Direct Energy, 2018a

4 ANTENNAS 87
(d) Which antenna do you expect to be more isotropic: antenna
a, antenna b, or would they be about the same? Explain your
choice.
(e) Which antenna is more likely to be used as a receiver than
a transmitter: antenna a, antenna b, or both antennas about
equal? Explain your choice.
4.3. Some speculate that alien civilizations might be able to watch TV
programs that escape the earth's atmosphere. To get an idea of the
likelihood for this to occur, consider an isotropic antenna in outer
space transmitting a 200 MHz TV signal.
Assume that the alien civilization uses an antenna with surface area
0.5 m 2 and has the technology to detect a signal with power as low as
5 · 10 −22 W. What is the minimum power that must be transmitted
for detection to occur at a distance of 1.0 light year?
4.4. Project ELF, described in Sec. 4.4.1, was an extremely low frequency,
76 Hz, radio system set by the military to communicate with submarines.
It had facilities near Clam Lake, Wisconsin and Republic,
Michigan, 148 miles apart [52]. Because these facilities were located
a fraction of a wavelength apart, antennas at these locations acted as
part of a single array. The length of all antenna elements was 84 miles
[52]. Assume it took 18 minutes to transmit a three letters message
using 8 bit ASCII, and assume signals travel close to the speed of
light in free space.
(a) Calculate the ratio of the distance between the transmitting facilities
to the wavelength.
(b) Calculate the ratio of the length of all antenna elements to the
wavelength.
(c) What was the speed of communication in bits per second?
(d) How many wavelengths long were each bit?