Dave Leeson W6NL, in "Physical Design of Yagi Antennas" (check Amazon)
advises that trusses should resist sway in two planes - horizontal and
vertical. Wind up/down drafts can cause as many problems as side on
force. Particularly since hams like to place antennas on top of hills.
So to really stabilize for wind loads a minimum of three guys are
needed. Two above or below the boom on outriggers perpendicular to the
boom and one on the opposite side of the boom to the mast.
Of course, if you have enough wind strength and are only reducing the
boom weight stress, only one truss is needed. For booms that is
possible, but it is unlikely for elements - wind loads usually are
significantly higher the the weight load. If your site has significant
icing, it is potentially another big weight load to manage.
Another piece of advice from him was to guy long elements at 6 to 12"
each side of the boom to reduce the element swaying motion on the
boom. These could be integrated into a three truss boom support, two
upward to a crossbar on the mast above the boom and one down to the
mast. I think Phillystran is the best choice for all of these.
Grant KZ1W
On 12/23/2012 6:22 PM, K8RI wrote:
On 12/23/2012 8:30 PM, Patrick Greenlee wrote:
Don't use nylon as it is too stretchy. Don't use polypropylene as it
lacks UV resistance (Good water ski rope though) Dacron is a good very
low stretch rope, preferably treated for UV resistance but you can put a
sleeve over it like cut from a throwaway garden hose . You can cover
the knots with black silicone caulk and prevent the knots from
weathering. Phillystran is an excellent material but...don't clamp it
or tie it in a knot. Use the approved methods of terminating it: 1. the
Chinese finger grabber type grips or the 2. the two part catalyzed
potting system. Not really practical to use the grips on a really short
piece of Phillystran. some argue against a conductive material but
commercial antennas use conductive material often. If you have the
ability to do the math or perform the tests you can find out what the
effect of a conductive material is.
For a relatively light weight boom I'd not hesitate to use clamps, but
remember you will lose about 50% of the rated strength. I use 3
clamps and a minimum torque, or clamping force for the small
Phillystran. I would prefer the potted ends when using larger cable
and heavier booms. OTOH with long booms and lower frequencies the
"Big Grip" ends should not create a problem. Multiple trusses for the
long boom. I don't think they would be a problem on 20, but won't
offer a guarantee. <:-))
73
Roger (K8RI)
73 Patrick AF5CK
-----Original Message----- From: Cqtestk4xs@aol.com
Sent: Sunday, December 23, 2012 7:10 PM
To: TOWERTALK@contesting.com
Subject: Re: [TowerTalk] beam antenna truss
The cheapest is to use is wire. However, this can cause interaction
accord
ing to some, with the Yagi.
Many use rope. On my M2 antennas Dacron is used, fastened to the
boom by
mean of an eyebolt through the boom.
Some guys use Phillystran in the same fashion.
Bill KH7XS/K4XS
In a message dated 12/24/2012 12:50:36 A.M. Coordinated Universal Ti,
w1dxh@aol.com writes:
Looking for info on best way to truss a long boom yagi. What is the best
material to use? How to secure to boom etc. Thanks for help.
73,
Steve W1DXH
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