Broadcast Writer

By Vicki W. Kipp
April 1, 2004
Society of Broadcast Engineers (SBE) Chapter 24 Newsletter

Have you ever seen a tower that is wider in the middle than it is at the top and bottom?

If you have, consider yourself fortunate. Only a few of these diamond-shaped towers exist.

This is the story of a diamond-shaped tower near Nashville, Tennessee.

Small Beginnings

This story began on October 5, 1925 when WSM-AM signed on the air with a thousand watts of power at 1060 kilohertz. The transmitter and antenna were installed in downtown Nashville. WSM broadcast from a horizontal long wire antenna stretched between two self-supporting towers, in a setup known as a “cage” or “flat top” antenna. A tuning house centered between the two towers fed a vertical source wire to the center of the horizontal long wires.

Edwin W. Craig, an insurance executive for the National Life and Accident Insurance Company, created WSM to promote the company while providing a public service. Craig drew the station’s call letters from the National Life slogan: We shield millions. WSM-AM was given a dreamy nickname: Air Castle of the South.

An informal live- performance country music show called WSM Barn Dance premiered on November 28, 1925. The live show attracted so many guests to the WSM Studio, that people soon joked that WSM stood for We seat many. Two years later, the immensely popular show was renamed “The Grand Ole Opry.”

Almost eighty years later, “The Grand Ole Opry” still attracts giant studio crowds and many radio listeners on Friday and Saturday nights. On December 6, 1982, WSM-AM became Nashville’s first stereo AM station.

The present day studio for WSM-AM is in the Gaylord Opryland Resort in Nashville. WSM-AM broadcasts a combination of country music and talk radio.

Growing Audience

By 1926, WSM-AM had increased its power to 5,000 Watts, formed an affiliation with the NBC Network, and expanded its broadcast day to eight hours.

WSM received Class 1-A clear channel status in 1932. The clear channel classification enabled WSM to increase power to 50,000 Watts. WSM-AM became the only station in the United States allowed to broadcast at 650 kilohertz.

After the 1981 Rio Agreement changed the original clear channel classifications, WSM-AM retained the protection that no other station within a 750-mile radius can broadcast at 650 kilohertz at night. WSM is one of America’s twenty-five clear channel stations.

The low frequency channel assignment and channel protection gave WSM an enviable nationwide coverage pattern. WMS-AM can be received in much of the US and Canada at night.

The change in designation to a high-power clear channel status was the impetus for WSM to build a new broadcast tower.

Enter Blaw-Knox

The Blaw-Knox Company of Pittsburg, Pennsylvania was formed on July 6, 1917 by a merger of the Blaw Collapsible Steel Centering Company and the Knox Pressed and Welded Steel Company. The city of Hoboken, Pennsylvania, which was home to an early Blaw-Knox factory, changed its name to Blawnox, Pennsylvania.

Beginning in 1927, the Blaw-Knox company began building radio towers. Diamond shaped antennas – also called dual cantilever center-guyed antennas – are usually referred to by their trade name, Blaw-Knox towers. The vertical diamond shaped WSM antenna tower (Figure 1) made by the Blaw-Knox Company was cutting-edge antenna technology in 1932.

WSM’s Blaw-Knox tower was built by a tower crew from Columbia, South Carolina. Upon finishing construction of WSM, the crew ventured on to build the WLW Blaw-Knox tower near Mason, Ohio.

Steel Landmark

Located about eight miles south of Nashville at the intersection of I-65 and Concord Road in Brentwood, Tennessee, WSM-AM is a middle Tennessee landmark. The WSM Blaw- Knox tower transmitted its first broadcast on October 5, 1932.

Figure 1. The pole at the top of the WSM-AM antenna tower holds a turnstile FM antenna for the now defunct WSM-FM 100.1 MHz station. The functionality of the open-air transmission lines (lower left) have been replaced by underground transmission lines.

The tower was originally built with 758 feet of square structural lattice and then 120 feet of tapered mast to measure 878 feet tall. At the time of construction, WSM was the tallest tower in the US. The steel members on the bottom part of the WSM tower are larger than the steel members on the top of the tower. There is a transition between the bottom linear taper and the top nonlinear taper at 680 feet. Where the top and bottom tapers meet, there are eight insulated steel guy cables attached. At the point of maximum force at the tower base, the tower rests on two porcelain insulators (Figure 2).

Figure 2. The porcelain insulators at the base, which support the entire weight of the structure, are predicted by one expert to support in excess of 300 tons. A very large spark gap (not shown) is behind the insulators.

