Most licensed medium wave stations use an antenna consisting of a tall vertical radiator (a tower) standing on an insulator. Buried just beneath the surface of the earth, 120 evenly-spaced wires radiate out from the base of the tower like spokes in a wheel. These ground radials serve as a counterpoise for the vertical radiator. For lack of a better term, we’ll call this kind of antenna the Classic Vertical.
The chart above shows the relationship of vertical size to performance with
the Classic Vertical. The chart plots the effective field strength (in
millivolts per meter) at a distance of one kilometer, using 1000 watts
of transmitter power, with a simple vertical antenna having a ground
system of 120 radials, each 1/4 wavelength long. This data was gathered in
carefully controlled experiments during the pioneer days of broadcasting.
Note that the effectiveness falls off sharply for antennas less than
0.1 wavelength tall. Peak performance occurs at a height of 0.625
wavelength. (0.625 = 5/8)
Medium wave signals travel by two modes: groundwave and skywave.
The groundwave hugs the earth, and the skywave radiates up toward
the ionosphere and then bounces down to earth hundreds of miles
away (if conditions are favorable). Maximizing the ground-wave is
desirable for local broadcasting, and the Classic Vertical does this
fairly well. Although horizontal dipoles and other types of antennas
were used by AM broadcasters in the 1920s and 30s, almost all licensed
medium wave stations in North America are now using Classic Verticals
or closely related designs.
However, due to budget constraints, airplane flight paths, and
other factors, they don’t always use a 0.625 wavelength antenna. 0.25
wavelength is very common. Any combination of antenna efficiency
and transmitter power that will deliver the required field strength
to the target community may be used.
Most low-budget radio stations are not able to build a full-size Classic
Vertical antenna for medium wave frequencies, due to size constraints. A 0.625
wavelength antenna is about 110 meters tall for a frequency of 1700 kHz,
and even larger for lower frequencies. A quarter-wave ground radial at
1700 kHz is 44 meters long, so it would take more than 5 kilometers of
wire to build a ground system consisting of 120 radials, not to mention
an open field at least 88 meters wide.
A single vertical antenna is omni-directional with regard to the
horizon; it radiates equal amounts of power to the north, south, east and
west. If two or more verticals are connected to the same transmitter,
they can produce a directional signal pattern, i.e they can radiate
most of the power in one or two chosen directions, or have a nearly
omni-directional pattern with a bite taken out of it. The distance
between the towers and the phase relationship of the energy fed to them
will govern the shape of the signal pattern. That phase relationship
is controlled by inserting inductors and/or capacitors between
the transmitter and the antenna system.
Directional vertical arrays are too complex to discuss in this
brief article, but if you want to learn about them in order to build one,
start by reading The ARRL Antenna Book and then move on to
Smith’s Standard Broadcast Antenna Systems.
You can make a
single vertical antenna slightly directional by grouping the ground
radials together instead of distributing them evenly around the base
of the radiator. For example, if all of the radials are on the north
and east side of the antenna, it will radiate slightly more power
toward the south and west. However, this is not recommended for
local broadcasting, because grouping the radials together increases
the amount of energy radiated toward the sky and decreases the
intensity of the groundwave.