09 July, 2011

Wire antennas for DX: Pair of doublets

At my summer cottage (58° N, 9° E), I have two crossed doublets with their centers on the top of a mast as shown in the image. Of course, I would like to have had a rotatable beam antenna, but lacking that, this is a wire antenna substitute with acceptable performance. I use them for DX-ing and participation in contests like the IARU HF contest in July. The first antenna is a 26.8 m (88 feet) doublet and the second one at 13.4 m (44 feet), which was erected last summer, is half this length. Switching between the antennas sometimes makes a difference of 4-5 S-units of signal strength, but usually less.

Of the many desirable properties an antenna can have, the doublet emphasizes having similar directivity patterns over a 4:1 frequency range. This is different from other multiband antennas like a trap dipole (W3DZZ) or a Windom which emphasize the ability to obtain a 50 ohm impedance and a low standing wave ratio (SWR) in all the usable bands. The doublet maintains its directivity pattern broadside to the antenna as long as the antenna is shorter than 1.25 wavelengths. Although different from 50 ohms, the antennas have lengths which make the impedance manageable for a tuner. Losses due to high SWR are minimized by feeding them with ladder line rather than coax. The ladder line is attached to homemade plastic spacers so that the distance to the metal mast is about 10 cm.

The lengths have been analyzed in papers by L. B. Cebik, W4RNL (You will need to create an account to access the papers "Suppose I Could Have Only One Wire Antenna. . .", "My Top Five Backyard Multi-Band Wire HF Antennas" (no. 2 in this list), and "The "Ideal" Back-Up Antenna for 80-20 Meters"). As the impedance is usually higher than 50 ohms, each ladder line is terminated in a 4:1 balun into the 2-input KAT-100 tuner of the Elecraft K2/100.

Their centers and feeders (20-25 m long) are attached to a mast which is about 12 meters above ground. The ends are attached to trees. The ends slope somewhat down from the center, but this still should give a height which is more than half a wavelength for the 20 m band and up. The 26.8 m antenna runs NW-SE and is the main antenna for reaching Europe and North America, the 13.4 m antenna runs NE-SW and is usually better for Asia and Australia. In the image above, the wires have been redrawn for better clarity, and they are even easier to see when clicking the image for a larger version.

The length in wavelengths for each amateur band is given in the table below. The bold numbers signify those frequencies where the doublet maintains the important property of a controlled directional pattern broadside to the antenna. At 1.25 wavelengths, the doublet is a double extended Zepp antenna (28 and 14 MHz), but some of the resources on the web on the double extended Zepp antenna seem to overlook the fact that it can be used at lower frequencies also. The longer the antenna is in wavelengths, the higher the gain it has, i.e. the narrower the mainlobe is. As the length goes beyond 1.25 wavelengths, the broadside mainlobe splits into several lobes. On the other hand, at the lowest frequencies, the mainlobe becomes so wide that the antennas are almost omnidirectional.
Antenna length in wavelengths. Bold numbers denote
lengths where the antenna has a single broadside mainlobe.
An antenna's impedance is highest as the length is around one wavelength so this should be avoided. From the table above one can see that the shortest doublet is 0.94 wavelengths in the 21 MHz band and it can therefore be hard to tune in this band. The impedance is also high for very short lengths, i.e. in the 80 m band. The longest antenna is usable even on this band, but the lengths are optimized for the higher bands as 80 m is not so much of use for DX in the light Nordic summer nights. I have however not experienced any problems with tuning such a short antenna at 80 m as reported by W8JI, but I may have a lucky choice of feedline length that transforms the high impedance there to something more benign.

The doublet is also a field-friendly multiband antenna as described by C. Lofgren, W6JJZ.

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