .\,/- , -- ---- ---- ---- ---- ~ -3 -3 array away from the forward direction, a change in w~ 7~ ''I \ the phase of current in the dipoles may be calculated CD -1 0 -10 ~ f-- , for the desired amoun t of slewing. -20 -20 / \ The phase advance <P p of the current in a dipole p g \ B, -30 -30 required to produce a slew angle f30 off the broadside ~ I / I, "_, I \ direction is given in degrees by ~ -40 -40 ~ I'/,1 /"\I~ "" -5:) 'Ji. Or 1~I~in ilY ~ -50 Odp . '"~ Ii: '\ <pp = -36 >: S111 f30 tf\! Vf;,Ii! ~ II,l \ \.- -60 \fi i!. ~ ! I' 1.11 i 1.1 11\1 \!\: -60 ~ 80 70 60 50 40 30 20 /0 0 10 20 ~ 40 5:) 60 70 80 90 where dp is the distance of the dipole from the AZIf.!JTHAL AfQE. DEGREES J center of the array, taken as zero reference for FIGURE 2. Computed mdiation paltems of a 90° corner~ phase. The negative sign signifies that the phase refiectoT antenna u sing a collineaT aTmy of 1 0 half-wave~ is retarded when the slew angle is in the direction length dipoles as the dTiven element, r Dipoles spaced 0.8X apart and 0.3X fr om the apex. The dipcle currents follow of positive p. a Dolph-Chebyshev distribution computed to limit the side lobes below 40 db. Computations were made to obtain the radiation ---- Jv.faximizcd in forward direction. patterns for a collinear array of 16 half-wavelength --- -- Beam maximum 10° off forward direction. dipoles, spaced a half-wavelength apart with the - - --- Beam maximum 20° off forwa rd di rection . main beam slewed 0°, 10°, and 20° off from the 1 forward direction. It was noted that, even with A corner-reflector type of antenna with a Dolph-' the beam slewed as much as 20°, the side lobes were Chebyshev dipole current distribution was co n-' still nearly 40 db down from the main beam level. structed, adj usted, and tested by the Boulder Figure 1 presents these computed patterns. Laboratories of the National Bureau of Standards. The adjustments and tests were performed in two phases. The work performed in phase I consisted ~ ,;~ rr-"'-'-'-'-'-"--.-r--.-r,~",TV-"· ·"";mf-f-\-l\', -'''-\\',-''---I---I---I---t-t----1 KI of the adjustments and measurcmen ts necessary to obtain side-lobe suppressions below - 40 (lb'l § -20 f-t-t-t-t-t-t--t--t-1--!t--f\--t\--t--t--t--t---H 10 The work performed in phase II consisted of the adjustments and measurements necessftry to slewi ~ -~ f-t-t-t--t--t--t---l-iH-H-rtt--ti--t+-, -t-+-+-+---H ~ the main beam 10° in addition to side-lobe suppres­ ~ 40 1---1---1---1---1---t-~~~~/ +~~~\+-+-+-+-~ 40 SlOns below - 40 db. Gain measurements were -~ N¥1\'/,rrn~' V r d!\l'n/"il' ,! IA I~'~':, .I·'f\-"'''' ' made for the antenna in both test phases, § ,50 ,Jf.;\t-/'Il I '. I:: Ii" III 11·1 , 'I', '~ 50 The terminology employed here conforms to com­ l \~ \i: IN \1: ~ ,\ i ii.!:i! U'J' ll-'JJ:X ~, ED mon usage. The width of the reflecting surfaces, ~ 00 ro ED 50 ~ ~ 20 ~ 0 ~ 10 ~ ~ 50 ED m 00 ~ liV, is the dimension parallel to the apex line, The AZI MUTHAL AMilE. OCGflEES length, L, is measured along the direction normal t~ FIGU RE 1. Computed radiation patterns in the E-plane of a collinew' array of 16 half-wavelength dipoles spaced half­ the apex line. The aperture angle, 0, is that formed wavelength apaTt with a Dolph-Chebyshev CU1'1'ent distribution by the two reflecting surfaces. The dipole position, computed to limit the side lobes below 40 db , S, is the distance of the driven element from the ---- Maximjzcd in forward directioll. apex. The dipole spacing is measured between the ---- - Beam maximum 10° off rorward dirrcLion. -- --- Beam lnaximum 20° off rorward direc tion. dipole centers. The radiation patterns for a collinear array of 10 2.
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