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

[Larry Banks]

If I remember, part of the reason for these huge arrays was due to them 
being inside of the Auroral Oval.  They needed them to compensate for the RF 
not getting out of the oval.  (Yes -- very technical terms...)

73 -- Larry -- W1DYJ





----- Original Message ----- 
From: "Gary K9GS" <garyk9gs at wi.rr.com>
To: <towertalk at contesting.com>
Sent: Tuesday, December 10, 2013 9:05 PM
Subject: Re: [TowerTalk] [MWA] OH8X 80-160 monster tower collapses


>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.
>>>
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>
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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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