ANTENNAS FROM THE GROUND UP

39. The Skinny Mini Antenna
or A Nearly All-Band Vertical Doublet

L. B. Cebik, W4RNL (SK)

Perhaps the ultimate crisis for a radio amateur who is used to stringing horizontal wires is to move into a home or apartment where horizontal wires are not possible. Of course, no ham ever gives up and continually looks for ways around the problem. However, let's suppose there is no escape: we must use a vertical antenna. Our question is this: how can we get the most performance from the least antenna on the most bands?

At the risk of sounding repetitious of episode #34, the answer is 44' of wire--but strung vertically and fed in the middle. With this antenna, we can work 40 through 10 meters with fairly low-angle signals, which is one of the reasons for using a vertical at all. The gain will not equal that of a Yagi or even of a horizontal wire 1/2 wavelength high or more, but we will make contacts.

The first question is why I cut off the wire at 44'. The antenna that we shall construct is going to be about 1.25 wavelengths at 10 meters. A 1.25-wavelength vertical doublet will have a very similar pattern to a 5/8-wavelength vertical monopole: both are at the dividing region between low angle patterns and high angle patterns that go over the skip angles of propagation.

Fig. 1 gives us some idea of how radical the change is in the elevation angle of the lower lobe. The vertical dipole at 28.5 MHz is 3' off the ground at the base. Hence the top wire is 3' above the listed wire length. There are several notable features about the patterns. First, note the increase in gain as the antenna becomes longer. The first pattern is for a 1/2-wavelength wire, while the second is for a 1-wavelength vertical. The third pattern is for the 44' wire, while the last is for a 1.5-wavelength wire doublet.

Second, note the appearance and growth of the second lobe as the wire gets longer. When the wire is about 1.25 wavelengths long, the lowest lobe has reached its strongest level. If the wire reaches 1.5 wavelengths long, the lowest lobe almost disappears and the second lobe becomes the strongest. For most amateur radio communications, its angle is too high for regular success.

We would have obtained a similar set of patterns had we replaced the wire doublets with 1/4-wavelength monopoles and ground-plane radials. When elevated, the ground-plane radials are simply the lower half of the doublet, but arranged in a manner so that their radiation self-cancels.

The following pages present elevation patterns for all of the HF ham bands from 40 through 10 meters. Since we are dealing with a vertical antenna, the azimuth patterns will all be circular. Thus, elevation patterns will provide us with the most necessary data. You can always picture a circle at the elevation angle of maximum radiation, as well as at any other elevation angle of interest to you.

Do not evaluate the antenna patterns solely by reference to the maximum gain values. A vertical has more to offer.

Although the gain levels will seem low, remember that the elevation angles of maximum radiation are also low. As well, your signal will only be down only by about an S-unit relative to a horizontal 44' wire at about 50' in the air. Under some circumstances, you may actually be stronger than a horizontal wire antenna.

Remember also that we are--according to our hypothesis--trying to operate from a location that might not have room for any other kind of antenna. Our goal is not to compete with long-boom Yagis and mutli-wire horizontal phased arrays. Instead, our goal is to develop the best possible signal that we can at the best possible communications angles in the least amount of space.

For the task at hand, then, a vertical doublet has much to offer and may well outdo many of the so-called mini-antennas, some of which offer gains that are less than 0 dBi by a considerable amount. So the vertical doublet does have a place among ham antennas.

At 20 meter, we just begin to detect the formation of the second higher lobe, which appears as a high-angle bulge in the elevation pattern. The absence of high-angle radiation also means a lack of receiving sensitivity to high-angle radiation. The result is a reduction in overall QRN, since most atmospheric noise that results from electrical discharges occurs at closer range and hence has a higher skip angle to the antenna. Above 20 meters, the second lobe meters grows, but noise tends to diminish.

The feedpoint impedances on the various bands tend to be quite moderate, well within the capabilities of most antenna tuners. The exception is 15 meters, where the antenna is close to 1 wavelength long, resulting in a high resistive component to the impedance. On the adjacent 17 and 12 meter bands, we find more moderate resistive values, but high reactive components. The parallel feedline for this antenna should depart the wire at right angles if possible for as long as possible to reduce direct coupling into the line.

Hanging the 44' wire is subject to many situational variables. Hence, there is very little useful general comment. However, if a 44' height--plus a little extra to clear the ground and ideally some further height to place the hot wire-end out of reach--is not quite possible, you may zig-zag the vertical wire as necessary. Two guide ropes might make the zig-zag more stable. Expect a slight reduction in performance, but no radical degradation. The 44' vertical can be an effective 40-10-meter antenna for the space-starved amateur

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