Friday, July 23, 2010

I’d like to set up a 30-meter antenna in my back yard ...

Q I’d like to set up a 30-meter antenna in my back yard, but I’m really tight on space. I’ve been told that I can use a technique known as ‘linear loading’ to reduce the size of a dipole. Can you enlighten me?

A What you’ve heard is true—linear loading can significantly reduce the required lengths of resonant antennas. For example, it is easy to make a resonant antenna that is 30 to 40% shorter than an ordinary dipole for a given band. The shorter length comes from bending back some of the antenna wire. The increased self-coupling lowers the resonant frequency.

NN0F constructed a linear-loaded dipole using 25-feet of common 450-Ω ladder line and capacitive end hats (see Figure 1). The end hats are simply 6-foot lengths of stiff wire. Both conductors of the ladder line at each end are soldered to the hat wires. At the middle of the antenna (12 feet 6 inches from the ends) you cut through one of the ladder line conductors and attach your 50-Ω coaxial feed line. Cut through the other conductor as well, but leave it open. This antenna should provide a good match (no tuner required) and it fits easily within most back yards.

Figure 1—A two-wire linear-loaded antenna for 30 meters using 450-Ω ladder line.

From QST July 2000