[TowerTalk] Roof top towers

[K7LXC@aol.com]

In a message dated 00-01-12 10:58:57 EST, DAVED@ctilidar.com writes:

> I just moved back to the city and unfortunately have a small lot where only
>  a roof top tower will work. My roof is 25' high and I'm planning on putting
>  a Glen Martin 17.5' roof top tower up so I can end up with about 45' or so
>  at this installation.
>  
>  Does anyone have any experience with these towers? Are they really able to
>  free stand under the wind loads that GM specifies? 

     I've installed a bunch of them and they are nifty antenna structures for 
situations like yours. Being 4-legged does give them considerable strength 
but all I have is anecdotal experience.

>  Does the '2x8x8 under the
>  rafter support' shown in the ARRL antenna books really work and hold up in
>  the wind?

      Sounds reasonable to me. For the 9-footers, I usually use 2x4's so for 
the bigger one a 2x8 sounds good. I would probably put the same stringer on 
top of the roof as well. If you don't, all you would have supporting the 
tower feet is the plywood over the roof trusses. 

> Should this really be guyed etc? 

      Depends. What kind of antenna system are you contemplating? What's your 
county windspeed? 

      I did guy one of the ones I installed but it was only because the roof 
didn't have any horizontal bracing on the trusses (it was old and sagging) 
and I wanted to spread the tower forces over as large an area as possible. 
You can guy it if you want - it's just the old 'belt-and-suspenders' 
approach. Plus I don't discourage being conservative with an installation or 
over-engineering something. 

> Am I making a mistake? 

      If I couldn't put up a ground-mounted tower, I'd put up one of these in 
a New York minute. They're great.

>  By the way GM advertises that this tower has a ladder built into one side.
>  The assembly instructions however state that GM does not recommend climbing
>  this tower. How am I supposed to get my antenna up there? With a 
helicopter?
>  
      That's a weasel clause; that is, if you climb it and get hurt, they can 
weasel out of any liability. Tower manufacturers pay BIG BUCKS for liability 
insurance so this is for their benefit. I wouldn't have any problem climbing 
it. 

Cheers & go for it!    Steve    K7LXC
Tower Tech

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