Thanks for the many comments on this. Great feedback!!!
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 10/7/21 8:09 PM, Mpridesti via TowerTalk wrote:
Bob
Seems to me, this the heavier duty truss I suggested and installed at your
place. Glad it continues to deliver!
Regards,
Mark, K1RX
On Oct 7, 2021, at 8:23 PM, kq2m@kq2m.com wrote:
I have three LJ205CA's (HG205CA's) on my 130' tower. They were all put up in
late 2008 - so 13 years ago. They have been through and survived three EF0 and
EF1 tornados, one macroburst, two Hurricanes (Hurricane Sandy had wind gusts of
110+ mph) and countless ice storms and vicious T-storms with hurricane force
wind gusts. I am convinced that the only reason they have stayed in one piece
is due to the Trusses that I constructed and the fact that I balanced the
antenna at the Boom to Mast Clamp of the antenna by bolting lead sheeting
inside the boom after the Truss was in place and both ends of the boom were
guyed with kevlar rope to the Truss. With the balance point at the clamp where
the antenna attaches to the mast, there is less stress on the antenna and
clamps hopefully prolonging its survival.
I cut a 3' length of steel pipe to be used as the Truss Mast which is bolted to
the antenna boom, and in the case of the 5L at 130', it is also bolted to the
rotator mast, essentially forming a rectangle of pipe bolted to the Truss and
antenna greatly reducing the potential back and forth movement of the antenna
in the boom to mast clamp during strong winds. Before I did this the stock
boom to mast clamp would get torn to shreds by the rocking motion in the wind
within 1 - 2 years and needed to be replaced. Now the antenna is happy and the
boom to mast clamp looks unscathed after many years.
Bolted at the top of the Truss mast, I have a 4" x 4" thick aluminum plate
which has on each side a large turnbuckle to which kevlar rope is attached tightly with
three saddle clamps with the other end through an Eyebolt that is bolted above and below
the boom and also which is also attached tightly with three saddle clamps. The same
thing is done on both sides of the antenna. Each eyebolt is bolted through both sides of
the boom approximately 1' from the end of the boom for maximum support. The turnbuckle
easily allows me to tension the Truss wires (kevlar rope) and make adjustments to the
antenna for sag while it is on the tower.
The Truss is the only reason that these antennas have survived despite the
winds being so fierce at my qth that the top 20 @ 130' has had several element
tips bent UP while the other side of the antenna has element tips bent DOWN!
(Thanks to the August 4th tornado last Summer).
BTW, I didn't read the manual regarding how to build the Truss or where to
attach the Truss ropes to the boom. I wanted to design something better and
heavier duty for my qth which gets extremely severe wind weather on a regular
basis.
73
Bob, KQ2M
On 2021-10-06 15:12, Mark - N5OT wrote:
"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@contesting.com] On Behalf Of
john@kk9a.com
Sent: Tuesday, October 05, 2021 11:02 AM
To: towertalk@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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