The 40-Meter Bobtail Curtain as An All-Band Wire Antenna

L. B. Cebik, W4RNL (SK)

Sture Lof, SM5DXV, described to me an interesting adaptation of the 40-meter Bobtail Curtain to all-band use. The use was interesting enough to pass along as a possibility for those who use a wire on all bands.

Sture did not provide dimensions, so I went back to the SCV series at this site and developed an optimized 40-meter (7.15 MHz) antenna. This frequency is at the high end of the European band, but in the middle of the U.S. Band. Actually, dimensions are not that critical, although the rough proportions will tend to provide the maximum gain on 40. As well, I selected the height for the same goal.

Fig. 1 provides the dimensions for the antenna, along with the three feeding options for 40 meters, for bands below 40, and for bands above 40. The technique that Sture passed along was to construct the center wire from parallel transmission line. By judiciously connecting the bottom end of the center vertical line, we can achieve an all-band antenna.

The dimensions are quite reasonable and only a little longer than an 80-meter dipole. The total length is 149' (74.5' each side of the center vertical), with vertical legs that are 33.5' long. With the top wire at 44' above average ground, the vertical tips are 10.5' above ground, a safe height to prevent accidental contact with these high voltage points on the antenna. However, for convenient feeding, we bring the center wire close to the ground. Therefore, some form of "fencing" to prevent contact is a wise precaution.

The Bobtail on 40 Meters

On 40 meters, the Bobtail curtain operates in its design mode--as three 1/4-wavelength verticals spaced about 1/2-wavelength apart. Instead of using individual feedlines properly phased, we use a top wire that provides the correct phasing. Because the currents on the top horizontal wire are equal and of opposite phase at any distance from the center wire, the horizontal radiation is largely self-canceling. The result is a vertically polarized broadside pattern.

The current magnitude distribution, shown if Fig. 2, shows that the peak current on the verticals is at the top of the element, a desirable location for a low angle signal. As well, the maximum current on the center wire is twice the value of the maximum value on the end verticals. Of special note is the current distribution on the center wire. With the wire extended down nearly to ground level, where we locate a matching tank circuit, we need have no concern about the center wire's extra length. The current distribution shows the minimum at the same height as the tips of the end verticals.

The most common way to feed a Bobtail is still the use of a matching tank circuit that resembles the left option in Fig. 1. The parallel tank circuit would be similar in appearance and component values to an old fashioned plate circuit for a tube amplifier. The coaxial feedline can either tap the low end of the coil, a few turns above ground--and a good RF ground is essential--or one may use a small link coil over the low end of the tank coil. The parallel transmission line forming the center vertical is shorted and connected either to the top of the coil or to a tap near the upper end of the coil--whichever provides the best match. If the system is near resonance, then the reactance will be high. With the extended center vertical, the resistive component is likely to be much lower. Hence, initial set-up will likely involve a good bit of juggling of the two taps and the setting of the variable capacitor. But once set, the match should cover at least half of 40 meters with a 2:1 SWR on the main feeder line. Be certain to weather-proof the assembly when you finalize the settings.

The Bobtail should provide about 5.1-5.2 dBi gain at an elevation angle of maximum radiation (Take-Off or TO angle) of 19 degrees. Fig. 3 provides a view of both the elevation and the azimuth patterns of the antenna. In this azimuth pattern, and in all others to follow, the horizontal wires of the antenna stretch from left to right across the graphic. Why many operators prefer a Bobtail to a dipole with the same top height appears in Fig. 4.

