HERTZ

Jun 25,2007 00:00 by admin
HERTZ
The term hertz is a measure of frequency, or the speed of
transmission. The frequency of electromagnetic waves generated
by radio transmitters is measured in cycles per second
(cps), but this designation was officially changed to
hertz (Hz) in 1960.
An electromagnetic wave is composed of complete
cycles. The number of cycles that occur each second gives
radio waves their frequency, while the peak-to-peak distance
of the waveform gives the amplitude of the signal
(Figure H-1).
The frequency of standard speech is between 3000 cycles
per second, or 3 kilohertz (kHz), and 4000 cycles per second,
or 4 kHz. Some radio waves may have frequencies of many
millions of hertz (megahertz, or MHz), and even billions of
hertz (gigahertz, or GHz). Table H-1 provides the range of frequencies
and their band classification.
The term hertz was adopted in 1960 by an international
group of scientists and engineers at the General Conference
of Weights and Measures in honor of Heinrich R. Hertz
(1857–1894), a German physicist (Figure H-2). Hertz is best
known for proving the existence of electromagnetic waves, which had been predicted by British scientist James Clerk
Maxwell in 1864.
Hertz used a rapidly oscillating electric spark to produce
UHF waves. These waves caused similar electrical oscillations
in a distant wire loop. The discovery of electromagnetic
waves and how they could be manipulated paved the way for
the development of radio, microwave, radar, and other forms
of wireless communication.
150 HERTZ
1 Second
Frequency
Valley
Peak
Complete
Cycle
Amplitude
TABLE H-1 List of Frequency Ranges and Corresponding Band
Classifications
Frequency Band Classification
Less than 30 kHz Very low frequency (VLF)
30 to 300 kHz Low frequency (LF)
300 kHz to 3 MHz Medium frequency (MF)
3 to 30 MHz High frequency (HF)
30 to 300 MHz Very high frequency (VHF)
300 MHz to 3 GHz Ultrahigh frequency (UHF)
3 to 30 GHz Superhigh frequency (SHF)
More than 30 GHz Extremely high frequency (EHF)
Figure H-1 Each cycle per second equates to 1 hertz (Hz). In this case, 3
cycles occur in 1 second, which equates to 3 Hz.
HERTZ 151
Summary
As interest in electromagnetic waves grew in the nineteenth
century, a physical model to describe it was proposed.
It was suggested that electromagnetic waves,
including light, were like sound waves but that they propagated
through some previously unknown medium called
the “luminiferous ether” that filled all unoccupied space
throughout the universe. The experiments of Albert A.
Michelson and Edward W. Morley in 1887 proved that the
ether did not exist. Albert Einstein’s theory of relativity,
proposed in 1905, eliminated the need for a light-transmitting medium, so today the term ether is used only in a historical
context, as in the term Ethernet