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Do you remember

In June of 1993, the FCC issued a Notice of Inquiry (NOI) into the rules regarding AM directional antenna performance and specifications. This NOI was meant to provide guidance in the new world of computer-operated systems and, in many cases, non-human supervised systems of the time.

The original rules and the long-gone Standards of Good Engineering Practice (that are now incorporated into the FCC's rules) were products of the 1930s, and the rules concerning directional antenna systems had been written mostly in 1939. Many of the DAs designed and installed from 1939 through 1965 were located in wide-open spaces, far from towns and other developed areas. This posed a problem for engineers. This resulted in rules that worked, but failed to provide the technical guidance and regulation that was required.

Therefore, in the early 1990s, the FCC adopted sweeping changes in the AM rules. These were designed to facilitate the introduction of the extra 100kHz at the top of the broadcast band (the so-called expanded band), and to reduce co-channel and adjacent-channel interference. The new rule changes reduced the amount of interference allowed. At the time, almost every RF engineer knew of at least one case where a directional pattern was designed to provide interference-free service, but that satisfactory operation was hard to obtain. The new NPRM was expected to seek a method of anticipating some of these problems, and also trying to find better ways of calculating directional patterns.


That was then

In the early 1940s, Gates introduced the model 1D broadcast transmitter, which a promotional flyer touted as “a moderately priced, high-quality transmitting piece of equipment.” It featured two doors near the top of the front panel that exposed practically every component in the top part of the transmitter. The full-size door provided easy access to the back.

Frequency control for the 1D was obtained from the 25-A Frequency Control Unit, which included the oscillator and two buffer stages and power supply with provision for operating two crystals, one being connected for heating only.

The 1D produced 1,000W, but could be reduced to 500W or 250W for nighttime operations. The transmitter consumed about 5.5kW or power and responded to frequency within 1½dB from 30Hz to 10kHz. The 1D had less than 3 percent distortion at 95 percent modulation.

It was designed to operate with a 60 to 300 ohm load and "coupled into concentric or unbalanced low impedance transmission lines and into most standard antennae with external matching units."


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The oldest transmitter

Last year, Radio magazine asked you to help us find the oldest transmitter that was in working condition. We located a Western Electric transmitter in Sharon, PA, that was installed in 1940. That transmitter is still used as a backup transmitter today. Now we want to find the oldest transmitter in use as a main transmitter; that is, a transmitter that is used everyday. Do you have it? Send a note to radio@primediabusiness.com and let us know. We'll share the entries in our December issue.




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