An item not discussed much is the mechanical resonances in tower +
antenna systems. I see this on my big tower when the 20m5L top moves
and causes oscillations in the bottom 20m5L. This is with double guyed
Rohn65 which the commercial tower guys commented as "very stiff tower".
With a crank up there is a lot less torsional stiffness and even slop
between the sections that permit some rotation between them.
The brake jams IMO might be a problem caused by coupled moments of about
the same resonant frequency. Other ideas posted re alignment also seem
like good things to check.
Making the boom horizontally stiffer on the 57b could be done by
mounting a cross arm above the boom on the mast (or a mast extension),
say 5' up and 6' long and boom guying from its ends to the 57b boom.
Leeson recommends triple guys, 2 as I mentioned (either above or below
the boom) and one on the other side as he mentions that updraft failures
of booms are pretty common. More likely I think for exposed hill/ridge
tops such as his QTH. I use the 1200 or 2000# Phillystran for boom guys.
Another choice is to switch to a DC motor rotator with a Green Heron
controller has variable ramp up/down speeds to insure minimal energy
stored in the system when stopping or controlled when starting.
Grant KZ1W
On 2/18/2017 9:57 AM, DAVE WRIGHT via TowerTalk wrote:
Greetings all,
I've been having a continuous problemw/my tower setup since installed in 1993'. I just can't
figure it out. I've mentioned this online in the past, & received much helpf/hams/clubs
& groups (K6EAG/TT/& NCCC) both local & abroad,thank you! However the system is
still having the same problem; itseems incurable. :(
The system is: Tri-Ex LM-354 tower (54'HD crank-up, w/tilt-over base), that keeps eating rotors. The Yagi isa Mosley “Pro-57b,” mounted
on various mast lengths/strengths(both short & long), 2” OD/1/4" ID f/aluminum. Have also tried differingmaterials (both steel &
thick-wall aluminum), w/a thrust bearing(used 3 in 23 years/both stainless-steel sealed bearings, &“open” bearings using marine
grease (local salt-air/SF, CAbay-side.) I've been using T2X rotors mounted IN tower f/the start; & they eitherdon't last long, or quit
immediately (last 2), “right after” arebuild.
Originally purchased the 1strotor new, & have had the last 2 rebuilt/exchanged several timesvia
the two popular repair/rebuild shops here in the US. The onlypossible problem I can see from the
ground is the Yagi “swings” abit back & forth after done turning/brake
initialization. (looks like the swing is too much?)
Other than that, nothing seen exceptwhen it starts to fail, it will not turn up on tower; even though itturns fine on the
ground. That is, until it becomes “stuck” forgood/then I send it off f/a rebuild. The repairs typically
mention“brake failure.” I have since purchased the “brake-delay”PCB, & the next rotor will
have it installed in box.
At this point I'm -almost- positiveit's an “alignment” issue(?), but I'm not
clear how proceed. Ihad assumed perhaps wrongly that once everything was bolted together,it's
functional/should work. I have now been advised that assumptionmight be incorrect. If someone
could point me to a link/site w/theironclad alignment procedure in writing/illustration/or
video, thatwould be perfect.
When functional, the system worksgreat! =304 worked/277 confirmed. Most w/this system,
& a fewwires/dipoles/ smaller Yagi's prior to this large Yagi. Problem is,it's been
down more than 60% since installation. Never had theseproblems w/a small tri-bander;
this just sucks! :( I'veabout given up; & haven't been on much at all. (99% HF.)
This record of failure is justintolerable, & now that my age is getting up there w/healthissues, I
won't be able to work on it much. Need help as itis. Any suggestions/troubleshooting discussion? I would
be reallyhappy to get on air & try f/the last few “rare ones,” &do a few more of
the popular contests before kicking da bucket. :)
Thank you all,
Dave K6JOX 73
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