In 1949 Central Broadcasting Company executives made the decision to acquire a new 50,000 watt transmitter for WHO-AM. At the same time, a new FM transmitter and antenna were to be installed at the Mitchellville transmitter plant. WHO-FM went on the air using a 3 kilowatt transmitter and antenna which produced 5 kilowatts of effective radiated power. The transmitter and antenna were located on the top floor of the Equitable Building in downtown Des Moines.
Orders were placed with Westinghouse for a new 50,000 watt AM transmitter. It would replace the RCA transmitter which had been installed at the Mitchellville transmitter site in 1933. A 50,000 watt FM transmitter was also purchased from Westinghouse. It would be coupled to a 12 bay antenna. The FM antenna would be mounted on the new modified Franklin design tower that had been created by the WHO research engineers. The new transmitter and 12 bay antenna would give WHO-FM 400,000 watts effective radiated power. With a 50,000 transmitter and 12 bay antenna creating an effective radiated power of 400,000 watts, WHO-FM would be one of the most powerful FM stations in the nation
Construction permits to install the new AM and FM transmitters were obtained from the Federal Communications Commission on April 26, 1949.
The Westinghouse AM transmitter was the first unit to be installed. It is a Westinghouse 50 HG-2. It has serial number 3. Low power testing of the new transmitter began on January 18, 1950. Then, on January 20, 1950, full 50,000 watt testing was underway. The first modulated tone with the unit was made on January 24, 1950; station break announcements were made on February 2, 1950; and then, there were test programs from the studio using the new transmitter early on the morning of February 3, 1950. The Westinghouse 50 HG-2 went into regular service on February 6, 1950, at 5:28:23 AM.
It would fall 1950 before the new, modified Franklin design tower would be installed and in service. The tower was in place by October 8, 1950, and it was being fed from the transmitter at 5:44:15 AM. Adjustments were necessary, and by October 9, 1950, the Franklin tower was operating as designed and intended.
Here are some statistics about the modified Franklin design tower from a 1952 History of WHO:
The new tower, designed in WHO laboratories over a period of five year's planning and proving, dwarfs its 532-foot predecessor with its 780-foot reach into the sky. There are 137 ½ tons of steel in the structure, 8,800 pounds of steel in each of the 3 top guy wires which measure 1 5/8 inches across; and 3,160 pounds in each of the three lower guys measuring 1 1/8 inches across. These guys, pulling the tower sung against the earth on a 1-foot square insulator, give the new tower a "thrust weight" of 423,890 pounds.
Clear Channel Communications Central Region IT manager Jeff Hansen worked at the Mitchellville transmitter plant from 1993 through late 1999. He helped keep the Westinghouse HG-2 on the air. He also knows how the modified Franklin design tower works:
The commentary from Jeff should begin with the sentence, "The Franklin
structure is very similar to a vertical dipole."
The Franklin structure is similar to a vertical dipole. Very briefly, here's how it works: the 6 inch transmission line carries the 50KW energy up to the 330 foot level where it is dumped into a tapped variable coil. The coil then feeds the upper half of the tower (above the insulators). As this energy travels up and down the structure, an RF voltage is induced in the lower half of the tower. A nitrogen pressurized variable capacitor (at the base) is in series with the tower base and ground. This capacitor is used to tune the lower half of the tower, and thus, set the phase relationship between the fixed upper half and the lower half of the tower. In the end, both the upper and lower half of the tower are excited with equal amounts of power in phase.
Each section of the tower is fed by the transmission line. Only the top half is fed directly by the transmission line.
The modified Franklin design is intended to "fill in" the area where the ground wave portion of the WHO signal ends and the sky wave begins. The WHO tower has recently received extensive maintenance and a complete renovation. The purpose was to allow WHO to continue to provide a high quality signal throughout the state of Iowa and most of the nation.
The Westinghouse FM transmitter and 12 bay antenna were the last items installed in the 1950 upgrade. They were placed in operation on December 6, 1950.
A June 14, 1951, WHO History states a quarter million dollars was invested in the new transmitters, tower, and FM antenna. In 1954, a television transmitter and antenna for Channel 13 were installed at Mitchellville. These allowed WHO-TV to begin broadcasting as Des Moines' first VHF commercial station (WOI-TV, Ames was the first television station on the air in Central Iowa. Channel 17, KGTV-TV went on the air in 1953, ironically using the former WHO Radio Blaw Knox tower to support is antenna. The Blaw Knox tower was moved to a location on Second Avenue on the north side of Des Moines. When KGTV-TV went off the air, the state of Iowa acquired the tower and used it for many years for the state radio communications system. The Blaw Knox tower was dismantled several years ago.
The Westinghouse HG-2 is still in place at Mitchellville. But after close to fifty years of faithful service, she is no longer has a transmission line connection. While the Westinghouse remains fully operable, Jeff Hansen notes that she has earned her rest and probably will never return to service.
Today the main transmitter for WHO is a Harris DX-50. It is totally solid state. Incoming audio is sampled at the carrier frequency. From this sample, a digital control signal is created. This signal is used to rapidly turn over a thousand high-power MOSFET transistors on and off at precisely the right time. The combined output of these banks of MOSFET's creates a modulation envelope at 50KW.
The backup is a Harris MW50C3. This transmitter came on line at WHO in the late 1970s. It uses "Pulse Duration Modulation" and a pair of 4CX35000A tubes to create the fully modulated 50,000 watt signal.
The Westinghouse FM transmitter was removed from the Mitchellville plant a number of years ago. The 12 bay FM antenna was dismantled as part of the recent renovation and repair of the Mitchellville tower and its structure.
WHO-FM was not successful as a 400,000 watt FM operation. Perhaps this is because during the 1950s and early 1960s, FM receivers were rare, especially in automobiles where a lot of radio listening was occurring at the time. Also, AM stations continued to be popular and adjusted their programming to meet the desires of the audience. Power of WHO-FM was reduced. The station was revitalized as KLYF-FM with 100,000 watts of effective radiated power in 1973 when its transmitter and antenna were located near Alleman, Iowa, the site of a 2,000 foot tower that also has transmitters and antennas for WHO-TV, KDIN-TV, WOI-TV and WOI-FM.
Visual tour of WHO ransmitter site (1991)
WHO main page
DesMoinesBroadcasting.com home page