Chimney mounts are used more frequently than other types of mounts, but they often are not the best option.
Although they are relatively easy to install, the smoke and gases from a chimney can shorten the life of the antenna and significantly impair its performance. A chimney installation is practical only if the chimney is sturdy and vertical. Never mount an antenna on a deteriorated chimney. During moderate to high winds, an unguyed mast taller than 10 feet can exert enough leverage to break off an unstable chimney. If you choose a chimney mount, use enough mast to place the antenna above most of the smoke and gases. However, to avoid over-stressing the chimney, do not mount the antenna more than 10 feet above the top of the chimney. If the height of the antenna must exceed 10 feet to receive satisfactory signals, the mast must be properly guyed.
Securing the chimney-mounted antenna and minimizing the stress on the chimney requires the mounting straps to be properly spaced. The top strap should be placed as high up on the chimney as possible. If the chimney has a crown or projecting cap, place the top strap directly under it. The bottom strap should be placed 4 feet below the top strap. If the chimney isn’t long enough to permit this, place the bottom strap as far down on the chimney as possible. If the mast must be 10 feet above the chimney top, don’t use a chimney mount unless you can space the straps at least 30 inches apart. For masts less than 10 feet above the chimney top, the straps should be spaced no less than 24 inches apart.
Assemble one end of each strap to one eyebolt using a strap clamp. Attach the eyebolt / strap assemblies to each bracket. Insert an eyebolt into the other end of each bracket. The nuts on each eyebolt should only be threaded about ½” onto the eyebolts. Run the straps around the chimney. Be sure the straps are level, with no kinks or twists. The easiest way to level straps is to line them up along the nearest course of bricks. Straps should be centered on the bricks – not over the mortar joint. Feed the free end of each strap through an eyebolt on the other end of each bracket. Pull each strap tight, line it up so that it is level, and then tighten it just enough to hold it in place. Before the straps are tightened completely, fasten the mast to the mounting bracket. (It is assumed that the antenna has already been clamped securely to the mast, and one end of the transmission line has been connected to the antenna terminals.) Align the mast so that it is vertical. Then completely tighten the mounting straps. Next, orient the antenna. Finally, tighten the clamps that hold the mast to the mounting. Be sure the clamps are tight enough to prevent the mast from being rotated by the wind load on the antenna.