A space saving antenna in the form of an upright
L has been around the amateur radio world for a long time. References are found back to a QST article
in the 60’s (Reference
1), the ARRL Antenna Handbook in the 70’s through 90’s
and to L. B. Cebik’s (References 3,4) and Craig LaBarge’s (Reference 5) writings in recent years. Commercial
designs are seen in the Buddipole TM
6), MFJ-1790 (Reference
7) and the CrankIR antennas
By the use of antenna modeling software (Reference 10), this article is to expand our general
knowledge relative to angle-of-bend and off-center-feed (OCF)
configurations of the L-Antenna.
The L-antenna can be looked at as a center-fed dipole
bent in the middle or… a vertical monopole with a single radial.
At an elevation one-half wavelength a straight vertical dipole
has about 1.6 dBi gain (decibel isotropic). The L form however has gain
around 4 dBi in the direction where
the radial is pointing. Depending
on ground conditions and mounting height, the impedance is around
40 ohms making the configuration a decent match for 50 ohm coaxial
Since the impedance of a vertical
dipole is around 70 ohms, there obviously is a 50 ohm point somewhere
between 0° down and
a bend of 90° horizontal… usually near 75° (or 15° below horizontal). Gain
at this point is around 3 dBi.
Interestingly, as the arm/radial
continues to be raised above
horizontal, the impedance declines to around 30 ohms, the
Standing Wave Ratio (SWR) approaches 2:1 but the gain of the vertical
monopole increases up to 5 dBi…. several times the gain of a vertical dipole.
In practice and in antenna
modeling studies it is observed that the impedance of a horizontal
dipole rises as the feed point moves away from center. Resonant frequency and gain remain the same.
Just impedance changes. This technique is used when tuning low-impedance
antennas like a J-Pole or when using a Gamma Match or Delta Match
on multi-element beams. Tall ground plane antennas with short
radials are also off-center designs.
Figure 2 following is a generalized impedance graph of what happens when feeding
Question: If an antenna
has low impedance fed at the mid-point, where is 50 ohms? For
example, a half-wave at 10 meters is 5 meters long. From the graph
the usual OCF ratio of 0.67 (2/3) looks like a good guess for
a starting point. That means we start with one arm at 0.67 x
5 meters = 3.35 meters long. The
other arm is 5 meters minus 3.35 = 1.65 meters long.