November 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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. Radio signals unless refracted or
reflected take a
great circle route from point A to point B. Long distance airline flights
take the same routes, which explains why flights from London to Los Angeles fly
over Greenland enroute. 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).
Zone website has a list of resources for determining the proper direction
for pointing your antenna to hit the intended geographical area.
How to plan your antenna installation to pull in those hard-to-get
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
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.
To orient your antenna, tape a piece of string between your location
and the area you want to pick up. Use a protractor to get the bearing in degrees
from true north or magnetic north.
Effective maximum pickup angle for a half-wave antenna is about
75° in each sensitivity lobe. Pattern in dashed area varies with height of antenna
above ground and other factors.
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
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
On the higher 17 - 21 mc. bands, the same antenna becomes an approximate 3/2-wave-length
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-wave-length
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
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
Finally, erect your antenna for maximum ( sensitivity toward the stations you
want to pick up.
The sensitivity pattern for the same half-wave antenna (top)
changes when it is used to receive signals on twice the frequency (center) and three
times the frequency (bottom).
Posted July 8, 2020(original