MLA·2B_antenna_review_1973.pdf
-/,- Surlotf.1.1-.L BYGORDONELIOTWHITE ANOTIJERinteresting military antenna has comealong in surplus.Themanufacturer will probably be flabbergasted to hearthatthese arc actually gettinginto the excess category, sincehe is selling thembriskly toUncle Sam, but at least one has gone the wayofall government goodies. This is the MLA-21B. designedbyAntenna Research Associates. Beltsville, Maryland. FigIis adrawingoftheMLA-2/B. a ratherodd-looking loop device which has the interestingcapability of handling a kilo- wattcontinuously over thehigh-frequency band3-25mHz. There is not a great deal of gainin the MLA-2, except above17mHz,butthe an- tennahas a beautiful elevationpattern that covers bothhigh-angleandlow-angle radia- tion,witha4mHz coveragethat is almost perfectlyuniform. (fig. 2) Thehorizontal patternis the typicalfigureeightofa loop, anditotTers excellent directivity, with an in- herentnoise-figureimprovementovera dipole. The patternnull israted as sharpas 20db. TheMLA2s longest suit is its size, offer- ing the performance of a half-wave antenna with a deviceno longer than54inches. 1502 StonewallRd. Alexandria, Va.22302 , 54i-1 Fig. 2-Radiation patterns forthe MlA2/B. Nowlet mesaythat I doubtthe average readerwillfind a11LA-2 in a surplus store anytimesoon, but the design of the antenna could be a starting point forsomevery use- ful amateur projects. From a commercial standpoint,A.R.I.ofcoursehasapatent (no. 3,588,905)but amateursmight usc it as a basis for their own experiments. The theory of thelILA-2is an impedance transformer from a50ohm transmission line to the nominal377ohm radiationresistance of free space.Theinput totheantenna is to a feed loop placed withintheprimary loop. The signal is inductivelycoupledtothepri- mary, which is the radiatingbody.Thereisno directconnectionbetweentheprimary (outer)loop, andthe feed loop. Tuningoftheprimary loopisaccom- plished by varying a motor-driven, remotely- controlled vacuum variable capacitor inserted in the topof the loop.Thevalue isvaried from IOta 1,500 picofarads. The loopitself isconstructedofthree and one-half inchdiameter tubing, and thewhole assembly is a ruggeddevice, capableofstand- ing up in 100 mile an hourwinds. Thefeed loopis matchedtothetrans- 0 Coo-Cen . Cond.etee 10 -1:;:=-I-=711.e 86COJanuary,1973
