I concur with Grant, I connect the boom truss on my homebrew Yagi's about
2/3 out. This is approximate as I try to maintain symmetry while also
avoiding any elements. I use DX Engineering ATBB-3 boom brackets, I do not
use Big Grips due to their size.
John KK9A
Grant Saviers KZ1W wrote:
Jeff,
The number of variables (and equations) is pretty large - and Leeson
enumerates these in his equations:
guy angle
section moments & lengths
boom slenderness
guy angle to boom
attachment point
guy spring constant
boom moment
Perhaps it defies "rule of thumb". However, it seems to me that
depending on the boom, somewhere around 2/3 out from mast for the guy
attach point is sensible. All depends on the taper/section moments. Guy
to boom angle of 20 to 30 degrees. My preferred very long boom/element
design is two guys each side to a single cross member on the mast about
24" each side (48" overall). This yields lateral wind support as well as
ice strength and sag reduction. Did this for a rebuilt 86ft 80m loaded
rotatable dipole. Some serious ice and wind over 10 years and still in
one piece at 100ft up. Leeson recommends 3 guys, two below the boom to
counteract wind lift and one above. Clearly needed for his QTH wind
conditions.
With larger angles of guy to boom, negative sag to the guy attach point
is possible without overstressing a slender column boom into buckling.
My 10m 5L 24ft booms are 3" diameter and are guyed, but the single guys
are not symmetrical since the boom mounting point is on rings and space
between elements is needed for tower clearance. The guy angle to boom
is pretty near 30 deg. Probably total overkill considering the 3" boom.
One error (I suspect) in my construction was using guy grips on the
Phillystran at the boom attach points. I suspect they are long enough
conductors to affect the performance. Not modeled though. A problem
unique to 10 and maybe 12m. Wire rope clips are ok on the smallest
Philly if thoroughly tightened and I have seen Nicopress compression
sleeves used successfully. Nicopress will be the fix (someday).
Grant KZ1W
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