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.

directional verticals

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.