|
March 1935 Short Wave Craft
 [Table
of Contents]
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
|
In 1935, not much was yet known
about the ionosphere. Its existence was first theorized in 1902 by
Arthur Kennelly and Oliver Heaviside, and
Edward Appleton proved
its presence in 1924 by conducting a series of broadcast experiments, but no direct
measurements were possible until rocket-borne instruments could be launched. An
Aerobee-Hi sounding rocket was launched in 1956 as part of the
International Geophysical Year (IGY) project that made the first
actual detection of ionized particles in what is now referred to as the D-layer.
It is therefore forgivable that Hugo Gernsback, normally spot-on in his theories
and postulations regarding RF propagation, incorrectly suggested in this editorial
that based on observed time measurements from Europe to the USA, radio waves may
vary in speed through the atmosphere by as much as a factor of two, that is, from
half the speed of light in a vacuum to the full speed. The actual explanation is
almost certainly that the waves took vastly different paths from the points of transmission
to points of reception.
Mysterious
Short Waves
 An Editorial By Hugo Gernsback
It is a well-known fact that the more we learn about a given subject, the less
we know about it in the end. Ten years ago, any radio engineer would have been cocksure
that radio waves, the same as all electromagnetic waves, traveled at the speed of
light, that is, 186,000 miles per second. These were known as facts, and no one
ever seriously questioned these "facts." But our latter-day scientists have the
habit of pulling out the props from almost any so-called "fact" and many of our
preconceived notions have a habit of tumbling about our ears in a most disconcerting
fashion of late.
Thus, for instance, Dr. Harlan T. Stetson told the American Association for the
Advancement of Science recently that radio waves, which had been assumed to travel
always at the speed of 186,000 miles a second, did not always do so! Indeed, he
found that sometimes they traveled at only half this speed, that is, about 93,000
miles a second.
Dr. Stetson found that signals from Rugby, England, transmitted to Annapolis,
Md., varied greatly in speed, while those from Bordeaux, France, to Annapolis did
not vary. These variations immediately raised havoc in several fields. In the first
place, scientists had become used to the idea that they had a most accurate and
unvarying ""yardstick" in the speed of radio waves, which they assumed to be 186,000
miles a second. They now found this yardstick no longer accurate.
To illustrate, radio has been used right along to plot the exact longitude, that
is, in other words, east and west position of any point of the earth's surface.
Thus, for instance, we are not certain now what the exact longitude of New York
is, and, as a matter of fact, it is no more exact now than before the advent of
radio.
In astronomy, where exact results are of paramount importance, the radio yardstick
is now found not to be accurate any longer, and this may have important considerations
and effects on astronomy. Of course, as far as the radio listener is concerned,
it makes very little difference if the program is delayed a fraction of a second,
and he does not particularly care about a slight delay, but to science in general,
it raises absolute havoc!
What are the reasons behind this apparent mysterious behavior of radio waves?
The answer is probably in the Heaviside Layer, or rather the electrified or conducting
air in the upper regions of our atmosphere. Thus, Dr. Alfred N. Goldsmith thinks
that waves from Europe to America traveling the southern route, encounter more normal
atmospheric conditions and travel at the usual velocity, that is, 186,000 miles
a second; while, on the other hand, other radio waves sent from Europe to the United
States travel through the Arctic regions, where they encounter an electrified or
conducting air, in the upper regions, which may have the effect of slowing up the
flight of the waves.
I personally have no fault to find with this theory and it probably will hold
true to a large extent. On the other hand, there is nothing absolutely original
with these findings, if we consider the following:
It has been known for many years that if you send a signal by cable across the
Atlantic there is a delay of about 1/10 of a second. The delay is caused by the
fact that the cable has a certain electrical capacity. We have a conductor inside
of the cable, then the insulation, and outside the ocean. This gives us a huge electrical
condenser. When trying to get a signal through this condenser we must first charge
the condenser. Now, as anybody knows who has done much work with condensers, it
takes a certain time to charge the condenser, and this accounts for the delayed
action of the signal. After all, the signal is only an electrical current and if
you try to push the signal through the condenser, you meet with a certain resistance.
Indeed, it is most interesting to know that the time delay increases as the square
of the distance, in other words, if you had a submarine cable going around the world,
that is, 24,000 miles, it would actually take 17.3 seconds to get the signal through
the cable.
If we consider the earth and the Heaviside layer as the two members or plates
of a huge condenser, and knowing further that the velocity of transmission of a
wave through a highly attenuated gaseous medium, such as that existing between the
earth and Heaviside layer, varies with the degree of ionization of such a medium,
it is apparent that there can be quite a radical change in the velocity of the wave
or signal transmitted between two such widely separated points as New York and London.
As pointed out by Ladner and Stoner in their excellent treatise, "Short Wave Wireless
Communication" "the reduction in the group velocity (referring to the transmission
of waves through an ionized medium, such as gas) is dependent upon the electron
density of the medium through which the group is travelling." Further these authorities
state - "The importance of atmospheric pressure (in regard to radio transmission)
lies in the fact that pressure determines conductivity and dielectric constant,
for although air at atmospheric pressure is almost a perfect insulator, at low pressure
it becomes ionized by the sun's action. The effect of ionization is to reduce the
dielectric constant and increase the conductivity of the gas in different ways to
different frequencies. A removal of the cause of ionization allows the gas to return
to its un-ionized condition, due to the recombination of charged particles, and
it is to be observed that the time of recombination and ionization may be a slow
process if the gas pressure is very low
Posted October 10, 2019
(updated from original post on 5/28/2015)
|
