Date: Fri, 23 Sep 2011 19:18:29 -0700
From: Grant Saviers <grants2@pacbell.net>
Subject: [TowerTalk] Element fastening - a data point from F12 75/80m
dipole
I'm converting a Force 12 180C 75/80m dipole from linear loading +
relays to the Seco Tornado variable inductor at the feedpoint (a custom
design) for full band coverage (85' long and EZNEC predicts a 25 ohm
feedpoint Z).
### I'm in the process of doing the same thing..albeit with the smaller
F-12 EF-180B. I bought it new in the box several years ago... and it never got
installed. The 180-B is 68' long. I think your 180-C is a lot longer like
aprx 83' ? I had Seco custom build me a 80-D drive.. but using plastic
coated CU
tubing instead of plastic coated AL tubing. I also dumped the LL wires and
relays
and instead used T bars, capacity hats, abt the size of a 10m ele. Each T bar
is located
at the ends of the 2" 'trunk section.' The balance of the loading is done
via the Tornado drive.
## The game plan was to use 5/16" dacron rope, one per ele half, to guy it.
In the original LL
set up, there was 2 x LL wires per ele half..... and a 8' cross piece of
tubing aprx 30" above the
80m dipole terminated them. That puts each LL wire 4' on either side of the
mast. Dunno if
I should be using 2 x dacron lines per ele half.... or if I can get away with
just one. It's almost the
same thing as guying a boom. IMO, putting a cross bar on the mast, and using 2
per side will not buy
you any lateral support at all, if the entire mess is broadside to the wind.
What is interesting is that every bolted joint is solid and every
riveted joint is loose.
### This is baffling to say the least. The 180-B appears to be nothing more
than
a main 2" trunk section...with a heavy duty 20 M REF stuffed into each end.
I own several
F-12 yagi's and never had a problem with loose rivets. I did note that years
ago, some folks were slopping
penetrox onto the rivets...before inserting them, and using the rivet gun on
em. When the pentrox washed out/dried up,
the rivets would be loose. The only thing that gets penetrox is the els /2"
trunk sections..where they overlap.
It baffles me why the rivets came loose at all. Rivets are better than bolted
sections for one main reason, they suck from the
inside out. Bolt's and screws just compress the joint. The more torque you
apply, the more you just crush the joint.
## On the EF-180B, some of the assy is already at the factory. Right where
that 2" section morphs down a whole bunch..they used
3 x rivets in a row [3/16" rivets] and on one ele half only on mine..all 3
were loose from the factory ! I replaced em all, then also added
9 x more rivets, so 12 in total. 3 x rows of 4 each...and 1 rivet every 90
degs. I coulda probably just used one rivet every 120 degs.
3/16" rivets are very strong to say the least. I can see why F-12 uses
1/4-20 SS bolts and nylocks for their various boom joints. Most folks
will not have the available pop rivet long handled device..to pull 3/16"
rivets. Although most small hand held rivet guns will handle a 3/16"
rivet, you gotta be hercules to pull em. I ended up buying the 'pop' brand
real long riveter, looks like 25" long garden shears, to easily pull
the 3/16" and 1/4" rivets. The same tool will also pull 1/8" rivets, but
is real awkward and slow for 1/8" rivets. I use the standard
small hand riveter to pull the 1/8" rivets.
## On my 20M boom, tit would make this 'squish squish' racket in high winds.
At the splice joints, done with 1/4-20 SS bolts, was the problem.
I supplemented each joint with 3 x 3/16" down each side..and one x 3/16"
rivet between the bolt heads..and another 3/16" between the nylock nuts.
The oem pair of 1/4-20 SS bolts ran vertical. End of joint problem. That
25" long riveter made by pop co, will easily do 3/16" and 1/4"
rivets..with zero
effort. The same co also makes another riveter, that fit's onto any small air
compressor hose [90 psi]. The air riveter is the real ticket, since it will
also catch all the spent mandrels..and is VERY fast..made for production
work..and will also easily pull any rivets up to 1/4" size.
The original design has two bolts in line for bolted sections (2.5" od
down to 1.25" od) and three rivets in line for all smaller od sections.
So for every riveted joint I am adding two bolts at 90 degrees in
between the rivets. The new bolt sizes range from 1/4-20 to 6-32, all
18-8 stainless and nylock nuts. Each joint is now tight.
## Don't forget to slop the marine grade 'never seize' goop on all SS
threads...ESP when SS nylocks are used. A few yrs back, I was using
3/8" SS bolts and mating SS nylocks..with no goop on the threads....and
had gauling problems left and right, when tightening em up. There was
still a gap between the Nylock and the tower leg...like 1/8" gap....and it
seized up solid.
They wouldn't tighten, and they wouldn't loosen either. Ended up cutting em
off
with a 1.25" dremel cut off wheel. It was only when SS nylocks were used that
the problems
started up.
I analyzed the original design with Yagi-Mech (without LL wires or guys)
and found that the 1" od section where the LL wires previously attached
is the weakest section, so am reinforcing that section with 0.875"
tubing (and I need to modify this section anyway). The unguyed survival
is now 75mph at 100' exposure B, no ice. With Phillystran guys
replacing the LL wires I expect 85mph survival, but that is very hard to
analyze.
### Since you dumped the LL wires, the stock solid fiberglass insulator is
no longer required, and either has an Al strap across it..or just toss the
insulator, and replace with thick wall AL tubing. [these are the insulator's
that are 1/2 way out on each side]. F-12 tells me the EF-180B was good for
well > 90 mph, no ice. [ that's the later versions of the 180-B]
later... Jim VE7RF
Grant KZ1W
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