ANTENNAS FROM THE GROUND UP
The basic 1/2 wavelength dipole, shown for scale at the left of Fig. 1, has been the subject of many past episodes, and we have looked at it in both straight and zig-zag forms. We have also devoted columns to the inverted-V and the inverted-L, which are the most common bent versions of the dipole. Perhaps it is time to look both backward and forward to examine the entire family of dipoles bent at a right angle. We shall look at 40-meter versions, since we are assuming a backyard with limited space.
In Fig. 1, we have almost all of the family members, both common and rare. The inverted-V is perhaps the most-used version of a dipole with a right-angle bend, since it suits the backyard with only one tall support. The upright-V has similar properties to its inverted brother, but needs two supports. However, those supports can be closer together than the ones needed for a straight dipole. At 40 meters, if a dipole needs 70' between supports, the upright-V needs only 50'. If we set all of the antennas with a maximum top height of 50', the two V antennas will have their lowest points at just under 25' above ground, a safe distance over the heads of family members and visitors. Since all dipoles have high voltages at their ends, safety must be a significant concern.
The upright-L is rarely used as a low-band bent dipole. However, it is useful as a utility antenna at 10 meters and up. Made from tubing, rod, wire, or a combination and set above a roof top, it can serve to capture the signals from both horizontal and vertical antennas, covering point-to-point and repeater communications.
More common is the inverted-L. Many operators feed this antenna at its base, close to ground. However, fed at the upper corner, the antenna can be set a safe height above ground, about 16' at its lowest point, and still meet the 50' top height limit of this exercise. The supports can be about 35-40 feet apart.
Less commonly used is the quadrant antenna, shown in a top view. It is an antenna for the yard with more supports than space. Both legs are at 50', but form a V, with bidirectional radiation. A space about 25' by 50' will hold the quadrant. What can we expect from the various members of the right-angle family of dipoles? To answer that question, the following pages present azimuth and elevation patterns for each antenna, cut to be nearly resonant at 7.15 Mhz. All of the models for the antennas use AWG #12 copper wire. The top-most height for the wires is 50'. Hence, we can expect the upright-L to have poorer performance than the other antennas, since half the antenna is at a low height. When raised (in a 6-meter or 10-meter version) to 20' up or so, the performance improves dramatically.
The following table give you the length of each of the two legs, plus the approximate feedpoint impedances.
The Right-Angle Family
Ant. Leg-Length Feedpoint Z
(Feet) (Ohms)
Dipole 33.55 88
Inv.-V 34 55
Upr.-V 34 46
Inv.-L 34.2 52
Upr.-L 34.2 42
Quadr. 34.2 51
Feedpoint impedance will vary somewhat with height, although the Vs and Ls will be less sensitive to height changes than the straight dipole, especially at top heights from .25 wavelength to 1.0 wavelength. For the Vs and Ls, the closer to ground, the lower the feedpoint impedance.
Of course, since every yard has its collection of objects that affect antenna operation, be prepared to prune the lengths slightly to bring them to a satisfactorily low SWR, if you feed them with 50-Ohm coax. For the antennas that place one end beyond reach, you may prune only the end in reach without disturbing the performance noticeably.
The azimuth and elevation patterns that follow aim to give you a set of reasonable expectations about the relative performance of the antennas. You may adjust your personal expectations by taking into account the likely affects of your trees, metal cables, buildings, tin roofs, and the like. Also account for any height changes from the 50' top height used here. A lower top height will generally reduce gain, raise the elevation angle of maximum radiation (TO angle), and further circularize the oval patterns. At 1/4 wavelength and below, all of the patterns are nearly circular.
The dipole has the highest broadside gain and lowest gain off the ends. The upright-V has good gain, but at a higher TO angle, while the quadrant has pretty good gain at the same TO angle as the dipole. The center-fed inverted-L has somewhat less gain, but at an overall lower TO angle, due to the radiation from the vertical leg of the antenna.
The quadrant antenna is less well-known in the U.S. than elsewhere in the world, but unfortunately, some claims for it just do not hold up. Some folks have claimed that it has a circular pattern, but in fact, at the 50' height (about 3/8 wavelength at 40 meters), the pattern is an oval with the strongest directions along the line that bisects the V of the layout. Nonetheless, it is a highly usable pattern for general communications.
The dipole takes the most space between supports. The invert-L takes the least, although the vertical leg should be well spaced from its support. The quadrant takes an intermediate amount of space and may still fit a small backyard with more supports than area. A further advantage of the quadrant is that you can bring the feedline down vertically from the antenna and minimize coupled common mode currents. For some installations, this feature may be very useful.
The Straight Dipole
Note not only the gain and the elevation angle of maximum radiation, but also the 6.6 dB reduction in signal strength off the ends of the wires. The antenna is oriented as if it were a vertical line in the middle of the azimuth pattern. The elevation pattern results if we look straight into the end of the wire and see how the pattern emerges broadside to the wire.
The Inverted-V
The azimuth pattern results from placing the wire so that the vertical peak is centered in the diagram and the two drooping leg make a vertical line on the page. To capture the elevation pattern, picture yourself as looking at the edge of the drooping wire pair so that the pattern shows what emerges broadside from the antenna. Note the reduced gain relative to the dipole and the greater radiation in the direction of the wire ends, giving a rounder azimuth pattern overall.
The Upright-V
Although this version of the V seems to have a higher gain than its inverted brother, note that the maximum radiation is at a considerably higher angle--almost too high for most skip path communications. Since the elevation pattern is still very broad, communications will still be good, but at a lesser strength than for the inverted-V for most skip paths. However, this antenna is a good candidate for NVIS use.
The Upright-L
The patterns for this antenna are mostly for reference to show what happens when the horizontal wire is too low for effective radiation. The pattern is something like that of a distorted 1/4 wavelength vertical. However, this antenna shows very much improved performance when the height of the bottom wire is raised to 1/2 wavelength or more above ground. Thus, it tends to work best on 10 meters and above, where it would make a small roof-top antennas or even an antenna to place outside the window of a high-rise apartment building.
The Inverted-L
With its horizontal wire at the top, the center-fed inverted-L shows a nice combination of vertical and horizontal radiation. The overall gain is not as high as some other bent dipoles, but the elevation angle of maximum radiation is the lowest of the group. Picture the vertical wire at the center of the azimuth pattern, with the open end of the horizontal wire pointing downward on the page. Hence, the kidney-bean shape of the azimuth pattern. Positioning the feedline to minimize coupling to the antenna legs can be a challenge.
The Quadrant V
Picture the quadrant in the azimuth pattern as forming a V with the open end to the right and the closed end to the left. Note that there is a very tiny difference in radiation, favoring the open end. However, the difference is so far below the level of being detectable as to make no difference in performance: the azimuth pattern is bi-directional. It has the same elevation angle of maximum radiation as the dipole, but the pattern is slightly rounder and the gain is slightly less. However, if this antenna fits your backyard, use it.
Updated 11-06-2003. © 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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