The elevation patterns are for a 40-meter dipole and the Bobtail, both with a height of 44'. This height is a bit above the average U.S. casual wire height (about 30'-35'), but the dipole pattern is still filled with very high angle radiation. Its maximum gain--6 dBi--occurs at about 44 degrees elevation. The slightly lesser gain of the Bobtail, however, occurs in just those regions favoring DX skip. In addition, the Bobtail has very little high angle sensitivity, and a large amount of QRN and other interfering signals are relatively high-angle incoming energy. For many, the Bobtail is a nearly ideal wire DX antenna, especially considering that it is cheap, low enough for easy maintenance, and durable. Although we need a good RF ground for the base of our matching tank circuit, we do not need a radial system for the antenna on 40 meters. Indeed, you can think of the end verticals as composed of two L-shaped dipoles, while the center vertical is a Tee.

If you do not quite have the 150' length needed for the idealized Bobtail, you can make it as short as about 125' by lengthening the verticals to about 38'. The best top height for the shortened Bobtail is about 50' above ground, which places the end vertical tips 12' above the soil. Once again, we can bring the center vertical all the way to the ground without disturbing the pattern. At most, this shortened (lengthwise) version loses only about 0.1 dB of gain or so.

However you build your Bobtail for 40, you will discover that setting the matching circuit benefits from a friend with a VHF/UHF transceiver and a field strength meter at least 10 wavelengths distance and broadside to the antenna. Tune for a combination of a good main feedline match and maximum signal strength. It is possible to obtain a good match with most of the energy circulating in the tank and not as much in the antenna.

Converting the Bobtail to 80 and 160 Meters

On 80 and 160 meters, we connect the feedline ends together, but set aside the matching tank circuit. We feed the combined lines against a ground radial system to form a shortened top-loaded vertical. We should use very low loss line to the ATU. It may even be possible to use an automatic ATU at the feedpoint.

The top loading consists of a modified Tee. The horizontal wires and the end verticals form the top hat for the vertical antenna. Radiation tends to be at a fairly high angle, and so the antenna is good mostly for local or regional contacts. Because the end wires interact with the central vertical, the pattern will change as we move across 80 and 75 meters.

Fig. 5 shows typical patterns for the ideally shaped Bobtail. As we move upward on the band, the spacing between verticals changes, and so does the overall current distribution. So the pattern can range from a figure 8, with most radiation in line with the antenna, to a clover leaf. The exact dimensions we use for the Bobtail will have a bearing on the exact patterns we achieve. Likewise, the impedance can range from 100 to 1500 Ohms resistively and -1500 to +5000 Ohms reactively. So the matching situation calls for the widest range tuner you can find or make.

Fig. 6 shows the general circular pattern that we achieve on 160 meters, where the current on the outer verticals is low and distorts the pattern very little from normal. Of course, the antenna is short, but the Tee hat is just about long enough to bring the antenna close to resonance. Close does not mean a very low reactance, but a manageable one. However, the resistive component will be low--about 10 Ohms before we add in loss factors resulting from the ground quality and the size of our radial system. Hence, a couple of hundred Ohms of reactance yields a very high SWR to match.

Fig. 7 shows the current distribution along the wires of our top-loaded short vertical at 1.85 MHz. At a height of less than 0.1 wavelength, the antenna cannot be expected to be a stellar performer. If you need DX performance on 160 meters, you will require a quite different design.

While certainly less than optimal for 80 and 160 meters, the adapted Bobtail still permits communications on these bands. Of course, one can build a Bobtail for 80 meters (about 296' long at 3.6 MHz) and shift these patterns downward by one band.

The Bobtail Adapted for Use on Bands Above 40 Meters

For all bands above 40 meters, the Bobtail opens up the double center vertical to use the transmission line as what it is: transmission line. so the antenna is no longer a Bobtail curtain. Instead, it becomes a center-fed doublet at 44', with drooping end sections. On some bands, the current in the vertical end sections is insignificant relative to pattern formation. On other bands, it will play a significant role.

The following table lists the modeled results for the new doublet. The maximum gain is the gain of the strongest lobe in the pattern. All of the patterns (except 30 meters) have multiple lobes, since the doublet is 216' long from tip to tip. The Hor. Angle indicates the angle between the plane of the wire and the strongest first lobe in the horizontal plane. The TO Angle is the elevation angle of maximum radiation. The feedpoint impedance (Feed Z) shows values taken at the junction of the wire and the transmission line that formerly was our center vertical.

