Rhombic antenna

Lore & Background

The rhombic antenna was designed in 1931 by Edmond Bruce and Harald Friis. It was most commonly used in the high frequency (HF) or shortwave band as a broadband directional antenna. Prior to World War II, it was one of the most popular point-to-point high frequency antenna arrays. After World War II, the rhombic largely fell out of favor for shortwave broadcast and point-to-point communications work, being replaced by log periodic antennas and curtain arrays.

Each of the four antenna sides is the same length – about a quarter-wavelength to one wavelength per section – converging but not touching at an angle of about 42° at the fed end and at the far end. The length is not critical, typically from one to two wavelengths (λ), but there is an optimum angle for any given length and frequency. A horizontal rhombic antenna radiates horizontally polarized radio waves at a low elevation angle off the pointy ends of the antenna. If the sections are joined by a resistor at either of the acute ends, the antenna will receive from and transmit to only the direction the end with the resistor points at.

The rhombic antenna can radiate at elevation angles close to the horizon or at higher angles, depending on its height above ground relative to the operating frequency and its physical construction. Its beamwidth can be narrow or broad, depending primarily on its length. The shallow radiation angle makes it useful for skywave propagation, the longest distance mode for shortwave, in which radio waves directed into the sky at the horizon reflect from layers in the ionosphere and return to Earth far beyond the horizon.

Reader's Guide

The rhombic antenna's significance lies in its combination of simplicity, high forward gain, wide bandwidth, and ability to operate over a wide range of frequencies. Its principal advantages over other antenna types are these characteristics, along with low cost, reliability, and ease of construction. The rhombic remains one of the least complex medium-gain options for sustained long distance communications over point-to-point circuits. It also handles considerable transmitter power, since it has essentially uniform voltage and current distribution. Multiple rhombic antennas can be connected end-to-end to form MUSA (Multiple Unit Steerable Antenna) arrays for receiving long distance, short wave, horizontally polarized downcoming waves.

However, the rhombic requires a large area of land, especially if several antennas are installed to serve different geographic regions or frequencies. It suffers from efficiency problems due to earth losses below the antenna, significant power-wasting spurious lobes, termination losses, and the inability to maintain constant current along the length of the conductors, with typical radiation efficiency in the order of 40–50%. The low efficiency significantly reduces gain for a given main lobe beamwidth when compared to other arrays of the same beamwidth. Despite these drawbacks, rhombic antennas are still used in cases 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.

Did You Know?

• It can be made unidirectional by terminating one acute end with a resistor.
• Typical radiation efficiency is in the order of 40–50%.