ANTENNAS FROM THE GROUND UP
The average operator is as concerned over where his or her signal is going as well as how well it is getting there. That is the premise for the following notes. Letþs also set up a scenario where the longest possible wire antenna an operator might set up in his or her back yard is a 40-meter doublet. Unfortunately, the 40-meter doublet, while being a very effective wire antenna, does not meet the requirements of the "where" part of our initial problem.
Fig. 1 shows in the simplest possible sketch the dimensions of a 40-meter doublet. The length can be varied by plus or minus a few feet without affecting performance. In fact, in the following performance table, only the impedances would change noticeably.
Let's assume that the yard has some high old trees and that we can get the antenna about 66' up, a half wavelength high on 40, where horizontals become very competitive with verticals.
The figures in the table represent some important operating parameters that are worth noticing. The gain, in dBi, is obvious. The TO angle is the elevation angle of maximum radiation, an indication of the most potent skip angle. The vertical beamwidth (VBW) and horizontal beamwidth (HBW) are indicators of the general departure from the line of maximum gain where we can still get good performance. The feedpoint impedance is at the antenna terminals. Since we would normally feed the antenna with parallel feedline to an antenna tuner, the impedance at the tuner terminals will vary with the line length and characteristic impedance.
The figures are not to be viewed as exact, but only as a general guide to what we can expect from the 67' long, 66' high doublet.
General Performance of a 67' 40-10 Meter Doublet
Freq. Max. Gain TO VBW HBW Feedpoint Z
Mhz dBi Deg Deg Deg R +/- jX Ohms
7.15 7.3 28 35 86 70 - j 10
10.1 8.1 20 23 70 275 + j 800
14.15 9.0 15 16 51 4670 - j 345
18.1 10.5 11 12 33 175 - j 860
21.2 8.4 10 10 33 100 - j 115
24.95 9.3 8 9 33 375 + j 730
28.5 9.5 7 8 28 3265 + j 375The 40-meter doublet has some good signal strength in its main lobes. However, the antenna will only be bi-directional broadside to the wire up to about the 17-meter band. For higher frequency bands, the lobes will begin to break up so that we can get 4-petal and 6-petal patterns, where maximum radiation is at some angle to the wire. See Fig. 2 for some representative overlaid azimuth patterns (40, 15, and 10 meters). If the antenna is broadside to Europe on 20 meters, then 15- and 10-meter signal may miss Europe altogether.
A solution to the problem of knowing where oneþs signal is going on all bands lies in a key fact: a doublet reaches a maximum length for a truly bi-directional pattern when it is about 1.25 wl long, that is, when it is about EDZ length. There is nothing absolutely precise in the length, but it must be well under 1.5 wl long, since at that length, we get 6 lobes. In contrast, the EDZ or 1.25 wl doublet has a main bi-directional lobe and some ears that are 10 dB down or more.
Fig. 3 shows a doublet that meets the condition of having bi-directional patterns on all bands from 40 through 10 meters. There is nothing magical about the 44' length. It might easily be 2' shorter or longer without noticeable change in performanceþexcept for the impedance at the feedpoint. Like any doublet, the antenna is designed for parallel feedlines and an ATU.
General Performance of a 44' 40-10 Meter Doublet
Freq. Max. Gain TO VBW HBW Feedpoint Z
MHz dBi Deg Deg Deg R +/- jX Ohms
7.15 7.0 29 35 94 25 - j 580
10.1 7.6 20 23 83 55 - j 100
14.15 7.7 15 16 72 195 + j 485
18.1 8.6 12 12 60 920 + j1565
21.2 9.0 10 10 51 4160 + j 155
24.95 10.4 8 9 40 520 - j1545
28.5 10.4 7 8 31 140 - j 650
The table shows that the range of impedances is well within the capabilities of most tuners if we select a good line length. The gain column shows that the antenna has slightly less gain on 40 through 10 meters than the longer doublet. However, a glimpse at Fig. 4 shows that all of our patterns are broadside to the wire itself. The EDZ-type patterns show up on 12 and 10 meters, while 17 and 15 meters show patterns typical of a 1-wl long doublet. At 40 meters, the antenna is about 1/3 wl long, about the minimum length from which we can get good performance and a feedpoint impedance whose resistance is not exceedingly low and whose reactance is not exceedingly high at the same time.
Because the antenna has relatively narrow horizontal beamwidths at the highest bands, we cannot get coverage of the entire horizon from placing two such antennas at right angles to each other. However, such a system would require at least three supports for wire ends (assuming that there is a common support for the ends of two wires). Why not rearrange the supports so that the three together support 3 44' doublets, as in Fig. 5.
The ends of such an array of 3 doublets need only a small amount of clearance from each other. A 6' wire-end-to-support distance is shown. Models of the situation show that there is no significant interaction among the doublets. This is true whether the feed points of the unused doublets are open or shorted.
In addition, the triangle can be considerably distorted before any interaction occurs among the active and unused doublets. Therefore, in planning such a system, you can alter the broadside vector to place signals more directly at desired target areas.
It is possible to use the system by running three lengths of feedlineþone from each antennaþto the shack and switching among the doublets at the operating position. This scheme has one major disadvantage. The individual feedlines are likely to have different lengths. Therefore, when switching among the antennas, trying to find the one with the strongest signal, one may well have to retune the ATU. Apart from the extra time involved, the switch-and-retune routine may obscure which of two doublets indeed provides the strongest signal.
A more convenient arrangement, although one requiring more work during the installation of the array, is to set up a central support for a remote switching box consisting of 3 double throw-double pole relays. Even for QRP power levels, the contacts should be sturdy and well spaced to simulate the feedline spacing. 12-volt relays are generally the safest, since the control voltages are low, but almost any coil voltage will do. From the shack to the relay box, you will need two lines: a common feedline and a control voltage line. Fig. 6 provides a general sketch of the system.
Of course, as with any remote system, you would need to take special precautions. First, be sure that the relay box is water-protected. This does not mean totally water tight, since a þweepþ hole is necessary to carry away any condensation. With open-wire feeders, a non-conductive (plastic) box tends to reduce the number of problems with unwanted couplings.
Second, be sure that you account for safety. The common feedline should be well elevated. The 12-volt (or more generally, the control voltage) line may be buried, if it is made from materials that are recommended for such use. The control voltage line should not closely parallel the feedline, lest it become a means of disrupting the balance of the feedline. Burying the voltage line works well.
For world-wide coverage, the only alternative to the 3-doublet system would be a single rotatable doublet. However, such a scheme would require a considerable investment in aluminum, a sturdy tower and concrete base, an expensive rotator, and some ingenuity in setting up the parallel feedline so that it does not couple disruptively to the tower or rotator, especially as you turn the doublet. In addition, a 44' element would likely require a mast extension and tension ropes from the peak to the mid-points of the element in order to reduce the sag stress on the element.
Nevertheless, such an arrangement is possible for advanced antenna builders. One element arrangement is shown in Fig. 7. The element-diameter taper schedule shown is likely only satisfactory up to about 60 mph winds. However, many other taper schedules can be used.
All of these ideas are icing on the original cake. Basically, the antenna concept is a broadside 44' doublet for 40-10 meters. It works.
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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