Extended Double Zepp Antenna: A 1.25 Wavelength Wire Gain Antenna

A half-wave dipole is the reference for most wire antennas, and it is hard to beat for simplicity. If you have a little more space, however, you can gain several decibels without a rotator, a tower or any metal other than wire. The extended double zepp, often shortened to EDZ, is a center-fed wire about one and a quarter wavelengths long that concentrates its signal broadside to the wire. This guide explains what the antenna is, how long to cut it for each band, what pattern and gain to expect, how to feed it, how it behaves on other bands and how to build and test one that actually works.

What the Extended Double Zepp Is

The name comes from the early Zeppelin antenna, a wire trailed from an airship and fed at the end. The double zepp is two half-wave elements in line, fed at the center, and the extended version lengthens each side so that the whole antenna is about 1.28 wavelengths long. Each leg is roughly 0.64 wavelength. The article on Zepp antennas describes the history and the family of related designs.

The reason for the extra length is the radiation pattern. A half-wave dipole has a broad figure-eight pattern. When the antenna is lengthened, the currents on the two halves reinforce each other more in the broadside direction, and the pattern becomes narrower, which concentrates the power. The gain over a half-wave dipole is about 3 dB, which is a halving of the power needed for the same signal in the preferred directions. That is an improvement that most operators notice, and it comes at no more cost than additional wire.

The price is directivity. The beam is narrower than that of a dipole, so the antenna favours directions at right angles to the wire and gives less signal along the wire. If you want to reach a particular region, orient the wire so that the broadside direction points there. The two directions are opposite each other, so the antenna serves a region and its opposite side of the world.

How Long to Cut It

The length depends on the band, on the wire and on the height above ground. A good starting point is a total length in feet of about 1,250 divided by the frequency in MHz, or about 380 divided by the frequency for metres. Because the real figure varies, cut the wire a little long, measure and trim. The table shows total lengths for common bands, calculated for the middle of the band with this rule.

The 20 metre version, at about 27 metres in total, fits many suburban gardens, and the 15 and 10 metre versions are small enough for almost any plot. For 40 metres the antenna is more than 50 metres long, which is a major undertaking, but a bent or sloping version still works reasonably. Make the two legs equal in length and, as far as possible, in a straight line, since sag and bends alter the pattern and the feed impedance.

Gain, Pattern and Comparison

The figures below are approximate values for the antenna in free space or well above ground. Real results depend on height and the environment, so use them for comparison and not as a promise. To compare more carefully, model your own installation, as our article on antenna modelling with EZNEC and 4nec2 describes.

• Half-wave dipole. Length 0.5 wavelength, the reference with 0 dB, broad pattern.
• Full-wave center-fed dipole. Length 1 wavelength, roughly 2 dB over a half-wave dipole with narrower pattern.
• Extended double zepp. Length 1.28 wavelengths, about 3 dB over a half-wave dipole, narrow broadside pattern.

Three decibels is not dramatic, but in practice it moves a marginal signal into readability. The narrower beam also reduces the noise and interference from directions along the wire. A dipole article gives the baseline description of the pattern, which is useful when you compare.

Height Matters

An EDZ benefits from height like any horizontal wire. At a quarter of a wavelength above ground, the takeoff angle is high, which suits regional contacts. At a half wavelength or more, the takeoff angle drops and the antenna works better for distance. If you must choose, hang the antenna as high as you can safely manage and keep it clear of metal objects and house wiring.

Feeding the Antenna

The feed point impedance of an EDZ is not 50 ohms, and it is not purely resistive. It is typically in the region of 100 to 200 ohms with a reactive part that changes with length, height and band, and the exact value depends on the installation. There are three sensible ways to feed it.

1. Open-wire or ladder line to a tuner. This is the most flexible option, because the line has low loss even with a high SWR, and the tuner transforms the impedance for the transmitter. The article on doublet antennas with ladder line covers this approach, and the guide to external antenna tuners helps with the tuner.
2. A 4:1 current balun and coax. This gives a convenient single-band installation, with the balun transforming the impedance to a value closer to 50 ohms. The SWR is rarely perfect, but a moderate value is acceptable on a short line. Our explanation of baluns and ununs shows which type to choose.
3. A matching section. A quarter-wave section of cable or a stub can transform the impedance if you know it accurately from an analyzer measurement.

Whichever way you feed it, the antenna is balanced, and the feed line must not carry common-mode current. Place a choke at the feed point or soon after the balun to stop RF on the shield, as the article on common-mode chokes describes. Without it, the coax becomes part of the antenna and distorts the pattern.

Multiband Behaviour

An EDZ is designed for one band, but it works on other bands, with different results. On a band at half the design frequency, the wire is only about 0.64 wavelengths long and acts as a long dipole with a different impedance. On higher bands the wire becomes longer in wavelengths, and the pattern splits into several lobes, some of which point in directions away from broadside. The gain in the strongest lobes can be higher than on the design band, but the pattern may have deep nulls in some directions. If you plan to use the antenna on several bands, model it for each and use balanced feed and a tuner, because the impedance varies widely.

