WWVB - North antenna

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Time signal on 60 kHz, Operated by NIST.

WWVB uses two identical antennas that were originally constructed in 1962, and refurbished in 1999. The north antenna was originally built for the WWVL 20 kHz broadcast (discontinued in 1972), and the south antenna was built for the WWVB 60 kHz broadcast. The antennas are spaced 857 m apart. Each antenna is a top loaded monopole consisting of four 122-m towers arranged in a diamond shape. A system of cables, often called a capacitance hat or top hat, is suspended between the four towers. This top hat is electrically isolated from the towers, and is electrically connected to a downlead suspended from the center of the top hat. The downlead serves as the radiating element.

Ideally, an efficient antenna system requires a radiating element that is at least one-quarter wavelength long. At 60 kHz, this becomes difficult. The wavelength is 5000 m, so a one-quarter wavelength antenna would be 1250 m tall, or about 10 times the height of the WWVB antenna towers. As a compromise, some of the missing length was added horizontally to the top hats of this vertical dipole, and the downlead of each antenna is terminated at its own helix house under the top hats. Each helix house contains a large inductor to cancel the capacitance of the short antenna and a variometer (variable inductor) to tune the antenna system. Energy is fed from the transmitters to the helix houses using underground cables housed in two concrete trenches. Each trench is about 435 m long.

A computer is used to automatically tune the antennas during icy and/or windy conditions. This automatic tuning provides a dynamic match between the transmitter and the antenna system. The computer looks for a phase difference between voltage and current at the transmitter. If one is detected, an error signal is sent to a 3-phase motor in the helix house that rotates the rotor inside the variometer. This retunes the antenna and restores the match between the antenna and transmitter.

There are three transmitters at the WWVB site. Two are in constant operation and one serves as a standby transmitter that is activated if one of the primary transmitters fail. Each transmitter consists of two identical power amplifiers which are combined to produce the greatly amplified signal sent to the antenna. One transmitter delivering an amplified time code signal into the north antenna system, and one transmitter feeds the south antenna system. The time code is fed to a console where it passes through a control system and then is delivered to the transmitters.

Using two transmitters and two antennas allows the station to be more efficient. As mentioned earlier, the WWVB antennas are physically much smaller than one quarter wavelength. As the length of a vertical radiator becomes shorter compared to wavelength, the efficiency of the antenna goes down. In other words, it requires more and more transmitter power to increase the effective radiated power. The north antenna system at WWVB has an efficiency of about 50.6%, and the south antenna has an efficiency of about 57.5%. However, the combined efficiency of the two antennas is about 65%. As a result, each transmitter only has to produce a forward power of about 38 kW for WWVB to produce its effective radiated power of 50 kW.