Direct Energy, 2018a

76 4.4 Antenna Characteristics
antenna designed to operate at 3 kHz looks quite dierent from an antenna
designed to operate at 3 THz. Wire-like antennas are used for signals
roughly in the frequency range 3 kHz f 3 GHz. Solid cone, platelike,
or aperture antennas are used to transmit and receive signals in the
frequency range 3 GHz f 3 THz [15, ch. 15]. To understand the need
for dierent techniques, consider the wavelengths involved. A signal with
frequency f =30kHz, for example, has a wavelength λ =1.00 · 10 4 m.
The length of an antenna is often of the same order of magnitude as the
wavelength. While we can construct wire antennas of this length, they not
portable. As another example, a wi signal which operates at 2.5 GHz
has a wavelength of λ =12.5 cm. Wire antennas which are this length are
easy to build and transport. However, wire antennas designed for signals at
higher frequencies can be dicult to construct accurately due to their small
size. For this reason, wire antennas are typically used at lower frequencies
while cone or plate-like antennas are used higher frequencies.
A human eye can detect electromagnetic radiation with frequencies and
wavelengths in the range
4.6 · 10 14 Hz f 7.5 · 10 14 Hz or 400 nm λ 650 nm
Antennas are not used to receive and transmit optical signals due to the
small wavelengths involved even though optical signals obey the same fundamental
physics as radio frequency electromagnetic radiation. Green light,
for example, has a wavelength near λ = 500 nm and a frequency near 6·10 14
Hz. An antenna designed to transmit and detect this light would need to be
approximately of length λ ≈ 250 nm. An atom is around 0.1 nm in length,
2
so an antenna designed for green light would be only approximately 2500
atoms long. Antennas of this size would be impractical for many reasons.
Another reason that dierent techniques are needed to transmit and receive
optical signals is that electrical circuits cannot operate at the speed
of optical frequencies. Techniques for transmitting and detecting optical
signals are discussed in Chapters 6 and 7.
When selecting an antenna, the range of frequencies that will be transmitted
or received as well as their bandwidth should be considered. Some
antennas are designed to operate over a narrow range of frequencies while
other antennas are designed to operate over a broader band of frequencies.
An antenna with a narrow bandwidth would be useful in the case when an
antenna is used to receive signals only in a specic frequency band while
an antenna with a broad bandwidth would be useful when an antenna is
to receive signals over a wider frequency range. For example, an antenna
designed to receive over the air television signals in the US should be designed
for the broad range from 30 MHz -3 G Hz because television signals