Around the outside is considerably easier as well. The antenna was assembled on
the ground and hoisted into place with me alone on a bucket lift. Getting to
the element ends would require either a lot of acrobatics standing on the roof,
or spending another $400 to rent the lift again. That was the single most
expensive element of the tower construction last fall even though everything
was purchased new except for the rotor and C3.
The elements are tapered so I can't use that formula. The longest are 1.000
down to 0.375. When I first heard the noise I thought it was construction
equipment working up the road, but there is nothing going on up there. 4-500hz
sounds about right. In the winter especially, we get 40 mph winds from the
west whenever a nor'easter passes by.
Thanks!
Al
AB2ZY
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Lux, Jim
Sent: Monday, March 15, 2021 8:52 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wind Noise
On 3/15/21 4:54 PM, Al Kozakiewicz wrote:
> I appreciate all the suggestions.
>
> I don’t think isolating the tower from the house is possible. Yes, I can slip
> some rubber or other material between the bracket and the house, but I
> probably over-engineered the anchors. They consist of threaded rod passed
> through the header in the gable end of the house along with two of the roof
> trusses. It is rigidly fixed with nuts and washers on each side of every
> member the rods pass through. The bracket is bolted to the rods.
In general, you're better off preventing the oscillation in the first place
than trying to isolate it once it's there.
> Rope through the elements sounds like the way to go.
Rope around the elements (on the outside) is far more effective at
killing the vibration. Inside just damps it, but the forces are still
driving it and the mass just changes the resonant frequency. Around the
outside stops the oscillation.
Do you have an estimate of the vibration frequency? That can help you
figure out what is vibrating.
F (Hz) = 3.26 * V(mi/hr) / d (inches)
For instance, a 1" diameter tube in a 10 mi/hr wind will shed vortices
at about 33 Hz
Typically, aeolian vibrations are most noticeable in mild winds (10
mi/hr) - when the speed gets up higher, you often have turbulence or
gustyness so it can't excite the "resonator" as well.
However, anyone who's been in a high wind (>40 mi/hr) can tell stories
about wires and cables singing (or screaming), during the gusts too..
0.25" at 40 mi/hr is 521 Hz - and that's pretty noticeable.
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