Your RF Cafe
November 1959 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 quick course on how to point antennas for over-the-horizon (DX) reception, and, if you also happen to have a license to transmit, for broadcasting. It covers how to determine the shortest straight-line path by stretching a string around a globe (remember those spherical maps that used to be a mainstay of every household and schoolroom?) and using a protractor (a what?) to get the angle. Author Edward Noll uses a simple 1/2-wave dipole antenna radiation pattern as an example of how directivity is affected by frequency (relative to the fundamental).
How to plan your antenna installation to pull in those hard-to-get foreign stations
By Edward M. Noll
A Short-wave receiving set brings the whole world right into your home. Listening to broadcasts from distant lands affords not only a challenge and technical thrill but gives you a general education as well. Despite the propaganda and slanted news you sometimes hear, you develop a better understanding of foreign countries, their people, and their problems. Short-wave radio offers good music, plays, and other enjoyable programs, and it enables you to study foreign languages first hand.
You can enjoy more short-wave programs if you plan your antenna installation carefully. One of the keys to consistent long-range reception is a directional antenna. Its orientation is often the prime factor in receiving hard-to-get stations or in improving the general performance of your listening post. Even a very simple antenna can provide an improvement in signal strength and reliability if it is "aimed" in the right direction. Also, a highly directional antenna can eliminate pickup from unwanted stations.
The Shortest Path. A short-wave signal takes the shortest path around the earth. But here it's easy to become confused. For example, a flat map would seem to indicate that the shortest route from, say, Chicago to Moscow would be eastward across the Atlantic and then across Europe. But reference to a globe would quickly show the shortest path to be over the North Pole.
Consequently, you don't put up a receiving antenna with maximum sensitivity toward the east when you want to pick up Europe and Asia. You turn the antenna toward the shortest route as the crow flies - north, over the top of the earth.
Choosing an Antenna. You may have noticed that most signals picked up from one direction are on one band while signals from another direction are on a different band. The explanation for this is simple. The directional sensitivity of any fixed-length antenna changes with frequency.
For example, a half-wavelength antenna at 6 - 7 megacycles has a figure-eight sensitivity pattern. However, the very same antenna has a nearly full wavelength dimension in the 11 - 15 megacycle spectrum and has a "four-leaf clover" sensitivity pattern.
On the higher 17 - 21 mc. bands, the same antenna becomes an approximate 3/2-wavelength type and the sensitivity pattern takes on a clover-leaf shape with a narrow "8" at its center.
Typical sensitivity patterns for half-wavelength, full-wavelength, and 3/2-wavelength dimensions are illustrated. Using these patterns, you can determine if a loop or a null faces the direction from which you wish to receive a special station.
It is important to remember that once the electrical length of an antenna exceeds 3/2-wavelengths it acquires many narrow sensitivity lobes. These multiple lobes make for spotty directional performance. In one direction there may be good sensitivity while just 10° to 20° on either side the response may drop off to almost nothing. In this case, one never knows if a loop or a null faces the desired direction.
Antenna Construction. Cut your antenna to meet your particular requirements. Some short-wave listeners prefer to cut the antenna to the lowest frequency band in which they are interested. Thus they have a figure-eight pattern on this band and a severely lobed pattern does not develop on the highest frequency band.
Some typical half-wave (λ/2) antenna lengths for use on the short-wave bands are: 62.4 feet at 7.5 mc., 52 feet at 9 mc., 42.5
feet at 11 mc., 31.2 feet at 15 mc., 27.5 feet at 17 mc., and 22.3 feet at 21 mc. Note that one half of the length given is located on each
half of the center feed point of a dipole antenna. Use 72-ohm coax for lead-in.
If mounting space is available, you may want to consider installing two antennas. The low-frequency version can be erected and oriented for good general coverage and perhaps to emphasize your favorite low-frequency s.w. stations. A shorter antenna can be used for the high-frequency bands.
Orienting Your Antenna. Lining up your antenna to get the signals you want is a simple matter if you use a world globe and a piece of string. Tape the string between your location and the city, country, or continent you wish to hear. Note the bearing from your location to the point you have selected in relation to true north. A protractor will be handy in reading the bearing in degrees.
Next, locate true north at your antenna mounting site. Use an accurate road map, courthouse or city-hall map to do this. Or true north can be found by shooting the North Star at night or the shadow of the high noon sun. A magnetic compass could also be employed; but remember that a compass points to magnetic north rather than true north.
Finally, erect your antenna for maximum ( sensitivity toward the stations you want to pick up.
Posted June 10, 2013