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Performance of the Modified Bobtail on the Upper HF Bands

Freq. Max. Gain Hor. Angle TO Angle Feed Z
MHz dBi degrees degrees R +/- jX Ohms
10.125 5.5 90 31 440 - j 1200
14.1 7.9 57 21 1800 - j 1500
18.118 8.2 43 17 2500 + j 200
21.1 8.5 28 15 300 + j 310
24.94 6.8 21 14 270 - j 125
28.1 8.5 39 11 1000 - j 800
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Not listed in the table is 80 meters. The user can operate the antenna as a low doublet with drooping ends on 80 meters. 44' is low and yields high angle radiation, but the pattern will be an oval that is nearly circular. So it may complement the use of the antenna in the hatted-vertical mode described above.

In the chart, bands with the lower gain values tend to have the most activity by the vertical ends sections of the doublet. We can check out 30 meters as an example.

Fig. 8 shows the azimuth pattern at the TO angle for 10.125 MHz. The total pattern is composed of vertically and horizontally polarized components. If you examine just the horizontal component, you will find a slightly distorted figure 8. However, the vertical end wires are active enough to produce radiation off the ends of the doublet. Hence, the total pattern is bi-directional with significant side lobes less than 10 dB below the strength of the two main lobes.

Fig. 9 gives us just the total patterns for the remainder of the upper HF bands from 20 through 10 meters. Each pattern uses the TO angle. 20, 17, and 10 meters show the least activity in the vertical end wires. Hence, their patterns have the fewest lobes--commensurate with the length of the antenna in wavelengths--and the deepest nulls in the direction of the plane of the antenna. In all cases, the antenna runs horizontally across the pattern.

15 and 12 meters show higher levels of activity in the vertical wires. The chief indicator is the ragged pattern outline, indicating many individual lobes, some created by the length of the horizontal wire, others created by the phase relationship between the two active end vertical wires. As well, the null that we would expect off the ends of the doublet is very shallow.

Let's open up the 12-meter pattern and take another look at how the lobes form. See Fig. 10. The vertically polarized component of the field obviously is at work in making the end null shallow. As we gradually move toward a position broadside to the wire, we can see that some lobes in the total field result from horizontal component lobes, others result from vertical component lobes, and still others are combinations of the two. The complexities of a pattern like this suggest some points to remember.

First, for wires with even modest geometric complexity, a random guess at the lobe structure based on what a linear doublet might produce is likely not to give too much guidance as to the pattern shape. If you model your antenna, be as precise as possible with every twist and turn to arrive at the most usable result.

Second, if you ask someone else about your pattern shapes, do not expect precision in the answer that you get. You will receive--in a best-case scenario--only as much precision as you put into the question.

However, once we reach the bottom line, we get the following result: if we can effect a match with the orders of impedance indicated in the chart, we have a doublet that will be as effective as most other single- wire doublets. For the ham with room for only one wire antenna, then, the Bobtail is not a bad choice.

However, let's remember that the antenna here emerged from a desire for an effective DX antenna on 40 meters. The other bands were bonuses. Indeed, if one also has a beam for upper HF DX work, then the ability to tune the Bobtail as a doublet on the upper bands gives one a good back-up antenna for when the rotator freezes or when an element breaks, or (more benignly) for a second receiver checking for openings on other bands.

One caution: only consider the bobtail curtain in any form if you have the room to keep the elements in a straight line and apart from each other as the dimensions suggest. Do not try to bend the bobtail. Even a shallow Vee shape with a 150-degree included angle (where straight is 180 degrees) will reduce gain by a full dB on the primary band. A 120-degree included angle will reduce performance to a nearly circular pattern with a gain level not much different from a single monopole.

So the all-band Bobtail is not for everyone. But it may serve well for some.

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