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! 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|