Rhombic Antenna: The Big Wire Diamond for HF Beams

Some antennas are famous because they are easy to build, and some are famous because they are enormous. The rhombic antenna belongs to the second group. It is a diamond of four horizontal wires, each one to several wavelengths long, strung from poles or towers high above the ground, and for decades it was one of the main tools for long-distance shortwave communication. Few amateurs have the land to build one, but the design is still worth understanding, because it shows how a directional antenna can be wideband, why traveling-wave antennas are terminated, and what the real costs of high gain look like. This guide explains how a rhombic works, what a terminated and an unterminated version do, what the advantages and disadvantages are, and whether a radio amateur might ever consider one. The technical description follows the standard reference article on the rhombic antenna.

What a Rhombic Antenna Is

The reference defines a rhombic antenna as a type of radio antenna made of four sections of wire suspended parallel to the ground in a diamond or rhombus shape. All four sides are the same length, and the length is not critical. It is typically from one to two wavelengths for each side, and for any chosen length and frequency, there is an optimum angle at which the sections should meet at the acute vertices. The reference mentions about 42 degrees at the fed end and at the far end as a typical figure, but the correct angle depends on the design.

A horizontal rhombic radiates horizontally polarized waves at a low elevation angle, off the pointed end of the antenna that is opposite the feed point. Its main advantages are simplicity, high forward gain, wide bandwidth and the ability to operate over a wide range of frequencies. It is fed at one of the two acute vertices, through a balanced transmission line or a coaxial cable with a balun. The opposite vertex is either left open or terminated with a non-inductive resistor. That one choice, open or terminated, changes the antenna’s behaviour completely.

How It Works

The easiest way to think about a rhombic is as four long wires that each radiate a pattern with main lobes at an angle to the wire. A long wire does not radiate evenly like a short dipole; with a length of a wavelength or more, the pattern splits into lobes that point toward the wire’s ends at angles that depend on its length. The diamond shape is chosen so that one of the main lobes from each of the four sides points in the same direction, and the four contributions reinforce each other. The reference illustrates this with a diagram of the radiation patterns of each segment, and states that using the correct vertex angle makes the lobes line up and increases the gain.

That geometry explains two things. First, it explains why the correct angle depends on the length and the frequency: the lobes of a long wire move as the electrical length changes, and the angle must be chosen so that they remain aligned. Second, it explains why the antenna has useful performance over a wide frequency range. As the frequency changes, the lobes shift only slowly, so the alignment degrades gradually rather than collapsing. The reference notes that the input impedance and the radiation pattern are relatively constant over a 2:1 range of frequencies, and that the impedance can be kept fairly constant over a range of 4:1 or more, with the forward gain increasing at about 6 dB per octave.

If you want to refresh the terms used for lobes and beamwidth, our article on the antenna radiation pattern explains them.

Terminated or Open?

The termination decides whether the rhombic is a one-way antenna or a two-way antenna.

The Terminated Rhombic

When the wires meeting at the far vertex are joined by a non-inductive resistor, the antenna becomes a traveling-wave antenna. The current wave that flows along the wires from the feed point is absorbed at the end instead of reflecting back. With the reflection removed, the radiation pattern is unidirectional, with the main lobe off the terminated end. That end of the antenna is oriented toward the intended receiving station or region. The cost is power. A resistor that absorbs the wave also absorbs energy that could have been radiated, and in a transmitting antenna it must be able to dissipate a large part of the transmitter’s power.

The Unterminated Rhombic

When the far end is left open, a reflected wave travels back along the wires. The rhombic then radiates in both directions, with two opposite lobes off the two acute ends, although the reference points out that it is not perfectly bidirectional. This version is simpler and wastes less power in a resistor, but it gives up some of the cleanliness of the pattern and the broadband behaviour of the terminated antenna.

Amateurs who have built Beverage antennas for low-band receiving will recognize the principle. A Beverage is also a terminated traveling-wave wire, and the resistor at the far end is what gives it a clear direction. Our article on low band receive antennas describes how that works in practice, and the reference article on the Beverage antenna gives the formal definition. A rhombic can be thought of as a high-gain, diamond-shaped relative of that family.

Improving the Efficiency With a Stub

The reference mentions an interesting way to recover some of the lost power. It is possible to improve the low efficiency and gain of unidirectional rhombics by replacing the termination resistor with a low-loss, balanced, resonant stub transmission line. The stub reflects the power that would have been wasted in the resistor back into the antenna with the correct phase to reinforce the excitation from the transmitter. According to the reference, this arrangement can increase the radiation efficiency of transmitting antennas to the 70 to 80 percent range, at the cost of increased complexity. The picture in the article shows a horizontal three-wire rhombic terminated with a resonant stub, not a resistor, for this purpose.

A Short History

The reference says that the rhombic antenna was designed in 1931 by Edmond Bruce and Harald Friis, and that it was most commonly used in the high-frequency or shortwave band as a broadband directional antenna. Before World War II, it was one of the most popular point-to-point high-frequency antenna arrays. After the war it largely fell out of favour for shortwave broadcasting and point-to-point communication, and it was replaced by log-periodic antennas and curtain arrays. Large log periodics provide wider frequency coverage with comparable gain, and curtain arrays give a cleaner pattern, the ability to steer in elevation and azimuth, much higher efficiency and higher gain in less space.

