[VHFcontesting] Stacking Question
K2DRH Bob N2KMA Fran
k2drh at sanasys.com
Tue Mar 20 21:57:45 EST 2007
I do mine vertically so they don't narrow the beamwidth. I mounted a 3' long by 1" OD by 1/8" wall aluminum mast horizontally across the face of my rotating Rohn 45G top section at around 110'. I requested that Dave K1WHS of Directive Systems make me custom mounts for over/under stacking with the proper stacking distance per his calculations and mounted a looper stack on each end of this 1" horizontal mast. One end has a pair of 47el 903 the other a pair of 55el 1296. The antennas are mounted above and below the 1" horizontal mast. Later I put a pair of 76el 2304 between the 903 and the tower and later still added a pair of 112el 3456 between 1296 and the tower. At first look it seems too close, but I have had no interaction issues with this configuration that is tucked between the bottom 6M antenna and the 222/432 stack H frame. No reason you couldn't do the same thing mounted to a regular 2" or so vertical tower mast. I'm sure if you ask Dave, he'll remember how he made my antenna mounting brackets.
Dave says that stacking his loopers side by side rather then over/under makes them less susceptible to "hot" ground noise because of the way the patterns plot out, but I don't seem to have much problem hearing with mine up as high as I do, regularly working out past 300 miles (for example EN41 to EN13 KM0T) under flat conditions using 10W and low noise preamps right at the power dividers. I found side by side loopers difficult to aim with a standard Hygain bell type rotor back when I was rotating a mast instead of a tower top. A pair of 55 el on 1296 was pretty sharp and the wide spacing between brake wedge notches (I'm thinking it was like 3 or 4 degrees) didn't help much either. While you probably could get away with it on 903 and 1296 if you had to, I personally wouldn't advise it with the really long boom loopers on 2304 and 3456 unless you have a more accurate rotor that is repeatable to within a degree or two. And don't forget that if you go over/under you have to reverse the gap on ONE of the driven elements.
73 de Bob K2DRH
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