November 1959 Popular Electronics
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.
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).
Your Antenna - Key to World-Wide DX
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.
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
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
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
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.
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.
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
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.
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.
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).