The rhombic has not disappeared, however. The reference notes that it remains useful where the combination of high forward gain and large operating bandwidth cannot be achieved by other means, or where a directional antenna is needed but construction and installation costs must be kept low. It is also used for reception, and the article gives the example of a monitoring station in the United Kingdom with three rhombics aligned for reception at three different azimuths. If you want to see a more modern wideband directional antenna that hams can build, read our guide to the log periodic dipole array.

Advantages and Disadvantages

The rhombic’s advantages are the reasons it was used so widely. It is simple, with no moving parts and no complicated phasing network. It is reliable, because it is mostly wire and poles. It handles considerable transmitter power, because the voltage and current distribution along the wires is essentially uniform. It is wideband, so one antenna can serve a range of frequencies. And several rhombics can be connected end to end in the arrangement known as a MUSA, a Multiple Unit Steerable Antenna, which was used for receiving long-distance shortwave signals.

The disadvantages are just as real. A rhombic requires a large area of land, especially if several antennas are installed to serve different directions or regions, or to cover widely different frequencies. It suffers from efficiency problems because of earth losses under the antenna, spurious lobes that waste power, losses in the termination and the inability to maintain constant current along the conductors. The reference gives a typical radiation efficiency of 40 to 50 percent, and says that the low efficiency significantly reduces the gain for a given main lobe beamwidth compared with other arrays of the same beamwidth. In other words, a rhombic is not the most efficient way to produce a given amount of gain, but it can be one of the cheapest and simplest.

How Big Is It at HF?

The size is the first obstacle, and it is easy to calculate. The wavelength in meters is roughly 300 divided by the frequency in megahertz. At 14.2 MHz the wavelength is a little more than 21 meters, so a rhombic with each side two wavelengths long has legs of about 42 meters, roughly 138 feet, for a total of four legs. At 7.1 MHz, the same design has legs of about 84 meters, or 277 feet. Even at 28 MHz, where the wavelength is about 10.7 meters, a two-wavelength leg is about 21 meters or 70 feet. The antenna also needs supports at all four vertices, and those must be high enough to give the right takeoff angle, which depends on the height above ground compared with the wavelength. Our guide to antenna height and takeoff angle explains why height matters.

The reference says that the rhombic can radiate at elevation angles close to the horizon or at higher angles, depending on its height above the ground relative to the operating frequency and on its physical construction. A low radiation angle is useful for skywave propagation, in which the waves directed toward the horizon reflect from the ionospheric layers and come back far beyond the horizon. That is why rhombics were so popular for long-distance circuits. A beam of this kind, pointed at one region and giving a low angle of departure, is what a DX operator wants.

Could a Radio Amateur Use One?

For nearly all amateurs, a classical rhombic is impractical. The land requirement alone rules it out for suburban lots, and a multiband station would need several different rhombics or a compromise design. The antenna is fixed, so it points in one direction without a rotator, and that is a strength for a circuit that always works the same region and a weakness for a station that wants to work the world.

That said, there are situations where the idea makes sense. A club with access to a large field could build a rhombic for a single band and a single region, perhaps for a special event or a receiving project. A DXpedition team, or a station on a farm or at a remote site, could consider a smaller design for a single band. Others will be better served by a Yagi, a quad or a log periodic. Our comparison of the Yagi and the hex beam shows how a more compact directional antenna compares. If you do plan any large wire antenna, model it first, as described in our guide to antenna modeling with EZNEC and 4nec2, because the lobes and the termination values depend on the exact geometry.

Practical Cautions

• Mind the safety clearances. A rhombic is supported by tall poles or towers, and a wire that falls across a power line is a serious hazard. Read our article on tower safety before working at height.
• Use a proper termination. The resistor must be non-inductive and rated for a large part of the transmit power. An underrated resistor will fail quickly.
• Plan the feed system. The feed point impedance is high compared with 50 ohm coax, so a matching network or a balanced line with a balun is needed.
• Check local rules. A large wire structure may need permission from the landowner and from local authorities.

The Bottom Line

A rhombic antenna is a diamond of four equal wires that combine four long-wire lobes into one strong beam off the far end. A terminated rhombic is unidirectional, wide in bandwidth and simple, but it wastes a large part of the power in the termination, and its typical efficiency is only 40 to 50 percent. It needs a great deal of land and tall supports, which explains why log periodics and curtain arrays replaced it in many professional services. For amateurs it is mostly a teaching example and a rare project for people with a field, but it explains traveling-wave antennas, wideband design and the trade-off between gain and efficiency better than almost any other antenna.

Frequently Asked Questions

What is a rhombic antenna?

It is an antenna made of four wires of equal length suspended horizontally in a diamond shape. It gives high forward gain and wide bandwidth, and it radiates horizontally polarized waves at a low elevation angle.

Why is a rhombic terminated with a resistor?

The resistor absorbs the wave at the far vertex so that it does not reflect back. This makes the pattern unidirectional, with the main lobe off the terminated end, at the price of wasted power.

How efficient is a rhombic antenna?

The reference gives a typical radiation efficiency of 40 to 50 percent because of earth losses, spurious lobes and termination losses. A resonant stub in place of the resistor can raise it to the range of 70 to 80 percent.

Which bands can it cover?

The input impedance and the pattern are relatively constant over a 2:1 frequency range, and can be held over a 4:1 range or more, so one rhombic can cover several nearby bands, though the best performance is at the design frequency.

Why did the rhombic fall out of use?

After World War II it was replaced in many services by log-periodic antennas and curtain arrays, which offer wider coverage or more gain and efficiency in less space.

Can I build one at home?

Only if you have a very large area and tall supports. For most amateurs, a Yagi, a quad or a log periodic is a more practical way to get a directional HF antenna.