[TowerTalk] Misc. Feedline / Guying / Anchor Questions from a Newcomer

[K7LXC@aol.com]

In a message dated 04/29/2000 5:47:43 PM Pacific Daylight Time, 
jnipper@southeast.net writes:

> After 25 years of using a Hy-Quad, I am going to put up a TH11DX, at 90
>  feet. My tower is a Rohn 25.  I have several questions that I know many
>  of you have expert experience on, and I would like to draw upon that
>  experience.  Some of these questions will seem quite elementary, but who
>  cares, huh??

       First of all, there's no such thing as a dumb question here. We all 
started not knowing anything.

     Second, Duval County is a 95 MPH windspeed zone. The TH11 is 12.5 sq.ft. 
The 25G is rated at 9.3 sq.ft. @ 90 MPH. Sounds like your proposed 
installation will be overloaded by about 25% and you need to re-think your 
installation.
>  
>  1. Which is the best thrust bearing for me to use for this set-up?

        What's at the top of the tower now? Flat section or tapered top? For 
flat sections, the Rohn TB-3 is the standard 2-inch TB. If you've got a 
tapered top, you don't need a thrust bearing. 
>  
>  2. How high above the tower should my mast go?  Stated differently, what
>   is the ideal amount by which the beam should clear the tower?
>  
      I allow about 6 inches above the thrust bearing. 

>  3. I am considering the Yaesu 800sdx rotor. Any real advantage of using 
>   the 1000sdx instead??

       The K-Factor (weight times turning radius) of the TH11 is 1936 ft-lbs. 
The G800 is rated at 1299 ft-lbs and the G1000 is rated at 2020 ft-lbs. so 
the advantage of the G1000 is that it can turn the antenna. 
>  
>  4. I think that I have pretty well decided on using Phillystran. Do many
>  of you fellows use it? Are there any drawbacks, except of course
>   the expense?
>  
      That's about the only down-side.

>  5. Which size Phillystran should I use ?  4000 lb, or 6100 lb? Should
>   I use the same size at all levels?

    The Rohn tower spec says 3/16" EHS so the HPTG4000 as a minimum.
>  
>  6. I will only be able to guy at about 60% (instead of ideal 80%), so
>   I plan to put my guy anchors in concrete for more strength. 

      If this is the case, you might want to use the HPTG6700 1/2" EHS 
equivalent. I would have an engineer calculate the stresses to make sure 
everything is appropriate on these shortened guy anchors.

>  I will likely have about 2/3 cubic feet of concrete for each guy.
>   Which Rohn guy anchor would you recommend ?
>  
        Anchor rod or anchor hole? As long as you meet the minimum (Rohn 4A), 
you can make it as big as you want.        

>  8. What is the ideal length of mast?? Perhaps a total of 10 feet, with
>   say seven feet down in the tower and about three feet above it?

       As long as it extends high enough above the tower to handle the truss 
wire, that'll work FB. Personally I like to have a couple of feet above so I 
can use the mast as a ginpole for attaching a haul rope pulley.

      You can only have AS25G plates at the junction of 25G sections or right 
under a taper top so your choices are 40-inches below the top of a tapered 
top or approximately 9 feet below the top of a tapered top or flat top. 

      If you're just going to have your antenna 6 inches above the top of the 
tower, having the rotator 40-inches down is fine.
>   
>  9. What is the best mast itself?  With this relatively short mast (short 
>   distance above the tower, mainly) is aluminum alloy ok??
>  
       Since your lever arm and bending moment are going to be very small 
with the antenna only six inches above the top of the tower, just about 
anything will work for a mast - even Schedule 40 or 80 conduit or pipe since 
it has around 30kpsi yield strength. My MARC (Mast, Antenna and Rotator 
Calculator) Program shows a bending moment of 3635 in-lbs and recommends a 
mast with a yield strength of 15kpsi and 0.120-inch wall. In other words, not 
much of a mast. 

     County windspeeds are on the Champion Radio Products website under Tech 
Notes. The MARC Program can be purchased there also.

Cheers,   Steve   K7LXC
Champion Radio Products
http://www.championradio.com

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