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December 1957 Popular Electronics
 Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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Here is a back-to-the-basics
treatise on AC and DC, plus an introduction to radio frequencies (RF). The author, Herb.
S. Brier, is a licensed Ham who presents a very high level treatment of the topics
for rank beginners in this 1957 edition. Remember that Popular Electronics
was a magazine intended to appeal to hobbyists with backgrounds in electricity and
electronics ranging from knowing how to insert batteries into a flashlight in the
proper direction (most of the time) to engineers and college professors. Part of
the publisher's mission was to introduce as many aspects as possible in order to
capture the interest of as many people as possible. They were pretty successful,
based on how long the magazine ran its course.
Among the Novice Hams
 By Herb S. Brier, W9EGQ
In discussing the theory behind questions appearing in the FCC amateur license
examinations (October and November issues), so far we have made no distinction between
direct current (d.c.) and alternating current (a.c.). This is because everything
that we have learned up to now is equally true for either type of current. Nevertheless,
there are many important differences between the two which must be understood before
you can learn much about radio. Anyone who learned how to do long division in school
has enough on the ball to master sufficient a.c. and d.c. theory to qualify for
an amateur license. So let's get started.
The Two Currents. Direct current, as its name implies, always flows in the same
direction-from the negative terminal of the power source to its positive terminal.
To most of us, flashlight batteries, portable radio batteries, and storage batteries.
are the most familiar sources of direct current.
Alternating current, however, starts at zero, builds up to a maximum value in
one direction, decreases to zero again, builds up to a maximum value in the opposite
direction, and again drops to zero. The whole cycle repeats itself over and over
many times per second as the terminals of the a.c. generator become alternately
positive and negative. The electric power delivered to our homes by the power company
is 60-cps alternating current.
Fig. 1 - The difference between the effective and peak values
of an alternating current.
Figure 1 shows how a.c. peak and effective voltages differ. The part of the sine
curve above the base line indicates that the voltage and the current are building
up in one direction (positive), and the part below the base line indicates that
they are building up in the opposite (negative) direction. The arrow pointing to
the right represents the passage of time.
Peak Value = 1.41 x Effective Value
Effective Value = 0.71 x Peak Value
A direct current has the same effective and peak value.
Obviously, if an a.c. generator is connected across a load, such as a light bulb,
the current flowing through the load will increase and decrease in step with the
volt-age. Consequently, maximum power can flow into the load during only a small
portion of each cycle. With d.c., however, full power is delivered to the load constantly.
Thus, a direct current of a given voltage will do more work per unit of time
than an alternating current of the same peak voltage - exactly 1.41 times as much.
But don't jump to the conclusion that the power company is making a 30% profit by
selling a.c. instead of d.c.
Alternating current is rated in terms of its effective value* - the amount of
work it will do in comparison to a unit of direct current. Therefore, one volt (effective
value) of sinusoidal alternating current actually has a peak value of 1.41 volts.
The effective value and the peak value of d.c. are always equal.
Standard a.c. meters are calibrated to measure the effective values of alternating
currents and voltages, rather than their peak values. Cathode-ray oscilloscopes
show the peak values as well as the actual shape of the a.c. wave. Peak-reading
vacuum-tube voltmeters also indicate the peak values.
Advantages of Each. The big advantage of alternating current over direct current
for utility power is the ease with which it can be sent long distances from the
generating plant to the consumer. It can be generated and distributed to strategically
located substations at very high voltages - approaching a million volts in some
installations. This means that every ampere of current represents thousands of kilowatts
of power. As it is the amount of current to be carried that determines the size
of wire that must be used in a transmission line, this high-voltage distribution
permits carrying a maximum amount of power on a given size of conductor.
* Also called the root-mean-square (r.m.s.) value after the mathematical method
of computing the effective value.
When Al, KN9IDZ, finishes an operating session, he folds up the
desk leaf and closes the door of his built-in wall cabinet, concealing his equipment.
At substations, the extremely high voltages are stepped down in huge transformers
to a few thousand volts and transmitted to neighborhood "pole" transformers, where
they are reduced to a safer 117 or 235 volts before the power is delivered to the
individual customers. Such voltage division is possible because passing a.c. through
a heavy-duty type transformer very slightly affects the amount of power available.
It simply changes the ratio between the current and the voltage. Thus, when the
voltage is stepped down, the current available is increased, and vice versa.
In contrast to a.c., once d.c. is generated at a given voltage, it is impossible
to change that voltage with something as simple as a transformer. You can reduce
both d.c. and a.c. by passing the current through a resistance, but this just wastes
the unused power. To raise or lower d.c., you can use the current to drive another
motor-generator (or dynamotor) to generate the desired new voltage. You can also
convert it to a.c.-step it up or down to the desired value-and then convert it back
to d.c.
Both systems are used to power mobile radio equipment from an automobile storage
battery. In vibrator-type power supplies, for example, the vibrator, which is actually
a vibrating switch, reverses the connections between the battery and the primary
winding of the power transformer 100 to 200 times a second, thereby converting the
d.c. from the battery to a.c. in the transformer, although not the sine wave a.c.
shown in Fig. 1. This a.c. is then stepped up to the desired voltage in the
transformer and reconverted to d.c. to power the radio.
The higher power requirements of mobile transmitters are often taken care of
by dynamotors driven by the battery and delivering 400 to 1000 volts, d.c.
Radio Frequencies. All radio signals are alternating currents which differ from
60-cycle power in frequency and in the fact that they are generated electronically
in vacuum-tube oscillators, built up by r.f. power amplifiers, and maybe modulated.
Amateur transmitters emit signals of frequencies between 1,800,000 cps and 148,000,000
cps and higher. It is the rapidity with which they oscillate back and forth that
makes them capable of being radiated into space from an antenna and of travel-ing
great distances before being intercepted by a remote receiving antenna. (Just for
the record, signals with frequencies as low as 10,000 cycles can be radiated, but
it takes antennas several miles long to do the job.) Also, tremendous amounts of
power are required to span even moderate distances at such low frequencies, while
low-power high-frequency transmitters are capable of sending a signal around the
world under favorable conditions.
Jim, KN1DCK, Fort Devens, Mass., with his Hallicrafters S85 receiver
and Heath DX-35 transmitter.
Frequency and Wavelength. Electrical waves travel through space with the speed
of light-186,000 miles or 300,000,000 meters per second. Consequently, the distance
that a radio wave will travel in the time it takes for it to go through a complete
cycle is equal to the distance traveled divided by the number of cycles, or: wavelength
in meters = 300,000,000/frequency in cycles. As radio frequencies are usually given
in kilocycles (thousands of cycles) or mega-cycles (millions of cycles), the formula
is often written: wavelengthmeters = 300,000/ freq.kc.; or
wavelengthmeters = 300/freq.mc. Conversely, freq.cycles
= 300,000,000/wavelengthmeters.
Substituting a few figures in the formulas reveals that a 3750-kc. signal has
a wavelength of 80 meters, a 7000-kc. signal have a wavelength of 42.86 meters,
and a 15-meter wave has a frequency of 20,000 kc.
This relationship between the frequency and the wavelength of a radio signal
is of great practical importance in designing transmitting antennas, which must
be of a certain length* for best results for a given frequency. Also, it is important
that any-one planning to take an amateur examination be able to determine frequency
when wavelength is given, and vice versa, because all classes of amateur examinations
contain questions requiring knowledge of this kind.
* Usually an integral multiple of an electrical half-wave-length.
Posted July 5, 2022
(updated from original post on
8/2/2011)
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