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FM Slide Rule Is a Treasure From the Past - Radio World

FM Slide Rule Is a Treasure From the Past

To bolster the enthusiasm and brio that came out of WWII, RCA sent customers this interesting device circa 1944
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World War II stalled many emerging technologies and social changes. Broadcasting was hit hard; the conflict essentially put the kibosh on nascent television and frequency-modulated radio for the duration.

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On the home front, the war was one of out-supplying the enemy and husbanding our resources. Broadcast engineers were useful and busy in this effort, providing communications expertise and solutions at the highest level.

Out of that chaos, victories overseas brought a new optimism that we were going to win the war. A better day with a return to normal life was coming.

Broadcasters started to speculate and make plans for that future.

To bolster the enthusiasm and brio, RCA, a major supplier of equipment at that time, sent customers this interesting device circa 1944. It is an FM slide rule coverage calculator.

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This relic came to me as a gift from longtime friend and audio connoisseur extraordinaire, Richard Robinson.

The intriguing device allows one to ascertain from antenna height (assume flat earth), transmitter power and bay count (think gain) to determine radius coverage for urban (1 mV) and rural (0.05 mV) as well as the area served in square miles.

The FM band at the time was in the 40 MHz area and in horizontal polarity only in this case, predicated on using a turnstile antenna where each bay gives about a gain of 1.

If this slide rule concept looks familiar, review the 47CFR73.333 figure 1 in the Code of Federal Regulations from days of old, which for years was the methodology for calculated coverage in the FCC’s regulations … essentially a paper nomograph version of this slide rule.

My romantic Latin American friends tell me that our dreams may cost nothing but are our most valuable personal possession. Perhaps an engineering forbearer sat at his station desk during those daunting days of war, and manipulated this coverage calculator while dreaming of the new radio dial to come.

INSTRUCTIONS

The original letter that accompanied this device bears the RCA logo at the top and reads as follows:

An RCA FM Coverage Calculator is forwarded herewith to assist you in planning an FM station. This rule has been developed by the RCA Engineering Department and has been made with materials available under wartime restrictions. We hope you will find it helpful. Additional copies of this rule are available at one dollar each.

To use the RCA FM Coverage Calculator, set the left hand mark of the wood slide above the height of the transmitting antenna (second scale from bottom). Next, select the power transmitter to be used. For each power transmitter there is a line on the wood slider with a scale marked 1, 2, 4 and 6. These indicate the number of layers in a turnstile antenna. Set the small metal slider with its center mark on the number of layers in the antenna. Read on the next to the top scale the radius of rural coverage and on the top scale the radius of urban coverage. These indications will be shown by the center marking of the metal slider. The bottom scale on the rule indicates the number of square miles of rural coverage. Small markings to the left of those indicating number of layers in the antenna allow for 10 percent loss in the transmission line.
Radio Corporation of America
RCA Victor Division
Camden, New Jersey

Charles S. Fitch is a registered professional consultant engineer, senior member of the SBE, lifetime CPBE with AMD, licensed electrical contractor, former station owner and former director of engineering.

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