Rhombic antenna
Encyclopedia
A rhombic antenna is a broadband
directional antenna co-invented by Edmond Bruce
and Harald Friis, mostly commonly used in HF (high frequency, also called shortwave
) ranges.
" diamond shape, with each side typically at least one wavelength
(λ) or longer in length. Each vertex
is supported by a pole, typically at least one wavelength high. A horizontal rhombic antenna (picture below) radiates horizontally polarised waves. Its principal advantages over other choices of antenna are its simplicity, high forward gain and the ability to operate over a wide range of frequencies.
It is typically fed at one of the two sharper angles through a balanced transmission line. Less commonly, it can be fed with coaxial cable through a balun
transformer
. The opposite end is either left open for bi-directional use, or terminated at the opposite sharp angle with a non-inductive
resistor
. It is directional towards the resistor end, so the termination end points towards the region of the world it is designed to serve. Even when unterminated (bi-directional) the rhombic is not perfectly bi-directional. This is because of losses in the system primarily caused by radiation, conductor resistance, and coupling to the lossy soil below the antenna.
The rhombic antenna, like other horizontal antennas, 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. Likewise, its beam can be narrow or broad, depending primarily on its length. A proper combination of size, height, and operating frequency make it fit for medium or long range communication.
A rhombic requires a large area of land — especially if several antennas are installed to serve a variety of geographic regions at different distances or directions or to cover widely different frequencies. The rhombic 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. Typical radiation efficiency is 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.
At the expense of system simplicity, it is possible to improve efficiency by recirculation of power wasted in the termination resistance of unidirectional rhombics. Use of a recirculating termination system can move efficiency into the 70-80% range by combining power that would have been wasted in the termination with the transmitter power. Such systems bring a low-loss balanced line back from the termination end to the feedpoint through a matching and phasing system. Energy that would otherwise dissipated in the termination resistance is applied in-phase with the excitation.
Prior to WWII, the rhombic was one of the most popular point-to-point high frequency antenna arrays. After WWII the rhombic largely fell out of favor for shortwave broadcast and point-to-point communications work, being replaced by log periodics and curtain arrays. Larger log periodics provide wider frequency coverage with comparable gain to rhombics. Distributed feed curtains or HRS curtain arrays provided a cleaner pattern, ability to steer the pattern in elevation and azimuth, much higher efficiency, and significantly higher gain in less space. However, rhombic antennas are used in cases where the combination of high forward gain (despite the losses described above) and large operating bandwidth cannot be achieved by other means.
The rhombic remains one of the least complex medium-gain options for sustained long distance communications over point-to-point circuits. Rhombics also handle considerable transmitter power, since they have essentially uniform voltage and current distribution. The rhombic's low cost, simplicity, reliability, and ease of construction sometimes outweighs performance advantages offered by other more complex arrays.
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
directional antenna co-invented by Edmond Bruce
Edmond Bruce
Edmond Bruce was an American radio pioneer best known for creating the rhombic antenna and Bruce array.Bruce was born in Saint Louis, Missouri, and raised in Cambridge, Massachusetts, Brooklyn, and Washington, D.C...
and Harald Friis, mostly commonly used in HF (high frequency, also called shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
) ranges.
Technical Detail
It is named after its "rhombicRhombus
In Euclidean geometry, a rhombus or rhomb is a convex quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.Every...
" diamond shape, with each side typically at least one wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
(λ) or longer in length. Each vertex
Vertex (geometry)
In geometry, a vertex is a special kind of point that describes the corners or intersections of geometric shapes.-Of an angle:...
is supported by a pole, typically at least one wavelength high. A horizontal rhombic antenna (picture below) radiates horizontally polarised waves. Its principal advantages over other choices of antenna are its simplicity, high forward gain and the ability to operate over a wide range of frequencies.
It is typically fed at one of the two sharper angles through a balanced transmission line. Less commonly, it can be fed with coaxial cable through a balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. The opposite end is either left open for bi-directional use, or terminated at the opposite sharp angle with a non-inductive
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
. It is directional towards the resistor end, so the termination end points towards the region of the world it is designed to serve. Even when unterminated (bi-directional) the rhombic is not perfectly bi-directional. This is because of losses in the system primarily caused by radiation, conductor resistance, and coupling to the lossy soil below the antenna.
The rhombic antenna, like other horizontal antennas, 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. Likewise, its beam can be narrow or broad, depending primarily on its length. A proper combination of size, height, and operating frequency make it fit for medium or long range communication.
A rhombic requires a large area of land — especially if several antennas are installed to serve a variety of geographic regions at different distances or directions or to cover widely different frequencies. The rhombic 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. Typical radiation efficiency is 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.
At the expense of system simplicity, it is possible to improve efficiency by recirculation of power wasted in the termination resistance of unidirectional rhombics. Use of a recirculating termination system can move efficiency into the 70-80% range by combining power that would have been wasted in the termination with the transmitter power. Such systems bring a low-loss balanced line back from the termination end to the feedpoint through a matching and phasing system. Energy that would otherwise dissipated in the termination resistance is applied in-phase with the excitation.
Prior to WWII, the rhombic was one of the most popular point-to-point high frequency antenna arrays. After WWII the rhombic largely fell out of favor for shortwave broadcast and point-to-point communications work, being replaced by log periodics and curtain arrays. Larger log periodics provide wider frequency coverage with comparable gain to rhombics. Distributed feed curtains or HRS curtain arrays provided a cleaner pattern, ability to steer the pattern in elevation and azimuth, much higher efficiency, and significantly higher gain in less space. However, rhombic antennas are used in cases where the combination of high forward gain (despite the losses described above) and large operating bandwidth cannot be achieved by other means.
The rhombic remains one of the least complex medium-gain options for sustained long distance communications over point-to-point circuits. Rhombics also handle considerable transmitter power, since they have essentially uniform voltage and current distribution. The rhombic's low cost, simplicity, reliability, and ease of construction sometimes outweighs performance advantages offered by other more complex arrays.
Advantages of Rhombic Antennas
- Its input impedance & radiation pattern are relatively constant over a 2:1 range of frequencies. Its impedance can be made relatively constant over a frequency range 4:1 or more, with the forward gain increasing at 6 dB per octave.
- Multiple rhombic antennas can be connected in an end-to-end fashion to form MUSA (Multiple Unit Steerable Antenna). MUSA arrays can receive long distance, short wave, horizontally polarized down coming waves.
- In addition to its use as a simple and effective transmitting antenna (as described above), the rhombic can also be used as an HF receiving antenna with good gain and directivity. For example, BBC Monitoring's Crowsley Park receiving stationCrowsley ParkCrowsley Park is a 160-acre country estate in South Oxfordshire, central-southern England, owned by the British Broadcasting Corporation .- Overview :...
has three rhombic antennas aligned for reception at azimuths of 37, 57 and 77 degrees.