At 08:26 AM 6/27/2006, Mark Beckwith wrote:
> >I would like to mount a Force12 LPT tower on a trailer.
>
>Wow, a tower trailer thread. Few things turn me on more in our great hobby
>than tower trailers. This will be long.
>
>I read and digested the whole thread and here's my $0.02. All of us owe a
>
>
>With very due respect to Jim Lux, Jim and I have both had the pleasure of
>using Wayne's trailers, and I will offer a slightly different perspective
>from Jim. I know Kelly's original question is about something relatively
>small. It must be noted, however, that before the Cellular industry was
>even imagined, Wayne had mounted a full-out motorized LM-470 on a tandem
>axle trailer, and built the thing out of structural steel at what we used to
>call "night school." That's a big tower trailer. Enough said.
>
>There are two schools surfacing: Big is sometimes very necessary, but small
>can be very useful. You won't put up a decent 40M contest antenna, which
>many of us in Southern California have done for many decades in many
>contests using Wayne's trailers, with a 30 foot tubular mast on a trailer
>small enough to fit by the house. Obviously. But the benefit of having a
>30 or 35 foot mast on a small trailer is HUGE. Kelly's got the right idea.
>In fact, after Terry's post, I have started to look at those freeway
>construction light rigs in a new way :)
>
>Jim was somewhat critical of Wayne's trailers (it should be noted that the
>trailer we have the most experience with I believe was built by Marty Woll,
>N6VI, alongside one just like it that Wayne was building at the same time -
>virtually identical - they built them together) but I point out that Wayne
>faithfully duplicated a commercially produced tower trailer, pretty much
>down to the inch.
That was my impression. I know Marty has worked on the trailer, but I
don't know if he originally built it.
> Jim says the erection could be exciting (sorry) but after
>a few times you get used to it (sorry again). I never thought it was much
>of a big deal.
My particular problem with the boat winch is that either by original
design, or subsequent modifications, the boat winch doesn't really have
enough pull to erect the tower. Partly, this is because of the angle at
which the cable leaves the winch and leads up to the tower base. As the
tower comes up, the angles are a lot better, but at the beginning of the
erection (when the loads are highest), the angle is the sharpest. This
leads to the remarkable sight of 3 or 4 people hanging on the base of the
tower to counterweight it during the initial lift.
Some sort of falling derrick or shifting pivot would make this much better.
The other problem I have with the particular design is with lifting a multi
hundred pound assembly with a single cable, with likely catastropic
consequences if the cable fails. This isn't like a telescoping tower,
where if the cable fails, the tower just collapses inside itself..
exciting, loud, but unlikely to get anyone hurt.
Considering the overall weight and cost of building such tower trailer, a
few hundred bucks for a simple hydraulic setup would result in a
significant improvement in safety, as well as ease of use. Just as the
trailer and tower itself can be (and probably were) scrounged, so could the
hydraulics. This stuff turns up surplus all the time.
> The boat winch always seemed to work great. Hydraulics
>would be cool, but in my opinion far more complicated. These two trailers
>had Tri-Ex MW-65 4-section 65 foot crank up towers mounted to them.
The CITARC trailer has a 3 section, but yes, it's around 60 ft fully extended.
>Jim was critical of the CG issues, but in my opinion, they come with the
>territory. You're holding over a thousand pounds of steel at about 7 feet
>above the ground, you're going to have CG issues.
There was another trailer stored at CITARC where the pivot point was above
the tower assembly, which had two significant advantages:
1) It lowered the CG by several feet
2) It also made for a better routing of the winch line to erect the tower.
I maintain there's no need to get into the territory of carrying the tower
7 feet up, if one is willing to arrange the tower a bit differently, and
perhaps use a different way of erecting it.
> I don't know the details
>of the "scary moments" alluded to, and I don't mean to imply that those guys
>are bad drivers, but I logged probably 2000 miles on those trailers in 14
>years, and I never felt uncomfortable towing them, not once. I also never
>had to swerve to miss a deer, or had to drive with a hellacious cross-wind,
>which could well cause problems with such a payload.
I would suspect that part of the problem is that the original design was
sort of on the edge of stability, and casual misloading of the trailer, or
an interaction with the tow vehicle suspension, or, for that matter, just
the choice of spring rates and shockabsorbers on the trailer makes it
worse. My most frightening trailering experience was not with this tower
trailer, but with a 4 horse stock trailer pulled by a jacked up Chevy
Blazer: talk about suspension system interactions...
>Another reason Kelly's got the right idea keeping it small and light :)
Indeed.. I think that there's a lot to be said for a more "purpose
designed" approach than taking a tower designed for permanent installation
and bolting it to a trailer. For one thing, since you're going to be doing
some sort of guying, you don't need the stiffness of a traditional crankup,
which is designed to be unguyed (and, in fact, will be loaded excessively
by the guy tension). You need enough stiffness not to buckle, and you need
enough axial strength to support the antenna and downforce from the guys.
Isn't there some sort of nifty military mast that works this way (AB544 or
something?)
And, in a portable installation, you can deliberately design for
non-catastrophic failures. Let it collapse (safely), and put it back up
again, when the wind dies down. Think of a mast with telescoping tubes and
friction locks. When the axial load gets too high, it just shrinks.
>But so do the tall heavy ones, which I think is Wayne's point for building
>them. I heard about the JPL/Caltech mishap where Marty's tower trailer went
>over and I was very sad - me an' that trailer were BUDS - but the good news
>is it's restored and back in service, thanks much to Jim and Mike's efforts
>(and a bunch of other dedicated club members from both institutions). This
>is by way of saying Jim's caveat about wind is a MAJOR DEAL. I wouldn't go
>as far as to say "assume it's going to get bent at some point" because with
>the right combination of rope guys and competent people you can get a 5
>element 20M yagi up to 65 feet on one of these.
I don't think you'll always get something bent, but I think that you should
design assuming that might be the case, so that you have some way to
recover from it. I have heard stories about crankups (non trailered)
getting bent in the wind, and not being able to be retracted. Bad enough if
this happens in your backyard. Really bad if it happens in a field
location, and makes you unable to move the beast.
> I think the rope guys are
>mandatory, tho.
Which gets into needing good ideas for portable guy anchors for large
towers. Pounding T-stakes or sharpened rear axles (called "bullpricks" in
the location set rigging business) into the ground gets old fast, and in
some places, it's not allowed anyway. I've toyed with the idea of
something like a very (!) large tripod that could be rapidly
erected. (actually, it's just these sorts of deployment problems that got
me started on portable phased arrays, but that's another story)
> The outriggers are nice but they basically help with
>getting the tower vertical and the antennas on it. Once you start cranking
>it up, you can get out of shape in the wind and have a very dangerous
>situation. Rope guys can give you some control over it, in my opinion
>enough control to head-off a catastrophic outcome.
>
>So, tower trailers are every FD guy's dream - and those of us that's used
>'em, well, they are a dream come true. They are very expensive to buy
>commercially, which is why most of the ones (all?) you see on ham operations
>have been home-made, so YMMV as they say. Wayne built some great tall ones
>initially, and then even Wayne saw the logic in building some short ones -
>Dino posted those photos of one of Wayne's short ones - Thanks, Dino.
>
>Can we get the Caltech/JPL guys to post a set of photos showing the trailers
>in use? I think there are a number of TT readers who would enjoy seeing
>them.
http://www.luxfamily.com/events/fd2004/ has pictures from 2004.
about halfway down is the tower erection. Note Marty's clever barndoor
hinge for the antenna mount. You can also see the "stake pounding" guide
in the foreground of one shot.
Jim
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