[TowerTalk] height of truss for converted HiGain LJ205

[Mark - N5OT]

"When all else fails read the instructions."

(ask me how I know this  :-)

73 - Mark N5OT


On 10/5/2021 10:38 AM, Ed Williams wrote:
> The manual I have on the LJ205BA states the boom support should be approx 3'
> above the boom and 16'4" from the boom support clamp. I hope this will help.
>
> 73, de KN4KL ed
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> john at kk9a.com
> Sent: Tuesday, October 05, 2021 11:02 AM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] height of truss for converted HiGain LJ205
>
> 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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