On 12/25/2012 2:47 AM, Jim Thomson wrote:
Date: Mon, 24 Dec 2012 12:30:26 -0500 (EST)
From: jcjacobsen@q.com
To: w1dxh@aol.com
Cc: towertalk@contesting.com
Subject: [TowerTalk] re beam antenna truss
Yo,
Steve is looking for ideas on how best to "truss" a boom.
I'm amazed that no one has also suggested that no matter what he ends up using,
he should make sure to use thimbles to protect the wire/rope/philly where it
goes thru the eye bolts.
73
K9WN Jake
## Putting a SS eye bolt through a thin wall boom is asking for trouble.
However it will work on a thick wall boom. What else that works good is to use
those wrap around,
cast aluminum irrigation clamps, they are at least .187” thick. Then replace
the lag bolt with a SS one.. and nylock nut..+ thimble!
I agree with all but the Nylock nut unless the insert is protected from
nature. I've had the the inserts completely disintegrated in less than
a year. If you use them, paint over with something to protect them.
73 Roger (K8RI)
Underneath the rounded head is a square... and this square
fits like a glove into the mating square hole in the cast AL clamp. You can get em cheap at any
irrigation supply place. But I have only seen them in 2”-3”-4”.
## leesons method of using a 3 point truss was also mentioned. KLM used the same trick on
it’s 58’ long 20m boom’s years ago. The trick is the outriggers + mast are all
120 deg apart. If not there is no uplift support.
## yet another method is to use say a 4’ wide cross bar mounted to the mast,
well above the boom, then use 2 x truss lines on each side of mast,, converging to a
common
point, towards the ends of each side of the boom. Although this looks good, and was
used all the time in the 70’s.... fact is, it provides zero side support.
## another variation of above is to use a short cross bar on the ends of the
boom...and again, use 2 x truss lines... but this time they converge to a
common point on the mast...
again, that scheme provides very little side support.
## IMO, build the boom such that it does not require side support, only
vertical support.
### On a really long boom,say a 60’ boom, you can run a truss line out on each side
of the mast, and terminate aprx 15’ on either side of the mast. Then run a 2nd pair
of truss lines
from the mast.... but terminate the 2nd pair at the extreme ends of the boom,
or close to the ends..... sorta like using 2 sets of guy wires on a tower
instead of one set.
## You will also put way less stress on everything if the truss line terminates on
the mast, well above the boom. 5/32” galvanized winch cable works great as a
truss line...as does
5/32” SS. I use nicropress crimps at the boom end, and also the mast end.
Then a small turnbuckle at mast end to tweak the tension. Use thimbles at all ends.
## I’m getting too old for this long boom stuff these days. 36’ is as long as I have on all
my yagi’s....including 40-20-15m. The difference in gain is so miniscule it’s really not
worth the effort.
I can more than make up the difference with a bigger amp, lower loss coax, or greater
height. A pair of shorter length boom yagi’s, stacked, will usually do as
well, or better, than a single
long boom monster, way up high.
Later........... Jim VE7RF
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