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

[David Gilbert]

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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