This is a technique commonly used on the main cables of suspension bridges as
well.
Al
AB2ZY
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Patrick
Greenlee
Sent: Friday, June 16, 2017 2:01 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna & Tower Wind Load Ratings
A common, easily applied, and often quite effective effective solution to
vortex shedding that is causing element oscillation is a spiral wrap of a
string around the element that is oscillating. The technique from which this
strategy has been borrowed is in common use to protect such items as smoke
stacks and other tall cylindrical structures, albeit not with string or small
cord. Modern automobiles often come with radio antennas with a spiral wrap.
Patrick NJ5G
On 6/16/2017 9:53 AM, Jim Thomson wrote:
> The below url pdf explains the 222-G spec in detail.
> http://www.towernx.com/downloads/TIA-222-G_Explained.pdf
>
>
> 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.
>
> Where the software falls down is when long eles bend at the tips. In
> extreme winds, F12 el tips will bend straight back, so the ele looks
> like a U. I call that...shedding wind. If you side guy the elements, and or
> construct them so the horz deflection is far less, then you also increase the
> load on the tower..and mast.
>
> Then there are other issues, like Torque imbalance...which is my favorite
> pet peeve. Easily rectified with either a counter weight,
> or a Torque compensation plate. Why antenna manufacturers dont incorporate
> TQ balancing techniques is beyond me.
> Perfectly Torque balanced... and you dont need huge amounts of torque
> in the rotor. Almost like dragging a parachute behind your car, then having
> to up the engine HP to compensate.
>
> Some other issues are how ele sections are joined, and how various boom
> sections are spliced or joined. I fabricate my own
> boom to mast plates similar to the F12 easy on double plate concept,
> but use .375 thick plates, and DXE hardware. Then its dead simple to
> install the yagi... plus it wont slip on any mast, not with the use of those
> DXE heavy duty mast clamps. Muffler clamps with
> the sheet metal saddles have been fubar since day 1...good riddance. Other
> yagi issues include overhead truss lines
> on booms, typ they dont terminate high enough above the mast. Putting a
> cross bar part way up the mast, with 2 x truss lines for
> each half of the boom is another wasted effort, it buys you nothing..and wont
> decrease the horz deflection one bit.
>
> Most these days are using the sum total of all the projected areas of the
> eles on a HF yagi..which is typ greater than the projected area of the boom.
> VHF + UHF yagis are typ the opposite in some cases..with the boom having
> more area. If you have something like a 80m rotary dipole stacked above a
> 3 el 40M yagi, then you have to calculate the sum total of the 3 els.....
> vs the sum of the 40m boom + 80m dipole. Since the 80m dipole is oriented
> in line with the 40m boom, you have to calculate both scenarios.
>
> If you have a f12, M2, mosely, hy-gain product..they have used effective
> area, and not projected area. Multiply X 1.5 to get reality. Better yet,
> enter
> each elements taper into either DXE yagi mechanical or Kurts yagi
> stress.... to do the calcs. They can also easily be done on a calculator.
>
> Ice..and Ice + wind never appears on any yagi spec..dunno why not...nor
> heavy wet snow. You can enter any amount of ice in the above software.
> It will spit out the Vmax
> for any thickness of ice. DXEs will also spit out horz deflection in any
> wind..with or without ice. And also vertical sag deflection..again, with or
> without ice. I have had
> a few cases where heavy wet snow fell off one half of the ele.
> Meanwhile, the other half of the ele is still at least 6 inchs OD, weighs a
> bunch...and puts stress on any ele to boom assy.
>
> I have seen pix or heard about almost 3 dozen bent mast horror stories since
> 1970. A partially bent or folded over mast..with yagis mounted at different
> levels along the mast is an absolute disaster....2nd only to the tower
> falling over. 1.9 inch OD sched 40, 36 ksi pipe does not work with a big
> ant mounted
> 8-10 ft above tower. But folks keep on trying.
>
> On contesting.coms home page...under strays last week, is a pix of an old 4
> el, 40M KLM...where the boom had rotated 90 degs..so all 4 els were vertical.
> Great, now the total windload is the els + the boom combined = disaster.
> Seen the same thing happen on a F12 620 / 340 yagi. Muffler clamps with
> sheet metal saddles dont work folks.
>
> 222-G is one thing..but how the yagi is put together is another issue.
>
> Jim VE7RF
> _______________________________________________
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