In a message dated 97-02-24 23:55:02 EST, na5b@worldnet.att.net (Ed
Gilliland) writes:
<< Which antenna on top?
Spacing?
Can the two different 40m yagis be phased?
M2 3 element 40m yagi
Wilson 2 element 40m yagi
M2 4 element 20m yagi >>
Okay, first things first. If you're in Montgomery County, you're in an
85 MPH wind zone and within 100 miles of hurricane oceanline. Everything
should be built to that spec as a minimum.
90 feet of 45G will take a windload of 22 sq.ft. @ 90 MPH so you've got
plenty of tower.
The best scenario is to have the 4L 20M antenna at the top of the mast
and the 3L 40M antenna right at the top of the tower where it contributes
very little to the mast bending moment. BTW, with the 20M4 @ 10 feet above
the tower and the 40M3L at the top of the tower, the bending moment is
approximately 36,000 PSI and you'll need a mast with a yield strength of
70,000 PSI and 0.250 wall.
As far as spacing, what does Mike recommend? Most manufacturers want
ten feet but less will work. Stacking the 40M antennas may be worth a try
but you've only got 1/2 wave or so spacing and that's pretty close. Try
modeling it to see what you get.
73, Steve K7LXC
TOWER TECH -- professional tower supplies and services for amateurs
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