Contemplating the physics at play with the WSM-AM tower is overwhelming. It is not unusual for passersby to call the station to report that the tower is tipping over. Watt Hairston, the Chief Engineer of WSM-AM, often gets calls of this nature. Watt concedes that “The tapered nature of the tower can create Diamond in the Sky the illusion that the tower is falling, although it’s been standing straight for more than 70 years now.”

Upon use, it was discovered that the tower was too long electrically. The daytime signal is carried by ground waves while the nighttime transmission is carried by sky waves. An interference zone with cancelled or distorted reception occurs when the ground waves and sky waves become equal. At a height of 878 feet, WSM’s very high angle of radiation caused cancellation between ground waves and sky waves 120 miles away in Chatanooga and Knoxville. This was a problem. The tower would have to be shortened in order to shift the signal cancellation zone to less populated areas.

A portion of the tower’s tapered top was removed in 1939 so that the tower now stood at 808 feet. The removed portion of the tower was then put in to service as a flagpole at the nearby Lipscomb School. The tower section flagpole was used until the school was rebuilt in 1996.

WSM’s sister tower, WLW-AM in Ohio, also had to be lowered in order to avoid having its interference zone fall over Indianapolis, Indiana.

FM Not Financially Feasible

A new mast pole was installed at the top of the modified WSM-AM tower in 1939 for an FM sister station in the 45 MHz band, W47NV-FM. WSM-FM’s allocation moved up to 100.1 MHz after WWII ended. The 45 MHz FM antenna was replaced with another turnstile antenna. WSM relinquished their FM license and ceased operation in 1952 because FM transmission had not caught on at that time and WSM wanted to start a television station instead. The 100.1 MHz FM antenna remains installed on the WSM-AM tower. Another later attempt at an FM sister station for WSM-AM succeeded. The WSM-FM station 95.5 MHz antenna is located on a guyed tower a few miles away.

Limited Production

Despite being effective radiators and impressive works of architecture, there were very few Blaw-Knox towers ever built. Shortly after WSM and WLW were constructed, builders determined that a uniform cross section tower could be built for about half the cost of a dual cantilever center-guyed antenna. By one count, there are five diamond-shaped towers remaining in the U.S.

The Blaw-Knox company exited the tower business in 1958. In addition to building towers, the Blaw-Knox company also built tools for the public works and highway construction industries. The Ingersoll-Rand company has a line of paving and road surfacing equipment called Blaw-Knox.

In the Doghouse

The WSM doghouse is an aged brick building with charming white double doors with windowpanes. The doghouse contains equipment used to tune the characteristic output impedance of the transmitter and transmission line to the input impedance of the WSM-AM tower. An inventory of the WSM Antenna Tuning Unit (ATU) includes extremely large metal capacitors, inductive coils, a dummy load, and grounding straps. The coils and capacitors form a coupling network (Figure 3). A Caterpillar generator and a backup antenna at the site help ensure reliability.

Figure 3. Inside the doghouse, a person can hear an emulated version of the transmitted audio in the vibrations of the tuning coils.

Transmitter Building/Museum

While the WSM tower was awe inspiring, the WSM-AM transmitter building is just as much of a treat. Besides the obvious contents of transmitters, the building contained old-fashioned treasures.

Two Harris Corporation transmitters, a DX 50 and a 3DX50 Destiny, occupy the main room. WSM alternates weekly between the two transmitters. The transmitters were among the newer items in the transmitter building.

The transmitter building is large by today’s standards, but it needed to be built big to house the first 50-kilowatt transmitter. Storage drawers containing all of the original blueprints the Blaw-Knox Company prepared for the tower. In Watt Hairston’s office, a blackboard is entirely covered by schematics, formulas, and computations intended to determine the impedance of WSM.

A spiral staircase connects the main floor to the basement and the present to the past. The machine shop in the basement contains a DoALL Countour Machine Job Selector with War Finish made after WWII. WSM engineers preferred to craft solutions on site (Figure 4). A group of filing cabinets hold records from the 1960’s and 1970’s of a fight to keep the assigned clear channels clear. An electrical box holds a maze of fabric-wound wires attached to pegs. On the shelves are old radios and antiquated electronic measurement tools.

Figure 4. WSM-AM Chief Engineer Watt Hairston shows off the site’s machine shop. In the past, WSM engineers would build any tower or transmitter parts that they needed.

If you find yourself near Nashville one day, keep your eyes open for a diamond-shaped tower on the horizon. 

Acknowledgements: Watt Hairston, Chief Engineer of WSM-AM; John Hettish, moderator of Tower-Pro forum; Fybush.com; http://www.j-hawkins.com; and WSMonline.com