In a message dated 99-11-17 23:07:34 EST, zippy@sunsetd.com writes:
> I see Rohn usually specifies the guy radius for their towers as
> 80% of the height. But near the back of the catalog, they have some
> drawings for Rohn 25/45/55 for use with TVRO (?) dish antennas, and show
> all of them with a 50% guy radius (with 3 way guying, even), with up to a
> 6' dish at the top of the tower. The drawings are in the Mounts section,
> pages MS-62 through MS-67. Any idea what's going on here? I'm looking at
> putting up anywhere between a 40' self-supporting tower and a 60' guyed
> tower. My site will be on a small piece of leased land at the top of a
> local hill, with future lease rates depending on how much space I take up,
> so I'd certainly like to look into why Rohn shows their towers with 50%
> guy radius in some cases. My antenna load will be very light, with just a
> 17" whip at the top and a small parabolic grid at the 20' level. What are
> TVRO antennas, and why does Rohn show their towers at a 50% guy radius
> when used to support TVRO antennas? I see the Rohn drawing specifies this
> layout for a 70MPH basic wind speed, and I'm in a 75MPH zone. Would I be
> ill advised to consider setting up a 60' Rohn 25 this way?
50% anchor radius or 80% anchor radius - they're just different
scenarios. You could have 30% or 120% too or anything inbetween. I don't know
the engineering reason for the difference but as you pointed out, the 50%
distance is for another application and not typical antenna installation that
we're used to seeing.
The TVRO applications are spec'ed per the ANSI A.58 standard whereas
the other tower applications are sped'ed to the TIA-222 Tower Structure
Standard - that might account for the difference.
Your proposed installation is pretty small - usually it's hams that are
overloading their towers that is the problem. You could use probably the 50%
spacing with no problem but if possible I would follow the TIA-222
configurations and specs in the 25G section of the book. I hadn't heard that
the TVRO drawings are a "mistake". Sounds like another reason to use the
drawings in the 25G section.
Cheers, Steve K7LXC
Tower Tech
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