Topband: Base Insulators for Verticals

Donald Chester k4kyv at
Sat Sep 9 17:06:34 EDT 2006

Using three separate insulators, one in each leg of a guyed tower, is about 
the WORST possible approach.  Most insulating material has excellent 
compressive strength, but poor tensile strength.  With three insulators, one 
in each leg, whenever the tower sways, there will be compression on at least 
one insualtor, but tension on at least one other.  Even a guyed tower will 
sway to a  certain extent during heavy winds.  The risk is that one or more 
insualators will crack, or separate from the casting that caps the end of 
the insulator.

You seldom, if ever, see such a jury-rigged base insulator on an AM 
broadcast tower.  The AM base insulator is instead a single insulator 
rigidly fastened to the bottom of the tower, with a pier pin either on the 
bottom of the insulator casting or the base plate, and a  hole in the other. 
  That way, the insulator can rock and twist on the base plate as the tower 
moves during heavy windstorms.  The insulator works exactly the same, 
mechanically, as the standard base plate/pier pin arrangement used with most 
commercial towers.

Ham radio installations are about the only tower applications where the 
bottom section of a guyed tower is commonly buried in concrete.  I recall 
corresponding with the Rohn tower co, back in 1980 when planning my 
quarter-wave vertical, and inquired about the special 25G section that was 
insulated in the middle with a separate insulator on each leg.  At that 
time, that section was MUCH cheaper (after accounting for inflation) than it 
is now.  Their reply was "we do not recommend using this section as the base 
insulator in AM-type tower installations."

Examine carefully the separate base insulators used in self-supported AM 
towers.  Instead of a single ceramic insulator bolted between each leg of 
the tower and the concrete base, each insulator consists of two hollow, 
cone-shaped insulators back-to-back, with a mechanical configuration 
including a metal rod extending through the interior of the assembly 
combined with a metal basket-like bracket, configured so that regardless of 
whether the force is exerted downwards or upwards, the ceramic insulators 
are compressed and never tensed.

A better solution, if a used broadcast tower base insulator cannot be found 
(expect to pay well over $1000  for a brand-new 25G base insulator, much 
more for a larger tower), would be to use one of the large porcelaine 
insulators used in electric power substations.  The most appropriate size 
would be 4-6 inches in diameter, and about 6" in length.  Have a heavy-duty 
base plate made to attach to the bottom of the tower, and bolt the insulator 
to it.  On the bottom end of the insualtor, have a second heavy plate made 
to bolt to the other end of the insulator.  Stack two heavy plates, if 
necessary, to achieve at least 1" of thickness, with a 1" hole drilled 
through the centre of the plates.  These plates will be bolted to the bottom 
of the insulator.  They need be no larger in diameter than the end cap on 
the insulator.  Then have a third plate made, to bolt to the concrete tower 
base pier.  Weld a piece of 7/8" diameter steel rod to that plate to form a 
pier pin.  It should be  long enough to extend almost entirely through the 
piece that is bolted to the bottom of the insulator.  The bottom tower 
section, with the insulator attached, is placed over the pier pin so that it 
the pin extends through the hole in the bottom of the insualtor assembly.  
This bottom section is temporarily guyed, until the tower is constructed up 
to the first permanent set of guys.  This will  allow the tower to rock and 
twist without stressing the ceramic insulator, and will cost much less than 
one of those sets of three tower leg insulators.

Don k4kyv

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