The AM Forum

The Inverted-L has come up several times recently in on-line and on-the-air conversations. I recommended making the L 5/16 - 3/8 wavelength (WL) long instead of 1/4 WL. The reason is that the longer L will raise the resistive component of the feed point impedance to around 50 Ohms. A inductively reactive component will be present but is easily tuned out with a capacitor. Modeling results are below for 160 meters, but the concept is valid for any band. Both versions of the L are modeled at 1.9 Mhz over ground with average conductivity (Rel. dielectric constant 13.000, conductivity: 0.00500 mhos/meter) and four radials, 131.5 feet long, elevated 1 foot.

A Quarter Wavelength L


65.79' feet vertical, 65.79 feet horizontal
FeedpointZ: 13.658 - j 0.287    VSWR(Zo=50 Ω): 3.7:1


A Three-Eighths Wavelength L


65.79' feet vertical, 127.5 feet horizontal
Feedpoint Z: 50.02 + j 587.77    VSWR(Zo=50 Ω): 140.1:1

As you can see, since the antenna is longer than 1/4, it becomes inductive. If we "tune out" the inductance with a cap with the exact reactance, we have a nice 50 Ohm load and the SWR is 1:1. The cap value would be about 143 pF.

Feedpoint Z: 50.02 - j 0.003VSWR(Zo=50 Ω): 1.0:1

The above numbers are illustrative. If you wanted to build such an L, these would be starting points. Local ground conductivity, number of radials and whether they are on the ground or raised, and the exact vertical and horizontal lengths will all play in final impedance. In general, if the vertical portion is shorter, the horizontal portion will need to be longer to get the impedance up to 50 Ohms.