[TowerTalk] Antenna & Tower Wind Load Ratings

[Kurt Andress]

Dealing with improperly designed things is always quite entertaining!
I  appreciate all the efforts put forth to do this with antennas their 
designers did not understand, because they were office dudes, not 
mechanical engineers that climbed towers and had to deal with what they 
had actually designed....
I went and spent a bunch of time to go study "Vortex Shedding" , maybe 
~15-20 years ago, as I saw it going on everywhere, on all towers, with 
almost every existing commercially built antenna.....

When, I finally understood how it worked, it was a no brainer to figure 
out how to try to defeat it! That needs to be done in the engineering 
office, by someone that understands it!

What is going on is that each size and shape of each antenna member is 
going to experience "Vortex Shedding" in the wind speed range physics 
decides it will do that with its shape, at the wind speed it will do 
that! What is gong on is that wind flow over each section remains 
connected on one side of it, and cavitates and becomes disconnected from 
it on the other side, this reverses at the necessary low wind speeds to 
become reversed from one side of the member to the other side of the 
member, after the first displacement movement....so each piece of an 
antenna wants to do this at just the right wind speeds each size of 
tubing wants to do that by itself!
Configuring these things to not do that is possible....

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Message: 4
Date: Fri, 16 Jun 2017 21:22:58 -0400
From: Steve Maki<lists at oakcom.org>
To: towertalk<towertalk at contesting.com>
Subject: Re: [TowerTalk] Antenna & Tower Wind Load Ratings
Message-ID:<4da959b2-e771-8b41-12e9-e86a3b29293e at oakcom.org>
Content-Type: text/plain; charset=utf-8; format=flowed

On 6/16/2017 10:53 AM, Jim Thomson wrote:

> Vortex shedding is  well known.   Eles vibrate  up + down like crazy, like 6 Inches in total,
> in just a light breeze.  Ask anybody who own a 204BA or any wilson product.   Those swedged eles are a pita..and so is
> having to put rope inside the eles..which is still in the current Hy-gain  204-BA manual.

While rope in the elements is not what I'd call an elegant solution; it
works, and works well in almost all cases to completely cure breeze
flutter without having to toss an existing antenna in the trash and
start from scratch.

-Steve K8LX


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