tower1.art

一个很小和天线计算程序

SUBJECT:  Tower modifications for redistributing antenna windloads.

       Many Amateur Radio Operators are restricted to a maximum size 
antenna because of the limitations their towers have.  The windload 
rating can cut a dream antenna in half because the tower is rated for 
something about the size of a ten meter beam.  But, that's all you can 
afford so you take your lumps and settle for something less than you 
wanted.  I had that problem and decided to do something about it.

       o       I could purchase a larger tower (BTW I finally had one
               given to me.  It's still in the back yard).

       o       Find a way to get the weight off the tower and on the 
               ground (Put the dynimite away!).

       Looking at the lastest ads, then looking in my wallet, then at
my wife and kids, then again at my 9-in triangular crank up and 22-ft
beam, then back at my wife, I decided to to the latter of the two (gulp).

       How in the world was I going to relieve the weight from the 
tower and still keep my beam up at the same height?  That 9-in trangular 
crank-up wasn't going to cut it in heavy winds.  It was an accident
waiting to happen.  At that point I was working on my "I need a better
tower" speech to my wife and remembered some of my statics and dynamics 
from college.  

       I checked on a couple of examples in the book and found that my
idea would work.  The solution?  A drive shaft/bearing modification. 
With the rotor moved to base of the tower and a vertical bearing set
situated just above the rotor.  The rotor would have no downward 
thrust upon it.  All it would have to do is turn the shaft.  The bearings
turned out to be easier then planned.  Edmond Scientific Catalog
had Lazy-Susan bearing sets for sale.  I purchased the 1000-lb table
type for about $6.50 each.  After packing them in grease, I was ready to
install them.  The drive shaft can be a pair of push-up masts for crank-up 
towers or water pipe for fixed towers.  The shaft is guyed within the tower
by PVC tubing. Each tube is about 2-3 feet in length and I used three
per forty feet.  The shaft pokes out the top for antenna mounting.  In
my case, two sections of the push-up mast poked out the top allowing
me to work and mount a few VHF/UHF antennas. 

       Here's the basic set-up:



              =================== 2m beam
                       ||
                       ||
     ====================================== HF beam
                       ||
                       ||
                      |||| - PVC tubing over shaft inside tower
                      ||||
                      ||||
                      ||||
                      ||||
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                      ||||
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                      |BB| - Bearing
                      ||||
                      |RR| - Rotor location
               -------|--|--------


       o The rotor only turns the shaft - no weight on the rotor.
       o The vertical weight is transformed to the bearing near the 
         bottom.
       o The weight of the antennas are on the shaft instead of 
         the tower.
       o The towers sole purpose is to guy the shaft and the forces
         directed to the tower are distributed along the tower's 
         length.


       This system has been working here for over two years.

-WS