Summertime is construction time. If you maintain an AM and the signal seems a little weak, it might be appropriate to inspect your ground system. Corroded, brittle and broken ground radials, such as those pictured in Fig. 1, may need repair or replacing.
Before retiring, Merrill Pittman was chief engineer for Annapolis Broadcasting’s WANN in Maryland. “Pitt” managed the ground-replacement project for this two-tower directional and shares pictures of the work.
Although the ground radials were in good shape, the ground screen had torn in sections, and radials needed to be re-connected to the base of the tower. This repair was not due to abuse; the site has always been well maintained. Like everything else, things do wear out, even buried ground screen. So often the ground system is simply forgotten because it’s buried.
The ground screen at the base of the tower may be torn, or perhaps it has disintegrated. A wholesale replacement of the ground system is costly and time-consuming. Whether you’re replacing the entire system or just replacing what’s under the tower, the project begins with the removal of what’s left of the old system.
In some cases, such as WANN’s, the radials are fine, but their connection at the base of the tower has been compromised.
Replacement of the base screening is straightforward. It begins by identifying the radials that connect to the screen or copper strap/ring. A good way to identify these before removing them (for reconnection to the new screen) is by flagging each radial wire with a piece of white electrical tape.
Once the radials are identified, the old screen can be removed. This process, seen in Fig. 2, means turning the station off, or in the case of multiple tower arrays, switching to another tower. You don’t want all that grounded metal around the base of a “hot” tower.
After removing the old screen, rake the base perimeter clear, add a layer of landscape fabric to prevent weed growth, overlapping the edges, and cover the fabric with a base of sand.
Usually, your new ground screen has to be pieced together to cover the entire tower perimeter as defined by the license. Using a piece of radial wire and wrapping the ends around the wire is a convenient way to accomplish this.
Fig. 3 demonstrates this method. Note the use of heavy leather gloves. The edges of the copper screen are sharp and they will cut your hand easily. In the figure, the ungloved hand holds the screen in place, while the gloved hand wraps the diamond-shaped end around the radial wire.
The completed bonding of two pieces of ground screen is shown in Fig. 4. Now it’s time to silver solder. Silver solder requires a hot flame. Use MAPP gas or a torch. Because the metals are new and not corroded, “finishing” them with 3M Scotchbrite, Crocus Cloth or similar product usually is not necessary. Apply silver solder flux liberally. Place a sheet of metal under the edge to be soldered. This will prevent the small pebbles of sand from exploding due to the intense heat of the torch.
Wear safety goggles, of course, and pay attention to your work. The copper screen will melt if it’s exposed to too much heat. Once you get the hang of using silver solder, it will flow nicely, thanks to the flux, and you’ll get a good, solid bond.
To keep the sand base intact, it’s not a bad idea to use pressure-treated 4×4 landscape ties around the perimeter of the tower. The landscape ties also provide a good base for the copper strap that encircles the base. The ground screen can be silver soldered to the strap, as shown in Fig. 5.
Each radial then can be folded over the strap, as you can see in Fig. 6, and silver-soldered to the tower perimeter strap. Note in the picture the piece of metal discussed earlier to protect the strap and radial wire while soldering and keep the sand and dirt from “spitting.” The metal used was an old “Danger High Voltage” sign that was replaced with new warning signs and tower registration signs ordered from The Antenna Site Store, www.antennaID.com.
Fig. 7 shows the completed, soldered radial. By the way, copper nails were used to anchor the copper strap to the landscape tie. This prevents dissimilar metal interaction.
The project is completed by covering the completed screen with a layer of small pea gravel or round stones. Crushed stone should be avoided, as their sharp edges can cut the new ground screen, undoing your hard work.
The copper strap that passes under the base insulator and down the concrete pier can be tied into the ground screen with another perimeter strap. Bonding this portion of the screen and the strap is identical to the method described above for the outside perimeter strap.
Important: Before repairing a ground system, check the specifications on the station license. Changing a ground system from the specifications of the license requires filing an FCC Form 301, as “improvement” of the ground system may affect the overall efficiency.
Replacement of an existing ground system with the same components should not require this filing.
Also measure the antenna impedance before and after the work is done. If the measurement shows a change greater than 2 percent in the antenna resistance component, or if the resistance varies more than 2 percent from the value on the station license, a Form 302 (modification of license) will need to be filed. Technical details can be obtained from your professional engineering consultant and are recommended before embarking on this kind of repair project.