WSM, the home of The Grand Ole Opry in Nashville, was one of the first stations to install a Blaw-Knox "Diamond" antenna - really two self-supporters stacked to create a very tall vertical antenna.
We turn to Watt Hairston, a Chief Engineer of WSM, who offers the following information on the Blaw-Knox towers, and answers the question of whether the big, fat "middle" was there for an electronic reason.
WSM - Blaw-Knox tower details
By Watt Hairston
At WSM, we have Blaw-Knox dual cantilevered (center guyed towers). Also known as the "Diamond Antenna," the
WSM-AM tower is at
I-65 and Concord Road, in Brentwood, TN, just south of Nashville.
Any tower structure exhibits many dynamics that result from gravity, wind and temperature cycles. These forces are constantly at work and result in movements vertically and horizontally of the structure. These movements (even slight) result in tremendous tensions where the structure attaches to a very rigid earth through its foundations. Most specifically, the horizontal twisting that results becomes a formidable "moment." To compensate for this, either the strength of the structures lower components (where the moving tower meets the non-moving foundation) must be massive enough to compensate for this "moment" or attached to the foundation through a hinge mechanism in the form of a pivot. Thus the tapered base we see on most towers of significant length.
With all this in mind, we can reason that for a tower of the height of 808 feet, the dynamics (movements) are integral to the mass of the structure. The higher the tower, the size and strength of the lower supports must grow at an exponential rate. If this tower were not tapered, the lower supports (where this twisting moment couples to the foundation) would have to be so large as to make the project extremely costly to accomplish. In the case of AM towers that are electrically insulated from the ground, the problem is magnified because the tower has to rest on porcelain insulators that would be located at the transition to the foundation. (The point of maximum force). In a pivot attachment, the moment is not critical because the tower can rotate on its base. This is why tower manufacturers use this mechanism to this date.
The WSM tower lower half is made up of larger steel members than the top section. Where the bottom taper is linear, the top is not. There is a transition at about the 680-foot level. Eight guy cables are attached at the joining section waist. The entire weight of the tower and the pull of the guy cables rest on a two-section series pivotal Lapp insulator that is designed to hold a 1,200,000 pound load. Each of the eight guy cables are stressed to 55,000 pounds and it is thought the entire weight of the structure is in excess of 300 tons on the base insulator. We are not certain of this total weight number but based on the guy tensions that we can document, we feel this figure is pretty close. This tower represents the construction methods and technology at the time.
The WSM tower was erected in 1932 and began operation from it late that year and is still in-use today. The tower was designed and manufactured by The Blaw-Knox Steel Company of Pittsburgh Pa. The original mechanical drawings of the tower are very detailed. The erection crew was from South Carolina.
Originally, the WSM tower was 878 feet tall. This included 758 feet of square structural lattice then 120 feet of tapered mast. By 1939, it was determined the tower was electrically longer due to velocity effect. This contributed to a very high angle of radiation that resulted in a groundwave/skywave cancellation (fade) over Chattanooga some 120 miles distant. This condition was alleviated by an adjustment in height to 808 feet.
(As a side note, part of the mast was erected as the flagpole for the adjacent Lipscomb School. It was in use there until 1996 when the original school building was replaced. 50 feet of the pole still exist on the top of the tower. It was used to support a turn style antenna for the FM station that WSM operated at this location from 1939 until 1952. The original antenna was in the 45 MHz band. After World War II, the FM allocation moved up to 103.3 MHz. The antenna was replaced with another turn style antenna. That antenna is still up there. WSM relinquished the FM station license in 1952 as television was on the scene and FM broadcasting was no where near profitability.)
Blaw-Knox went out of the tower business in 1958 after several disastrous installation accidents.
There are several of these types of towers still in use besides the WSM tower. The same crew that installed the WSM tower installed the WLW tower near Mason Ohio. In fact, the crew went directly to Ohio when they finished WSM. There are three shorter versions of these towers at WBT in Charlotte, NC. Blaw-Knox built taller towers as well. The WSMV-TV tower is 1369 feet tall and has a 12-foot width face. This tower supports the antennas of WSMV-TV, WSMV-DT, WSM-FM, WRVU, WNPL and WZTV-TV. It is the classic pivotal base sectional guyed tower.
Prior to using the Blaw-Knox tower, WSM used a "cage" type antenna that was supported by two self-supporting towers. A wire was strung
between the towers and a vertical feed wire fed the center from a tuning house directly centered between the towers. This was also referred
to as a "flat top" antenna. Dr. George Brown et. al, of RCA fame, impressed the FRC with a vertical, series fed antenna that used 120
equally spaced quarter wavelength radials as 360 degree counter-poise. There have been no improvements on this basic design to date.
Another frequently asked question is “why didn’t they use a 90 degree tower?” The answer is: Part 73.182(6)(0) of the FCC rules
require a minimum field of 362 millivolts/meter/km at 1 kilowatt for class “A” AM stations. This field can only be developed by a
radiator in height of 164.4 electrical degrees or higher and 120 ground radials with an average length of 90 electrical degrees.