Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 67
interests of both types of users are best served by selecting the shorter of the two great
circle segments connecting the two points of interest, although that is not always the case
for radio communications, as we shall see shortly.
Later in this book we take up directional antennas—antennas deliberately designed
to maximize radiation in one direction or a small range of directions in exchange for
accepting reduced radiation in other directions. With the exception of broadcast stations
and government or commercial point-to-point stations, most directional antennas
are intended to be rotated either electronically or mechanically. However, some highly
directive MF and HF antennas for transmitting (the rhombic) or receiving (the Beverage)
are too large to be rotated. Instead, one or more of each is constructed with its direction
of maximum signal fixed on a desired region of our globe. An amateur low-band
DXer in Maryland might choose to install a Beverage receiving antenna aimed at Europe.
By referring to the great circle map of Fig. 2.35, he would conclude (correctly) that
his Beverage should be oriented so as to provide maximum pickup of incoming signals
centered on a true compass bearing of somewhere between 45 and 50 degrees. An amateur
in Japan (roughly the same latitude as Washington, D.C., and Lisbon), on the other
hand, would not aim due east or west to favor Europe on his receiving antenna; instead,
he would first consult a great circle map centered on Tokyo and then aim his Beverage
25 to 30 degrees west of north!
Ionospheric Fading
It should be apparent from the preceding paragraphs that ionospheric communications
are statistical in nature, rather than deterministic. One contributing phenomenon is
Âfading—time-varying signal strength and phase at the receiver site.
Perhaps the most audibly dramatic form of fading—selective fading—is what we
often hear when listening to analog voice transmissions on the MF and lower HF bands
at night. Especially at night, at distances beyond the local ground-wave reception zone,
the received signal will often consist of ground wave, sky wave, and multiple reflections
of the sky wave from the ionosphere. Thanks primarily to the unceasing motion of
the ionosphere, the received signal is an ever-changing mix of the transmitted signal
arriving by uncountable numbers of paths, each having its own (time-varying) amplitude
and phase relative to original signal. But the most distinctive aspect of this fading
is caused by the fact that the ionosphere is dispersive and its reflection characteristics are
different for even slightly different frequencies. Since an analog HF voice transmission
consists of frequency components spread over a 3- to 20-kHz range around the carrier
frequency depending on the service (amateur versus broadcast, for instance) and mode
(amplitude-modulated double sideband with carrier versus single sideband suppressed
carrier), the received signal is an eerie distortion of the original, similar to the flanger
feature of modern-day sound mixers. It is such a distinctive sound that a pop music
single released in 1959—“The Big Hurt” by Miss Toni Fisher—went all the way to number
three on Billboard magazine’s top 100 by replicating that sound.
Fading from any cause is a serious problem that can disrupt reliable communications
and severely reduce intelligibility. Its effects can often be overcome by using one
of several diversity reception systems. Three forms of diversity technique are popular:
frequency diversity, spatial diversity, and polarization diversity.
In the frequency diversity system of Fig. 2.36, the transmitter delivers RF with identical
modulating information to two or more frequencies simultaneously. Because the
two frequencies will almost certainly fade differentially, one will always be stronger.