What Is a Rhombic Antenna?
A rhombic antenna is a directional wire antenna formed by four conductive elements arranged in a diamond-shaped structure. It can be viewed as two V antennas connected end to end, with one acute vertex used as the feed point and the opposite vertex normally connected to a terminating resistor.
Unlike a resonant dipole, a properly terminated rhombic antenna operates primarily as a traveling-wave antenna. RF current travels from the feed point along the four wire sections toward the terminating load. The electromagnetic fields produced by these sections combine to create a concentrated beam along the major axis of the rhombus.
Rhombic antennas became well known for long-distance HF communication because they can provide wide frequency coverage, useful forward gain and relatively stable directional characteristics. Their large physical size, however, makes them more suitable for fixed installations than compact communication platforms.
Basic Structure
A conventional rhombic antenna consists of four wires of equal length suspended above the ground. Together, these wires form a horizontal rhombus with two acute angles and two obtuse angles.
The antenna is fed at one end, while the opposite end is connected to a non-inductive terminating resistor. This resistor is selected to approximate the characteristic impedance of the antenna structure and absorb the remaining traveling-wave energy.
The main design parameters include:
- Length of each wire section
- Acute or apex angle of the rhombus
- Installation height above ground
- Feed-point impedance
- Terminating resistance
- Conductor diameter
- Target frequency range and propagation path
The optimum angle is not a universal fixed value. It depends on the electrical length of each side and the desired elevation and azimuth direction of the main beam.
Figure 1. Basic configuration of a terminated horizontal rhombic antenna.
How Does a Rhombic Antenna Work?
Each side of the rhombus behaves as a long-wire radiator. As current propagates along the conductors, each wire section produces a directional radiation component.
When the antenna dimensions and angles are properly selected, the fields radiated by the four sides reinforce one another in the forward direction. In other directions, some field components partially cancel. The combined result is a relatively narrow main beam along the long diagonal of the rhombus.
The terminating resistor plays an important role. By absorbing energy that reaches the far end, it reduces current reflection and prevents a strong standing wave from forming on the conductors. This helps the antenna maintain more stable impedance and radiation characteristics across a relatively wide frequency range.
The trade-off is efficiency. Part of the transmitter power is converted into heat in the terminating resistor rather than being radiated. Antenna design therefore requires a balance between directivity, bandwidth, impedance stability and radiation efficiency.
Frequency Range and Electrical Size
Traditional full-size rhombic antennas are most closely associated with HF communication. At these frequencies, the antenna sides are often several wavelengths long, allowing the structure to produce significant directional gain.
A rhombic antenna is not defined by one fixed frequency range. Its dimensions can theoretically be scaled for different frequencies, but the structure becomes extremely large at lower frequencies and requires tighter mechanical tolerances at higher frequencies.
Compact rhombic and rhombic-loop structures may also be used at VHF, UHF or microwave frequencies. However, these scaled designs can have different feeding, polarization and radiation characteristics from a traditional terminated HF rhombic antenna.
Figure 2. Example of a compact rhombic antenna structure for higher-frequency reception.
Terminated and Unterminated Rhombic Antennas
A terminated rhombic antenna normally produces a unidirectional radiation pattern. Most of the useful radiation is directed from the feed point toward the terminating end.
If the terminating resistor is removed, current is reflected from the open end. The antenna then develops a stronger standing-wave component and can produce a bidirectional pattern, with major radiation in two opposite directions.
The unterminated configuration can avoid power loss in a terminating resistor, but its input impedance and radiation pattern generally vary more significantly with frequency. It may also show a poorer front-to-back ratio and less predictable broadband behavior.
Radiation Pattern and Polarization
The radiation pattern of a terminated rhombic antenna is characterized by a forward main lobe and multiple side lobes. The exact beam direction and beamwidth depend on the side length, rhombus angle, installation height, operating frequency and ground conditions.
Increasing the electrical length can improve directivity, but it can also produce additional side lobes. These unwanted lobes may radiate energy toward unintended directions and reduce the practical advantage of the main beam.
A horizontally installed rhombic antenna is primarily horizontally polarized in the main radiation direction. However, the complete three-dimensional pattern may also contain vertical polarization components, particularly in the side lobes and at directions away from the main axis.
Figure 3. Illustrative unidirectional radiation pattern of a terminated rhombic antenna.
Feeding and Impedance Matching
The rhombic antenna is normally a balanced structure with relatively high input impedance. It is often fed using a balanced transmission line or through a suitable impedance transformer and balun.
When coaxial cable is used, the transition between the unbalanced feed line and balanced antenna must be designed carefully. Inadequate matching can increase reflected power, introduce common-mode current and distort the intended radiation pattern.
Engineers should evaluate:
- Input impedance
- Return loss and VSWR
- Terminating resistance
- Forward gain
- Front-to-back ratio
- Beam direction
- Side-lobe level
- Polarization
- Power-handling capability
For high-power transmitting systems, the terminating resistor must be able to dissipate substantial RF power safely.
Advantages
The main advantages of a rhombic antenna include:
- Relatively wide operating bandwidth
- Useful directional gain
- Stable impedance when properly terminated
- Simple wire-based construction
- Suitable for long-distance HF links
- Capable of handling high transmitter power with appropriate components
- Multiple rhombic antennas can be arranged for wider coverage or switching between directions
Limitations
Its principal limitations include:
- Requires a large installation area
- Needs several tall support structures
- Part of the RF power is lost in the terminating resistor
- Can produce significant side lobes
- Performance is affected by height and ground conditions
- Mechanical maintenance can be demanding
- Not suitable for most compact or mobile platforms
Typical Applications
Rhombic antennas have historically been used for:
- Long-distance HF communication
- Fixed point-to-point radio links
- Shortwave receiving stations
- Ionospheric and skywave communication
- International communication networks
- Radio monitoring and signal intelligence
- Research into long-wire and traveling-wave antennas
Although modern communication systems often use log-periodic antennas, broadband arrays, reflector antennas and electronically controlled antenna systems, the rhombic antenna remains an important example in antenna theory.
It clearly demonstrates how conductor length, traveling-wave current, geometric angle and field superposition can be used to control antenna directivity and bandwidth.
RF MISO provides antenna and microwave component solutions for RF testing, communication, radar, antenna measurement and system integration. Understanding classic antenna structures such as the rhombic antenna helps engineers evaluate important parameters including frequency range, gain, polarization, beamwidth, VSWR, side-lobe level and radiation efficiency.
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Post time: Jul-24-2026
