Zepp Antenna: End-Fed Half-Wave With Open-Wire Feeder

The name sounds like something from an old airship hangar, and that is exactly where it comes from. The Zepp antenna was developed in the early twentieth century to let radio operators on Zeppelin airships trail a long wire behind the craft and feed it from the cabin. The idea outlived the airships: a half-wave wire, fed at one end through a length of open-wire line, is still a practical way to cover several bands from a single wire, especially when a coaxial cable would suffer large losses or when you simply have the parts in your junk box. This guide explains how the Zepp works, why the length of the feeder matters so much, which tuner you need, and when a different antenna, such as a centre-fed doublet or an end-fed half-wave with a transformer, serves you better.

The Basic Idea

A half-wave wire has a high voltage and a low current at its ends, and a low voltage and a high current in the middle. The impedance at an end is therefore high, of the order of 2,000 to 4,000 ohms, depending on the height and the wire diameter. In the classic Zepp antenna, the half-wave wire is connected to one conductor of a two-wire open line at its end, while the second conductor of the line hangs down parallel with the first and is not connected to the radiator at all. This is called voltage feed, because the antenna is fed at a point of high voltage, and it is the reason why the antenna is sometimes named the end-fed Zepp.

In the original design the feeder is a quarter of a wavelength long, or an odd multiple of it. A quarter-wave line has the interesting property of transforming impedances: it turns the high impedance at the antenna end into a low impedance at the transmitter end, and the other way round. At the transmitter the line therefore presents a low impedance in series with a reactance that is close to zero, which a series-tuned circuit can match to the radio. A line of a half wavelength, in contrast, repeats the antenna impedance at the other end, so the transmitter sees a high impedance that needs a parallel-tuned circuit. This is why vintage transmitters had switchable tuning for the two cases.

Why the Feeder Length Matters

With a centre-fed doublet and balanced ladder line, the line is a means of reaching the tuner, and its length is flexible. The Zepp is different. Because one wire of the line is connected to the antenna and the other is not, the currents in the two conductors are not equal and opposite, and the feeder itself radiates. The currents also vary along the feeder in a pattern determined by its length and the frequency. At the lengths that are multiples of a half wavelength, the transmitter sees a very high impedance, and at odd multiples of a quarter wavelength, a low one. In between the impedance contains a large reactance.

For the builder this means choosing the feeder length with the bands in mind. A feeder that is a quarter wavelength on 80 metres is about 19 metres of open-wire line, and the same feeder will be roughly a half wavelength on 40 metres and three quarters on 30 metres. The impedance seen at the radio then swings between low and high values from one band to the next, and the tuner must deal with both. Many people cut the feeder to an odd quarter wavelength on the lowest band and accept that on the others a tuner with a wide range will be needed. The tuner discussion is in the article on choosing an external antenna tuner.

Dimensions for Common Bands

The wire should be a half wavelength on the lowest band you want to use. The half-wave length is given by the same rule as for a dipole, about 143 divided by the frequency in megahertz, in metres, with the end effect and the velocity factor of the wire included, as shown in the guide to the dipole length formula. On 80 metres at 3.6 MHz the wire is about 39.5 metres, and on 40 metres at 7.1 MHz about 20 metres. The feeder length is about one quarter of a wavelength multiplied by the velocity factor of the line. Open-wire line has a velocity factor of 0.95 to 0.98, so on 80 metres the quarter-wave feeder is a little under 19 metres, as described in the article on velocity factor.

The wire used for the radiator can be ordinary stranded copper, and the open-wire line is made either from 450 ohm ladder line bought ready-made, or from two wires held apart by plastic spacers at intervals of 10 to 15 centimetres. The spacing and the diameter set the characteristic impedance, typically 300 to 600 ohms. A higher impedance line has lower losses at a given mismatch, which is why the open-wire line is so effective at high SWR. The article on ladder line versus coax compares the loss figures.

Radiation From the Feeder

The unbalanced current in the feeder is the Zepp’s honest weakness. The radiation of the feeder adds to that of the wire, and because the feeder usually runs down vertically or at an angle, its radiation is partly vertically polarised and distorts the pattern of the antenna. In a good installation this can be useful, since a part of the signal leaves at a lower angle, but it also means that RF is present on the feeder near the house and the shack, which can create interference and RF in the shack. The feeder should be kept away from metal objects and from electrical wiring, and it should leave the antenna at a right angle to the wire for some distance.

At the radio end, the presence of common mode current means that the tuner must have a good ground or a counterpoise, and that chokes are of limited use on an open line. The correct treatment is a balanced tuner with a built-in balun, or a design that has a clean ground, as described in the article on baluns and ununs. A feeder that radiates is not a catastrophe, but it makes the antenna less predictable, and a modern builder who wants a quiet and predictable installation often chooses a centre-fed doublet instead.

