Chapter 3: The Inverted L

The inverted L is a fairly simple wire antenna, shaped like an upside-down
letter L. It is held up by insulators attached to ropes which are tied to
whatever supports are available: chimneys, trees, what have you. Performance
is best when the vertical section is made as tall as possible.

The inverted L is a fairly simple wire antenna, shaped like an upside-down
letter L. It is held up by insulators attached to ropes which are tied to
whatever supports are available: chimneys, trees, what have you. Performance
is best when the vertical section is made as tall as possible.

The inverted L is basically omni-directional. It is good for reaching
both local (ground-wave) and distant (sky-wave) audiences; it radiates some
of its energy with horizontal polarity and some vertically polarized.

ground

The ground side of the antenna consists of a set of ground radials
or a couple of good ground rods driven into moist earth.

tuning

The impedance of an inverted L is not too far away from 50 ohms if
the overall length is about 5/16 of a wavelength, and in that case it
can be fed directly with 50-ohm coaxial cable without the need for
any elaborate impedance matching transformers. To estimate the size
of a 5/16-wavelength wire antenna, use this equation:

length (in meters) = 299,800 / f * 0.3125 * 0.99

    f = frequency in kilohertz

    example: for 1700 kHz, 54.56 meters

If you are using more than a couple of watts of power, you’ll
want to get an SWR meter and insert a variable capacitor at the
feedpoint to tune the antenna for minimum SWR. The capacitor can
be of the air dielectric type for powers up to about 100 watts. Be
sure to mount the variable cap in a dry, dust-proof container. The
maxiumum capacitance may need to be as low as 200 pF or as high
as 800 pF. Ham radio operators on the 160 meter band often use
a 150 pF or 365 pF air variable capacitor, shunted with a few 100 pF
mica transmitting caps to bring the assembly up to the needed capacitance.