ANTENTOP- 01- 2017, # 021

The Three L-Antennas

The Three L-Antennas

Wide- Equal - Tall

Dick Reid, KK4OBI

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 (Reference 2) 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 (Reference 6), MFJ-1790 (Reference 7) and the CrankIR antennas (Reference 8). 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 cable.

Since the impedance of a vertical dipole is around 70 ohms, there obviously is a 50 ohm point somewhere between 0 degree down and a bend of 90 degree horizontal... usually near 75 degree (or 15 degree 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.

Off-Center-Feed (OCF)

 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 off-center.

 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.

The L-Antenna

So simple. So easy to tune. Just hang up a wire or put up a pole and run another adjustable length wire or pole off to the side. Connect your coax at the bend. It really does not matter how you connect the coax. It really does not matter how long is the vertical arm. Simply tune by adjusting the length of the side arm. The total length of the two arms will end up around a half wavelength.

Figure 1

Angle Vs Impedance for L- Antenna

www.antentop.org

Page-12