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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