A wire antenna most efficiently transmits (or detects) radio waves which have a

Question

A wire antenna most efficiently transmits (or detects) radio waves which have a wavelength 4 times longer than the wire.
You are building a shortwave radio and You have exactly 10 meters (30 feet) of wire for which you can make an antenna. The shortwave radio can run at a number of frequencies. Which frequency should you choose so that the antenna broadcasts most efficiently? Assume that the speed of light (and radiowaves) is 300 000 kilometers per second.

A) 15 MHz
B) 7.5 MHz
C) 3 MHz
D) 1.5 MHz

Explanation / Answer

B. 7.5 Mhz

With 10 meters of wire you want to transmit at a wavelength of 40 m.

f = c/ = (3 * 108 m/s)/(40 m) = 7.5 * 106 Hz    (Hz are inverse seconds)

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