September 2000 QST

By David Blaschke, W5UNMBAThe Mighty Big AntennaEverything really is bigger in Texas—including antennas!“What the heck is an MBA?”It’s not my college degreethat I am referring to, butrather the Mighty Big Antennaarray that I constructed here at the W5UNhomestead back in 1985 for the purpose ofdoing some serious 2-meter Amateur RadioEME (moonbounce) operating. Yes, I’mtalking about the monstrous antenna systemshown on the cover of this issue.The MBA was first introduced to theworld in an article that was carried in theJanuary 1989 issue of QST, four years afterit was constructed. In the 16 years that havepassed since I first built the MBA, manyexciting and interesting things haveoccurred here at W5UN on 2 meters. Theastounding thing to me, though, is the factthat the antenna array is still standing andgoing strong after all this time. Of course,there have been several close calls fromhurricanes that have, fortunately, passedthis location by thus far.The Price TagOne of the first questions I am alwaysasked by visitors is, “How much did itcost?” My answer is that it cost a wholelot less than if the government had built it!But seriously, it was not all that expensive,considering its size and complexity. Iscavenged most of the materials (except forthe Yagi antennas themselves) from localjunk and scrap metal yards, where I literallypaid pennies on the dollar for most of theitems I needed. The tower sectionscomprising the main boom came from atower that I spotted a few miles up the roadthat had nothing on it. I stopped to ask theowner what he wanted for it, and he toldme, “You take it down, it’s yours.” So someluck was involved, too. The coaxial cableused to interconnect and phase all theindividual Yagis is aluminum-jacketed 75-Ω cable television hard line (CATV) that Igot without cost from a local cablecompany. They were throwing away spoolsof cable with less than 100 feet left on themduring a cable installation project.The only things I purchased new were28 September 2000the Yagi antennas, the 12 40-foot lengthsof 3-inch aluminum irrigation tubing, coaxcable connectors, standard bolts, nuts andU bolts. Counting everything, I estimatethat I spent less than $10,000 by the time Ifinished the MBA. Now that may sound likea substantial sum of money, but it was reallyquite cheap considering the 16 years ofpleasure (and exercise) I have received fromusing and maintaining the array. I alsoThe MBA is so large, it is impossible tocapture the entire array in a singleground photo.The author settles in for another eveningof moonbouncing at W5UN.became a pretty fair welder in the process.The Nuts and BoltsLet me tell you a little about MBA’sperformance and what it may mean for you.The array is capable of boosting power by30 dB (referenced to a simple dipoleantenna). In other words, with 1500 Wdelivered to the array, the effective radiatedpower is nearly 1.5 million watts. The beamis sharply focused, with a 3-dB beamwidthof 1 3 /4 degrees in the E plane and about 6degrees in the H plane.Mechanically speaking, the boom, madefrom Rohn-25 tower sections, is 155 feetlong. A stripped-down Ford pickup chassisserves as the MBA “rotator.” I rotate thearray around a 358-foot track by driving theFord’s rear differential with a 1 /3 horsepowerdc gear motor. (I paved the wheelpaths with concrete after the Ford got stuckin the mud a few times.) It takes about 15minutes to rotate the array completelyaround the track. Elevation aiming ishandled by a winch and cable system thatcontrols up and down movement. The mainboom is mounted on hinge plates attachedto the three mounting masts, allowing full0 to 90° aiming capability.The way my property is situated, I havea clear view to the eastern horizon onmoonrise and a nearly clear view to the weston moonset. There are woods to my north,but they present no problem, as I rarely aimthe antenna in that direction. I haveoccasionally made long-distance (over 1000miles) scatter QSOs with stations to thenorth, but terrestrial operation with MBAis infrequent. Occasionally, someone willrequest a meteor scatter schedule during themajor meteor showers, but I have foundMBA to be a poor performer for that mode,perhaps because of its very narrowbeamwidth. If only I could aim the array atthe meteor burn, I’m sure results would beimpressive, but I haven’t found a computerprogram to predict that yet.When EME operation is underway hereat W5UN, things are highly automated.Three computers do most of the work. One