December 1957 Popular Electronics
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Here is a back-to-the-basics treatise on AC and DC, plus an
introduction to radio frequencies. The author, Herb. S. Brier,
is a licensed Ham who presents a very high level treatment of
the topics for rank beginners. Remember that Popular Electronics
was a magazine designed 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
we pretty successful, based on how long the magazine ran its
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
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.
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.
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
Value = 0.71 x Peak Value
Fig. 1. The difference between
the effective and peak values of an alternating current.
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 voltage. 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.
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.
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.
When AI, KN9IDZ, finishes an operating session,
he folds up the desk leaf and closes the door of his built-in
wall cabinet, concealing his equipment.
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.
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 traveling 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
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 megacycles (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
anyone 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-wavelength.
Posted August 2, 2011