[TowerTalk] [MWA] OH8X 80-160 monster tower collapses

[Gary K9GS]

I can't speak to the 160M signal of OH8X but I have worked them before 
on 80M, 5 ele Yagi on the same tower.  The signal was nothing to write 
home about.  Other SM/OH stations had better signals during the same 
time.  Maybe they had vertical arrays?


On 12/10/2013 11:47 AM, David Gilbert wrote:
>
> I don't really understand the intricacies of the electron 
> gyro-frequency effect, but from what little I do understand the 
> difference between vertical and horizontal polarization can be 
> significant for some paths on 160m.  The only comments that I've every 
> read concerning the OH8X 160 array was that the signal from it was 
> somewhat underwhelming relative to the expectations for it.
>
> I worked one of the 3B9 DXpeditions on 160m with 100 watts from my QTH 
> in southern Arizona using a 2-element horizontally polarized wire yagi 
> that I ran across a canyon behind my house.  It was about 200 feet 
> above the canyon floor, with a sloping terrain extending miles beyond 
> that, and resulted in the longest QSO they made on 160m.  The path was 
> open for me for about 50 minutes and I thought that was pretty 
> special, but Bob Brown (NM7M) later sent me an email to tell me that 
> he had calculated that horizontal polarization for that particular 
> path had about 10.5 db disadvantage compared to vertical polarization 
> (due to the gyro-frequency effect).   No way I could have put up a 
> comparable vertically polarized yagi, but still ... 10+ db is a huge 
> deal on 160m.  The OH8X monster yagi was only rated for 12 dbi gain.
>
> It would seem that a well-designed super array of vertical elements 
> might possibly be a better option (cheaper, simpler, more reliable) 
> for the OH8X team than replacing the big yagi array, although for some 
> paths vertical polarization might even be worse than horizontal.   As 
> I say, I'm not an expert.  My only point is that the 12.9 dbi gain 
> spec for the now deceased antenna doesn't necessarily tell the whole 
> story.
>
> 73,
> Dave   AB7E
>
>
>
>
> On 12/10/2013 7:43 AM, john at kk9a.com wrote:
>> The whole tower rotated, harmonic oscillation may have been the
>> destructive factor. It would definitely be nice to use star guys on a
>> structure like this.
>>
>> I wonder how well the 160m yagi worked compared to a large vertical 
>> array.
>>
>> John KK9A
>>
>>
>> To:    towertalk at contesting.com
>> Subject:     Re: [TowerTalk] [MWA] OH8X 80-160 monster tower collapses
>> From:     Jim Lux <jimlux at earthlink.net>
>> Date:     Tue, 10 Dec 2013 05:45:50 -0800
>>
>> On 12/10/13 5:34 AM, Djordan (personal) wrote:
>> I wonder what the original design spec was for the system 75mph or 
>> 100mph...
>> Wonder what the cost differential would have been to raise the spec 
>> 25 mph.
>>
>>
>>
>> On a large system like this, often times the limiting aspect/failure 
>> mode
>> is not a simple strength to resist aerodynamic drag effect, but some
>> interaction, or a dynamic effect.
>>
>> The Tacoma Narrows bridge did not fail because it wasn't strong 
>> enough. It
>> failed because it wasn't *stiff* enough and the design had significant
>> wind induced torsional loads. One might say that the "Q" was too high,
>> although the aerodynamic design was also such that the wind excited the
>> oscillation in the first place.
>>
>> Until some sort of failure analysis is done, we don't know if perhaps
>> there was a failed component, etc. That is the design accommodated the
>> expected loads, with a factor of safety to account for manufacturing
>> variability, but it was that non-zero probability of failure that bit
>> them.
>>
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>>
>>
>>
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>
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-- 


73,

Gary K9GS

Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com
CW Ops #1032   http://www.cwops.org

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