-------- Original Message --------
Subject: Re: [TowerTalk] height of truss for converted HiGain LJ205
Date: 2021-10-08 07:26
From: kq2m@kq2m.com
To: Mpridesti <mpridesti@yahoo.com>
Hi Mark,
The Truss that I was referring to was what I created in 2008 - when I
built all three LJ205CA's (HG205CA's).
The "L" brace between the Truss mast and the Rotator Mast was what I
asked you about in our telephone call in late 2011 when we were
discussing the Hurricane Sandy damage that you used to provide an
Insurance company estimate for the antenna work. I proposed the idea to
you and asked what you though about it - you confirmed that you thought
it made sense to use that reduce the torque exhibited on the antenna in
the violent winds. You thought that it would work and then after I
built it, you then installed it when you were here with Webb the
following Summer to fix my antennas. Yes, it has made all the
difference!
73
Bob, KQ2M
On 2021-10-07 20:09, Mpridesti 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|