Donald Chester
2006-09-09 21:06:34 UTC
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
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
