[Antennas] /Concrete/Towers-One Ham's Experience

[W3HKK@aol.com]

I believe that if you wait long enough, any tower will fall..either due to 
high winds, or mechanical failure of some type.   We cant protect against 
everything.  But through proper conservative design, installation, and 
maintenance, we can  extend the life of  a tower system  into many decades.  
Bad things happen one day, one month or many decades  down the road.

I have erected  three 60 ft towers ( or more accurately, one tower at 3 
qth's) ,  each  supported by  a 4x4x4ft cube of  commercial truck delivered   
concrete, in  clay soil. 
I allowed one week to cure before erecting the tower,  with 8 sf of antennas 
on top.  One  was up for 5 yrs,  the other  for 4 yrs,  and the last  for 17 
years. 

 No cracks or deterioration was noted in any base.  The tower base section 
had 3  legs each about 3-4ft long that set in the  curing concrete and help 
anchor it in place-  and serve as rebars I suppose.  The dense soil certainly 
aided a slow cure.  Never used any other reinforcement in the concrete base, 
but I did  guy the tower even tho it was a self-supported ( aluminum)  tower 
preengineered for 10 sf of antenna at 100 mph, in a region where the 100 yr 
recurring wind speed was 80 mph.    You dont  want to underestimate wind 
loads over time, so you DO  want to be conservative,  to allow for  
corrosion, design defects, the fact that  record winds occur regularly, and 
because  other factors like  ice loading or falling tree limbs can present 
unusual load combinations not normally anticipated by hams.

My tower/antennas were a major investment that I wanted to protect as best I 
could. 
I used a 20 ft length of high strength 6061-T6 1/4" thick aluminum allow 
mast, with 7 ft set inside the top tower section,  designed to hold 10 sf 
antennas in 100 mph winds.  Tho many  wind storms  reached  into the 70 mph 
range, in temperatures between +90 and -25F, making  my multiple antennas 
dance uncomfortably in the wind, the tower never had a problem.  A couple of 
the 80 mph designed antennas had  clamps loosen and elements turn along the 
boom axis,  and had to be  taken down and  reoriented.  Once a U-bolt on the 
boom to mast clamp broke  ( perhaps from over-torquing when it was installed) 
 as the antenna approached its 20th birthday,  I never lost an antenna or 
sustained any tower damage.

Each tower leg was grounded by #4 copper wire into three 8 ft ground rods  
spaced about 6 ft apart.  Lightening struck my tower or close to it 3  times 
that I know of, in 25 years.  Damage included: loss of a QF-1 audio filter by 
pass caps( melted and  found laying on the bottom of the cabinet with  both 
wire leads melted thru);  the parallel sides of a clothes washer and drier 
had the enamel  blown off in the exact center,  due to a capacitive like 
discharge from a strike seeking ground; and the blower motor on my gas fired 
furnace,  a few feet away didnt start up that Fall and had to be repalced.  
Good   grounding of tower and gear and all coax entering the shack helps 
bleed off charges before they become distructive.  

I guess  my conservative design    dates back to the first home made 15M  
quad I erected on a 24 ft mast made of 2x4's, and guyed.  I was in a hurry to 
put it up.  It worked great.  Then I took a drive  that same afternoon. I 
came back home  to find it crumpled on the ground, like a giant dead spider, 
the vicitim of a thunderstorm I never saw. 

Bob