ANTENNAS FROM THE GROUND UP
An often-neglected multi-band wire antenna is the 1/2þ inverted-L. The antenna consists of a half wavelength of wire at the lowest frequency of interest and can be fed either at the upper corner with parallel feedline or at the base of the vertical arm, usually via an L-network to a coaxial feedline to the transceiver. Fig. 1 shows the options.
An 80-meter 1/2wl inverted-L requires about 65.5' horizontally and the same amount of vertical space. For a full size antenna, the height needs to be slightly more to ensure that the bottom of the vertical arm--a high-voltage point--is out of reach from anyone. If the fundamental frequency is on 40 meters, the antenna will be half the size.
For odd installation situations, the antenna can be distorted in either the vertical or horizontal dimension. Adding more horizontal and less vertical length tends to make the horizontal arm more dominant, with a reduction in lower-frequency, lower-angle radiation. Lengthening the vertical arm and shortening the horizontal arm does the opposite, with slight reductions in gain on the upper HF bands.
Top-corner feed usually requires parallel transmission line to a wide-range balanced antenna tuner. Often, however, hams feed the antenna through an L-network placed right at the base of the vertical arm and well-grounded for RF. Although a good RF ground is essential, a ground plane immediately beneath the antenna is unnecessary and in fact does little if any good.
Installation variations, if moderate, also create few problems for operation of the antenna on all bands. The horizontal arm can be modestly sloped downward or bent downward at its outer end. Likewise, a center-fed version of the antenna might have the lowest part of the vertical arm bent to one side to maintain a safe height above peopleswl heads.
The following pages present azimuth and elevation plots of a 1/2wl inverted-L cut for the middle of 80 meters and used on all the HF bands. The patterns and data are representative and will vary with the specifics of your situation. However, notice the absence of very high- angle radiation on any band. Moreover, even though the gain is almost always less than that of a 135' doublet, the patterns are smoother, with fewer and shallower nulls. That feature alone makes the antenna a good general operating aerial.
Center-Fed
Gain: 1.98 dBi
T-O Angle: 44°
Feed Z: 65 + j4 Ohms
Base-Fed
Gain: 2.03 dBi T-O Angle: 46°
Feed Z: 4800 - 1060 Ohms
Center-Fed
Gain: 4.08 dBi
T-O Angle: 26°
Feed Z: 6500 + j710 Ohms
Base-Fed
Gain: 5.12 dBi
T-O Angle: 28°
Feed Z: 920 + 825 Ohms
Center-Fed
Gain: 4.05 dBi
T-O Angle: 20°
Feed Z: 150 - j495 Ohms
Base-Fed
Gain: 4.36 dBi
T-O Angle: 24°
Feed Z: 190 + j280 Ohms
Center-Fed
Gain: 5.38 dBi
T-O Angle: 14°
Feed Z: 1850 - j2330 Ohms
Base-Fed
Gain: 5.28dBi
T-O Angle: 15°
Feed Z: 360 + j325 Ohms
Center-Fed
Gain: 7.02 dBi
T-O Angle: 36°
Feed Z: 170 - j255 Ohms
Base-Fed
Gain: 7.16 dBi
T-O Angle: 41°
Feed Z: 475 + j440 Ohms
Center-Fed
Gain: 6.74 dBi
T-O Angle: 9°
Feed Z: 700 + j1375 Ohms
Base-Fed
Gain: 6.22 dBi
T-O Angle: 10°
Feed Z: 230 + j165 Ohms
Center-Fed
Gain: 6.62 dBi
T-O Angle: 8°
Feed Z: 215 - j525 Ohms
Base-Fed
Gain: 6.64 dBi
T-O Angle: 8°
Feed Z: 200 + j175 Ohms
Center-Fed
Gain: 7.54 dBi
T-O Angle: 7°
Feed Z: 535 + j1055 Ohms
Base-Fed
Gain: 7.63 dBi
T-O Angle: 7°
Feed Z: 195 + j90 Ohms
Updated 07-18-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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