Wednesday, July 7, 2010

“shunt-fed tower.” What is that?

Q I worked a fellow on 160 meters recently and he said that he was using a “shunt-fed tower.” What is that?

A Anything can be made to radiate RF if you feed it properly including towers. The shunt-feeding system shown in Figure 4 is used by K6SE to get his 70-foot crank-up tower to radiate on 75 and 160 meters. The beam at the top of the tower isn’t a problem. In fact, it provides some beneficial top loading.

In Figure 4 you’ll notice two wires running up each side of the tower and connected to variable capacitors. This is the actual shunt feeding/matching system. You tune the system for resonance by first adjusting the capacitors for the lowest SWR on the desired band, then increasing or decreasing the spacing between the wire and the tower to reduce the SWR further. Shunt feeding a tower in this fashion requires a very good ground system for optimum performance, but the results can be impressive on the low bands for DX work. You’ll find more shunt-feeding methods described in ON4UN’s Low-Band DXing, available from the ARRL.

Figure 4—The details of the shunt feeding/matching system designed by K6SE and described in The ARRL Antenna Book. The 1.8 MHz feed (left side) connects to the top of the tower through a horizontal arm of 1-inch diameter aluminum tubing. The other arms have standoff insulators at their outer ends. The 80-meter side (right) is similar, connecting to the tower at a point 28 feet above the ground.

From QST December 1999