ANTENNAS FROM THE GROUND UP
In the last episode, we examined a collection of right-angle dipoles. Among the collection was the so-called "quadrant" antenna, a flat-top V-shaped version of the dipole. Like all of the other antennas in the collection, inverted and upright Vs and Ls, the quadrant was resonant at 7.15 MHz. Fig. 1 shows the basic elements of the quadrant, using a top-down (or bottom-up) perspective. Since we shall be looking at some more azimuth patterns, the sketch indicates the heading that correspond to those on the patterns. 0 degrees will always indicate the direction away from the open face of the V-shaped antenna.
The basic antenna will be a 40-meter version constructed from AWG #12 copper wire, which is typical of backyard antenna materials. #10 through #14 wires would not perform differently in any significant way.
The chief application of the antenna is for the small yard with more supports than ground area. The antenna occupies an area of only 25 by 50 feet. Gently sloping the wires downward from the presumed 50' height for this exercise would not create large changes of performance. However, if that slope approaches a 45-degree downward angle, then the patterns might well look like compromises between the ones we shall see here and the ones in the inverted-V collection in episode 30.
Our aim in this installment is to see what kind of an antenna the quadrant might become in all-band use from 40 through 10 meters. There are two reference points for you to use in looking at the patterns.
1. Examine the patterns for the 44' doublet in episode 34 of this series. The 44' wire is essentially 1.25 wavelength at 10 meters and progressively shorter as we move downward in frequency toward 40 meters. At 7 MHz, the antenna is only about 3/8 wavelength long. Still, the antenna is a relatively good performer as wire antennas go. It produces bi-directional patterns on all frequencies from 7 through 28 MHz. The advantage of using this scheme is that you always know in what direction your signal is going.
2. Examine also the patterns in installment 3, which dealt with the 135' center-fed doublet. You will need to make an adjustment in your thinking if you want to apply these patterns as comparators to the ones in this exercise. What applies to an 80-meter doublet on 80 meters also applies to a 40-meter doublet on 40 meters. Each antenna is about 1/2 wavelength long and thus develops the same kind of pattern. Make some adjustment for the height of the wire above ground in terms of a wavelength. A 40 meter wire at 50' up is like an 80-meter wire 100' above ground. As the height of the antenna increases, as measured in terms of a wavelength, the elevation angle of maximum radiation, the take-off or TO angle gets lower.
The chief adjustment, however, has to do with making frequency changes. An 80-meter (135') doublet is about 2 wavelengths long on 20 meters. A 40-meter (70') doublet is about 2 wavelengths long on 10 meters. What determines the number of lobes in an azimuth pattern is the antenna length as measured in terms of wavelengths. So a 1.5 wavelength wire (an 80-meter doublet on 30 meters or a 40-meter doublet on 15 meters) will have 6 lobes. A 2 wavelength wire will have 4 lobes, each nearly 45 degrees off broadside to the wire.
I am calling your attention to these patterns because a 40-meter straight doublet (about 67-70 feet long) has patterns that begin to break into lobes that are no longer broadside to the wire as the wire length exceeds about 1.25 wavelength. Hence, from 15 through 10 meters, the standard 40-meter doublet will display lobes at somewhat odd angles to the wire. Unlike the 44' wire, where the direction of the main lobes is well-known, the 40-meter doublet will have lobes that point in slightly different directions from 15 through 10 meters. The quadrant antenna, even though at 40-meter wire length, has nothing but bi- directional patterns. On 10 meters, side lobes become quite evident, but not to the demise of the main lobes down the apex of the V. Now the open end of the V is about 48', quite close to our 44' straight wire that produced similar results. Hence, the pattern comparisons grow ever more interesting.
40-Meter Feedpoint Z: 50 + j1 Ohms
30-Meter Feedpoint Z: 180 + j 810 Ohms
20-Meter Feedpoint Z: 4200 - 2900 Ohms
17-Meter Feedpoint Z: 210 - j 750 Ohms
15-Meter Feedpoint Z: 115 - j 75 Ohms
12-Meter Feedpoint Z: 275 + j 820 Ohms
10-Meter Feedpoint Z: 4470 - j 760 Ohms
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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