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 45
Ionospheric Propagation
Following on the heels of seminal discoveries by Hertz, Marconi, and other pioneering
radio scientists of the late 1800s, the early 1900s found experimenters from all walks of
life constructing simple radio transmitting and receiving equipment. By 1909, radio
clubs had been formed at a few colleges around the United States, and by 1914 the
American Radio Relay League (ARRL), dedicated to these amateur experimenters, had
been founded. Commercial, government, and amateur stations shared the airwaves in
willy-nilly fashion until around 1912 when the United States began to regulate radio. In
those days, “good DX” might mean the ability to communicate between, say, Boston
and New York City, and the prevalent assumption was that the longer wavelengths
were superior for extending those distances. Forced to shift from wavelengths as long
as 1000 m (well below the bottom of today’s AM broadcast band) to 200 m (roughly
around 1500 kHz, near the top of the AM broadcast band) by the new regulations, amateurs
grumbled a bit but dug in and went back to figuring out how to attain better and
better DX distances. As the distances increased, however, the experts were finding it
harder and harder to explain how it was possible.
World War I brought a halt to all amateur transmissions, and when the war was
over the U.S. government was inclined to keep it that way forever. But the ARRL convinced
Congress and various federal agencies of the merits of radio amateurs, and in
the fall of 1919 amateurs returned to the airwaves. In 1921 and again in 1922, the ARRL
sponsored transatlantic receiving tests, and in both events too many stations were heard
for it to be a fluke: 30 or so the first year and 10 times that number the following year!
Spurred on by these encouraging results, the ARRL and others began to investigate
what it would take to convert these successes into two-way communication across the
ocean. Despite the almost unanimous opinion of the best scientists of the day that the
shorter wavelengths would prove to be useless, some hardy experimenters plunged
below 200 m and, within a year, had shown that the shorter the wavelength, the better
the DX results!
Textbooks and magazines of the era were rife with theories to explain these totally
unexpected results. Ultimately, the “correct” theory turned out to be one originally advanced
in 1901, when Arthur Kennelly of the United States and Oliver Heaviside of
England independently proposed the existence of a “reflecting” layer circling the earth
some miles above it. Soon named the Kennelly-Heaviside layer, it is now known as the
E layer (Fig. 2.26) of the ionosphere.
Thus, it is the ionosphere that makes intercontinental radio communications possible
on the MF, HF, and lower VHF bands (between roughly 0.3 and 70 MHz). Included
in the range of frequencies for which ionospheric propagation is possible are the AM
broadcast band, many traditional maritime frequencies, amateur bands from 160 to 6 m,
and the first few channels (2, 3, and 4) of the analog broadcast television spectrum. True,
in some cases (especially at the upper end of the range) “possible” does not mean
“probable”, but the potential is there during sunspot maxima or during brief periods of
unusual solar activity. The author recalls the winter of 1957–58, when he saw firsthand
a neighboring amateur make two-way contact with six continents in one evening on the
6-m band (50 MHz), using a 5-W AM transceiver and a three-element 6-m beam dangling
from the I-beam running through his basement!
Kennelly and Heaviside described the unseen layer as a reflecting layer; in truth,
the sparseness of the ionization and the variation of ionization with altitude make