[TowerTalk] Sidemount yagis

[Barry Kutner]

Rich,
Not quite correct. The square foot rating is total, wherever antennas 
are mounted. Kurt, K7NV, has the calculations on his Web site 
proving this for guyed towers, though it intuitively doesn't seem 
correct.

Side mounts create a lot more turning moment (aka torque) on the 
tower when the wind blows. As an example, the Rohn HDBX 
towers have a sq ft rating, but then they go on to say don't use any 
antenna with a boom longer than 12 (?) ft.  This is due to torque. I 
believe is the total sq foot rating would need to be derated when 
using a side mount. Maybe one of the PEs can explain in more 
detail...

73 Barry

On 4 Apr 00, at 15:34, Richard Boyd wrote:

> 
> Yeah, but that square foot rating is at the top, right?  I've never
> seen figures on how sidemounting yagis decreases the allowable "at the
> top," and how much sidemounted at various spots on the tower, and
> depending on the guying of course, is possible, IN ADDITION TO what's
> at the top.
> 
> To take an extreme, something sidemounted a foot, or 5' or 10' above
> the ground wouldn't add much wind loading at all to the tower, I
> wouldn't think.
> 
> I know this point could be said better and I know what I'm saying is
> completely unscientific...but scientific answers have to start with
> theoretical speculation, right?
> 
> 73 - Rich, KE3Q
> 
> 
> ----- Original Message -----
> From: <K7LXC@aol.com>
> To: <WD4K@bellsouth.net>; <Towertalk@contesting.com>
> Sent: Tuesday, April 04, 2000 1:19 AM
> Subject: Re: [TowerTalk] Sidemount yagis
> 
> 
> >
> > In a message dated 04/03/2000 5:05:19 PM Pacific Daylight Time,
> > WD4K@Bellsouth.Net writes:
> >
> > > I would appreciate some opinions on sidemounting rotating gate
> > > type
> mounts
> > >  for large yagis on Rohn 25. Conditions: Tower is 74', guyed at
> > >  64', 3
> sets,
> > >  Top two are Philly and bottom is 1/4EHS at abt 32', just under
> > >  where
> the
> > >  gate will be mounted. Want to mount a C31XR at this level, 85lbs.
> > >  Other option is a long mast with ground mounted rotor with tower
> > >  standoffs.
> Any
> > >  advice from anyone with any experience with KT34's, C31's or
> > >  similar
> yagis
> > >  would be VERY welcome!  Replies to me directly are probably best
> > >  and I
> will
> > >  be happy to summarize to anyone interested.
> >
> >       I think the *magic* dimension between elements is about 31
> >       inches.
> If
> > you split the difference, there's about 15 inches from the clamp to
> > the nearest element. I know that a TH7DX (19 inches from the clamp
> > to the
> nearest
> > element) will turn around 45G which is 18" across the face. So the
> > best I
> can
> > do is give you a "maybe" without building a fixture and trying it
> > out.
> Sorry.
> >
> >      What else is going to be on the tower? That 25G @ 70 MPH will
> >      take
> about
> > 15 sq.ft. total and one C31XR is 10.7 ft2. Doesn't leave much left
> > over.
> >
> > Cheers,   Steve   K7LXC
> > Tower Tech
> >
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> 
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--
Barry Kutner, W2UP              Internet: w2up@mindspring.com
Newtown, PA         FRC         alternate: barry@w2up.wells.com

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