[TowerTalk] Class 2 and Class 7 Poles

[Chuck O'Neal]

Interesting.  Years ago (25 or more)  I did some antenna
work for a friend who had pole mounted yagis.  I found it
far more tiresome to climb the pole and work on the
antennas!  After that experience, I would not consider one
unless it was mounted with a pulley and used only as end
support for wire antennas!  Yes, I was replacing lots of
broken things that vibrated apart and fatigued, including
antenna elements.  I remember while climbing it and when
working at the top it had a resonant frequency when I hit it
hard.  I also remember that it torqued more than a guyed
tower when the wind hit the antennas.
YMMV
73,
Chuck...

----- Original Message ----- 
From: "W0UN -- John Brosnahan" <shr at swtexas.net>
To: "towertalk reflector" <towertalk at contesting.com>
Sent: Monday, May 17, 2004 5:21 PM
Subject: Re: [TowerTalk] Class 2 and Class 7 Poles


> I have always loved poles since the days of operating from
my
> DX Elmer's station (W0AIW/W0AR) which had a 90 ft pole
with
> a 4L 20M quad.   Big stuff for the 1950s and 1960s.
>
> But here is an issue that raises a question about their
use----
> much to my surprise.
>
> I mounted one 8L 15M Yagi on a guyed 80 ft Rohn 55 tower.
The
> Yagi has a 60 ft long x 3 inch x .125 wall diameter and
(half) elements
> that are 6 ft of 1 x .058 wall and 6 ft of 7/8 x 0.58
wall.  And 1/4 inch
> dacron ropes was used for vibration dampening.
>
> I also mounted a second, IDENTICAL, Yagi on a 55 ft
(unguyed) wooden
> pole.  Both Yagis were rotatable although the
tower-mounted Yagi
> was preferentially used to JA, while the pole-mounted Yagi
was
> preferentially used to South America.  And both tower and
pole had
> similar exposure to the elements since they were about 150
ft (max)
> from each other.
>
> Both Yagis used new aluminum tubing from the same
manufacturer
> and batch lot.
>
> The pole was a 65 ft western red cedar, Class 1,  pole
with a top
> diameter of about 11 inches with a butt diameter of 30
inches.
> Western red cedar are considered the cadillac of
poles--used
> mostly y the government because the are expensive.   The
pole was
> about 6 years old when I moved it to my site and installed
it.  And
> the mount was an old Telex pole mount made from angle
iron.
>
> Rotators on both were mounted about 5 ft below a top
thrust
> bearing and the antennas were both mounted just above the
> thrust nearing with no other antennas.  So the antennas
and
> mounting were VERY similar if not identical.
>
> Over the next 6 years I had FOUR half-elements work
fatigue,
> fracture, and fall off for the pole-mounted Yagi and there
were NO
> fractures on the tower-mounted Yagi.
>
> This seems statistically significant and makes me wonder
about
> vibration modes in a pole versus a guyed tower.  My
"guess"
> would have been that the guyed tower would have had a
higher
> Q and would have better supported high-frequency
vibrations (the
> kind that seem to be associated with crystallizing the
metal that
> results in metal fractures, and that the unguyed, wooden
pole would
> have tended to dampen out some of the Yagi vibration due
to its large
> mass and ability to flex in the wind.
>
> So this has raised a QUESTION in my mind about poles but I
don't have
> a strong position AGAINST poles yet.
>
> As usual, FWIW--John   W0UN
>
> (Nice to see so many of my friends at Dayton!)
>


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