[TowerTalk] height of truss for converted HiGain LJ205

Grant Saviers grants2 at pacbell.net
Tue Oct 5 00:54:50 EDT 2021


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

(ref: Leeson - Physical Design of Yagi Antennas)


On 10/4/2021 19:31, Jeff Blaine wrote:
> Without doing any actual engineering statics work, is there a rule of 
> thumb for truss height for helping to offload boom sag?
> 
> I've got a LJ205 that's been recycled to a 10m OWA.  The back end of 
> that antenna is very heavy relative to the front and I'm planning to put 
> a truss on it to help the sag.
> 
> Normally I would move the existing center mast mounting point to a 
> balance point that would leave the two boom halves even.  And then put 
> the truss near there, or integrated into the mounting point.  I'm not 
> sure if the boom is of uniform construction or not.  At this point I'm 
> thinking that using the mast for both the mounting point and the truss 
> will work although there will be a persistent moment around the mount. 
> Not ideal, but this is not a very big antenna and given the original HG 
> design did not have a truss integrated, adding one in this case may be 
> overkill.
> 
> Appreciate any comments the board may care to offer.
> 
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk


More information about the TowerTalk mailing list