[TowerTalk] height of truss for converted HiGain LJ205

[kq2m at kq2m.com]

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 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