Your RF Cafe
December 1957 Popular Electronics[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.
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 course.
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.
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.
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.
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.
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.
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