Pattern and Performance

On its fundamental band the Zepp wire radiates like a half-wave dipole: a figure-of-eight broadside to the wire, with a gain of about 2 dBi in free space. On higher bands the wire is several half waves long, and the pattern develops lobes that point closer to the wire’s direction as the electrical length grows. A 40 metre wire used on 15 metres is three half waves long, with main lobes roughly 35 to 40 degrees from the wire. These patterns are a normal feature of long wires, and they can be an advantage if the lobes happen to point at the area you want to work. The shape of the patterns follows the rules in the article on antenna radiation patterns, and the wire shape also matters if it is bent or sloping.

Height matters as it does with any horizontal antenna. At half a wavelength above the ground the low-angle radiation is useful for DX, and at a quarter wavelength or less it favours steeper angles. For a wire that is used on several bands, the height in wavelengths changes from band to band, so the angle of radiation changes as well. Keep the far end of the wire away from trees and metal, as it carries the highest voltage and is sensitive to the insulator and the surroundings.

The Zepp Compared With Other Wires

A centre-fed doublet with ladder line is very close to the Zepp in appearance and in the way it covers several bands, but the line is balanced and radiates much less, so it is usually the better design. The doublet antenna with ladder line and the G5RV are the descendants of this family. The end-fed half-wave with a 49:1 transformer, described in the article on the end-fed half-wave antenna, solves the same high impedance problem at the end of the wire with a transformer, and uses coax, which is easier to route and install.

What the Zepp offers is simplicity and low loss. There is no transformer to saturate or overheat, no balun to fail and no coax loss on a mismatched line. If you already have a long run of ladder line and a wire, it can be put up in an afternoon. For portable and emergency use, a Zepp made of a few metres of ladder line and a long wire is light and cheap. Its needs are a capable tuner and tolerance for a bit of RF around the feeder.

Practical Installation Tips

Support the radiator as high and as straight as practicable, with a strong insulator at the end and at the feed point. Connect the wire to one conductor of the ladder line with a solid joint that does not carry the weight, and leave the other conductor of the ladder line unconnected, with a small insulator at its top. Let the line leave the radiator at a right angle and keep it clear of gutters and walls. Bring the line into the house through a window feed-through or an entry panel, with a drip loop on the outside, and avoid twisting the line, because twists change its impedance.

Tune with low power first and watch the tuner for the point of lowest SWR. If the tuner finds a match only at one of two settings, choose the one that has the lower capacitor voltage and the lower inductor current, because a poor match on a half-wave feeder can place thousands of volts on the tuner. Check the temperature of the tuner after a few minutes of transmission, and keep digital modes at a reduced power, since they stress the components more than speech does. Finally, test for RF in the shack with the antenna in use by touching the metal of the microphone and the tuner with the back of your hand at low power, and if there is a tingle or a burn, improve the ground before you continue.

The Bottom Line

The Zepp antenna is an end-fed half-wave wire connected to one conductor of an open-wire feeder, which is traditionally a quarter wavelength or an odd multiple of it. The voltage feed gives a high impedance at the antenna, which the line transforms to a low impedance at the transmitter, but the feeder carries unbalanced current and radiates. Cut the wire to a half wave on the lowest band, choose the feeder length with the other bands in mind, use a tuner that can cope with both low and high impedances, and keep the feeder away from the house. A centre-fed doublet or an end-fed half-wave with a transformer is often the more predictable option, but the Zepp remains a simple, low-loss multiband wire.

Frequently Asked Questions

Why is it called a Zepp antenna?

It was developed for Zeppelin airships, from which a long wire was trailed and fed through a length of open-wire line inside the craft.

What is the difference between a Zepp and a doublet?

A Zepp is fed at one end, so only one conductor of the line connects to the wire and the feeder radiates. A doublet is fed at the centre with a balanced line, so radiation from the line is small.

How long should the feeder be?

Traditionally a quarter wavelength, or an odd multiple, on the lowest band, multiplied by the velocity factor of the line. On other bands the tuner must deal with whatever impedance results.

Can I use coax instead of open-wire line?

Not directly. The end of a half-wave wire has a very high impedance, so coax would need a matching transformer, as in an end-fed half-wave antenna.

Does a Zepp need a tuner?

Yes. The impedance at the radio varies widely from band to band and depends on the feeder length, so a tuner that handles both low and high impedances is required.

Is a Zepp antenna noisy?

It can pick up and radiate noise on the feeder because the line is unbalanced. Keep the line away from wiring and use a good ground at the tuner.