Many operators build an EDZ for 20 metres and then find it useful on 10 metres and 15 metres as well, with a tuner. That is an inexpensive way to get a gain antenna on three bands, though a dedicated design for each band will give better results. If you are choosing between a trap dipole and an EDZ on a single plot, think about what you want: the trap dipole is shorter and easier, while the EDZ gives more signal in a chosen direction on its main band. Many operators therefore keep a simple dipole for general use and add an EDZ for the band and the direction that matter most to them, such as the daytime DX band toward a favourite region.

Building and Testing It

1. Choose the band, calculate the length and add 3 to 5 percent for trimming.
2. Use stranded or copper-clad steel wire, with a strain relief at the center insulator and ropes at the ends. Our guide on wire for HF antennas compares the choices.
3. Hang the antenna as high and as straight as you can, with the legs equal.
4. Connect the feed line and the choke, and measure the SWR or impedance at several frequencies across the band.
5. Trim both legs equally in small steps, until the reactance crosses zero near the middle of the band.
6. Note the final length, the height and the feed method, so that you can reproduce the antenna.

Compare the antenna with a reference dipole if you have one, by using a reverse beacon or a network of receivers to see the reports. Differences of a few decibels are visible when you switch between antennas quickly.

Layouts When Space Is Tight

A perfectly straight wire between two tall supports is the ideal, but few gardens offer it. The antenna tolerates compromise better than many people expect. If the plot is too short, you can let the ends droop, with the center high and the outer sections sloping down toward the supports, which makes a shallow inverted V. The pattern changes a little, the gain falls somewhat and the feed impedance moves, but the antenna still works. You can also bend the ends at a corner, provided that the bend is gentle and that the wire does not run close and parallel to other wires or metal. Avoid folding the wire back on itself, because the currents cancel and the gain disappears.

For the longer bands, consider the antenna as a project for a field day or a portable operation, where a lightweight wire on fibreglass poles can be put up for a weekend. A 20 metre EDZ weighs very little, and a pair of 10 metre poles is enough to hold it well above the grass. If you use it temporarily, label the wire ends, because you will want to reproduce the same length, and keep the center support strong, since the weight of the feed line hangs there.

Safety Notes

Keep the wire far away from power lines, and plan the fall path of any support before you raise it. The voltage at the ends of a high-power antenna can be high, so keep the ends out of reach of people and animals, and use insulators designed for the load. Do not transmit with the antenna lying on the ground or touching trees, where the loss is high and the wire may get hot. If you run a legal limit amplifier, check that the balun and choke can handle the power at a high SWR, because that is where they overheat first.

Common Mistakes

• Expecting 50 ohms. The feed impedance is higher and reactive, so use a balun or a tuner.
• Unequal legs. The pattern tilts and the feed becomes unbalanced.
• Ignoring the choke. Without it, RF returns on the coax and spoils the pattern.
• Pointing the wire at the target. The best signal is broadside, at right angles to the wire, not along it.
• Cutting to the exact calculated length. Always trim by measurement, because height and surroundings shift the resonance.

The Bottom Line

The extended double zepp is a simple wire antenna that gives about 3 dB more than a dipole by being about 1.28 wavelengths long. It needs space, support points and a balanced feed with a choke, and it rewards you with a narrower beam that suits the direction you choose. Calculate the length from 1,250 divided by the frequency, cut long, feed with ladder line and a tuner or a 4:1 balun, and trim by measurement. Point the broadside at the region you want to reach, hang it as high as you can and model it if you plan to use it on other bands. For stations with room but no tower, it is one of the best uses of a spool of wire.

Frequently Asked Questions

How much gain does an extended double zepp have?

About 3 dB over a half-wave dipole, in the directions broadside to the wire. The exact value depends on the height above ground and the installation.

How long is an extended double zepp?

About 1.28 wavelengths in total, roughly 1,250 divided by the frequency in MHz in feet. For 20 metres this is about 88 feet or 27 metres, with each leg half of that.

What impedance does an EDZ have?

It varies with length and height but is typically in the range of 100 to 200 ohms with some reactance. Use ladder line and a tuner or a 4:1 balun to match it.

Can I use an EDZ on more than one band?

Yes, with a balanced feed and a tuner, but the pattern changes on each band and may have deep nulls. Model it first to see which bands are useful.

Which direction does an EDZ radiate?

Mostly broadside, at right angles to the wire, in two opposite directions. Little energy goes along the wire.

Do I need a balun?

If you feed with coax, yes, use a current balun or a choke at the feed point. If you feed with ladder line to a tuner, a balun or choke at the tuner output is still good practice.