HF portable wire antennas Clear Springs Press




Portable Wire Antennas

Ground, Ground Plane and Counterpoise

         A ground is a valuable station asset. An electrical ground with appropriate lightning protection hardware is a necessary safety asset. An RF ground is required for some antenna configurations.

         An Electrical Ground and an RF Ground are Not the Same Thing

         An electrical ground provides a low resistance path to earth. The ground in your house wiring is an example. A low resistance path to earth at DC or low frequency AC may not be a low resistance path to earth at HF frequencies. The wiring between your equipment and the earth ground acts as a combination of antenna and transmission line components.

         To be a good RF ground, the ground rods have to be placed at the base of the antenna. Even the ground rods may not provide a good RF ground because the soil may have poor conductivity or be dry. To make a really good reliable ground, you will have to install a ground plane in addition to the ground rods.

         A ground plane consists of a group of wires either directly on the ground or buried in the ground and extending in all directions from the base of the antenna. The length of the wire radials is typically one quarter wavelength or more at the lowest operating frequency of the vertical antenna. The RF signal is coupled to the ground through capacitive coupling as well as direct electrical contact. The efficiency of the antenna increases as the number of radials increase. Many permanent installations will use two to three dozen radial wires distributed evenly around the base of the antenna. Adding a ground rod to an existing ground plane system will not improve its RF performance but is still necessary for safety and lightning protection.

A Ground is Not Portable

         Some antennas require a good ground and / or ground plane in order to function. Long metal rods and the tools that are required to pound them into the ground are too heavy to carry. So, where a ground is required, a counterpoise or ground plane wires are used as a substitute. This will give good results with some antenna types and potentially poor results with others.

         The antennas that will work well with a minimal ground plane are those with a high impedance feed point. At the high impedance point, antenna current is minimal and antenna voltage is high. An example is an end fed half wave dipole.

         Those that require a good ground plane are those that have a high current at the feed point. An examples is a quarter wave vertical fed at the base. With a poor ground or minimal ground plane, this antenna will perform poorly.

         A counterpoise is a single wire on the ground serving as a substitute for a ground or ground plane. A dipole with one leg on the ground, a vertical antenna with a quarter wave wire on the ground instead of a ground, or a random long wire with an equal length of wire lying on the ground instead of a ground are examples. Individuals who operate pedestrian mobile with a vertical whip antenna extended above the radio pack generally have a wire of appropriate length training on the ground behind them serving as the other half of the antenna. This is called a "trailing counterpoise".

         A counterpoise wire may or may not be electrically connected to the antenna circuit. A counterpoise wire may be placed on the ground to act as a ground in place of low conductivity earth. Laying a horizontal loop of wire on the ground below a horizontal loop antenna is an example. Putting reflector wires under a low hanging dipole is another example. These counterpoise wires on the ground do not serve as half of the antenna, but do enhance the antenna performance by increasing the apparent ground conductivity and shaping the antenna radiation pattern in an advantageous manner.

         Balanced antenna systems like any of the dipole configurations do not require a ground, ground plane or counterpoise. This is one reason that these antennas are favored for portable operation.

         Stations operating mobile typically use the vehicle body and chassis as the ground plane or counterpoise. At the lower HF frequencies, improved performance can sometimes be achieved by attaching a long wire counterpoise to the antenna base or vehicle chassis while the vehicle is stationary.

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