What is a Rhombic?
Updated: Aug 1
The ARRL and RSGB Diamond Logo is in fact, a Rhombic Antenna
The Rhombic antenna was designed in the 1931 by Ed Bruce and Harald Friis.
It was commonly used in the HF shortwave point to point and broadcast as a broadband directional antenna, it is also known as a diamond antenna. See 1931 patent, US 2285565A1 I (aktuellum.com)
There are two types of the rhombic antennas, the resonant rhombic antenna and the terminated rhombic antenna. The resonant rhombic antenna is bidirectional, but the terminated rhombic antenna is unidirectional. Generally, the antenna is terminated with a value equivalent to characteristic impedance thereby causing the non-resonant condition to establish. Making the radiation characteristics of the antenna are unidirectional. When the power through the feed lines either the 2-wire transmission line is provided to the antenna. Then the generated current travels through the legs of the rhombic antenna (Traveling Wave).
These currents flow through the antenna and generate radio waves that progress in one direction through the legs of the antenna.
Two great books to read are, Rhombic Antenna Design by A.E. Harper of Bell Labs, and W6AM, which shows Dow Wallace’s antenna farm after WW 2 to the 1980s.
The key concept with traveling-wave type antennas like a Rhombic or Vee Beam (V-Beam) is that there are no standing waves, on the antenna itself, the current and voltage levels are the same everywhere along the antenna conductors, but you still have to match it to the fed-line.
The Rhombic antenna is a wide-band progressive traveling-wave (fast-wave) antenna, made of two acute-angle V-beams placed end-to-end and terminated in an open circuit or in a resistive load. Each side of the antenna is made of two legs of length "L" and as a whole the antenna has the shape of a rhombus, that is, the opposite angles are of the same value. The non-terminated rhombic antenna is bi-directional, whereas the terminated Rhombic antenna is directional. The Rhombic antenna is useful over a wide frequency range. Although some changes in gain, directivity, and characteristic impedance do occur with a change in operating frequency, these changes are small enough to be neglected. A rhombic antenna design works best at a height of one-half to a full wavelength at the lowest F.
So no waiting for a rotator to turn, the system has every direction, every band, every time.
The Rhombic is an equilateral parallelogram shaped antenna, it has two opposite acute angles. The tilt angle, θ is approximately equal to 90° minus the angle of major lobe. Rhombic antenna works under the principle of a fast travelling wave antenna. It is setup in the form of a rhombus or diamond shape and is normally suspended horizontally above the surface of the earth, but can be made vertical. It works great for long-distance F-layer propagation due to low vertical radiation angle, however it does have some higher radiation lobes which were thought as wasted power for long point to point use but for ham radio this help fill in closer in coverage, but overlooked in modeling and a major advantage of the antenna for ham radio use, for making more QSOs.
In designing the Rhombic it has to be kept in mind that length of all the four conductive wires must be equal, ranging between one wavelength, to over four. However, the opposite acute angles of the rhombus must be equal.
To avoid reflections of the traveling wave the opposite end of the antenna feed line is terminated with a properly adjusted resistor. This leads to cause the absence of standing waves in any of the legs of the antenna. The value of load resistance is generally around 600 to 800 ohms which is also the impedance.
Rhombics' input impedance and radiation pattern are constant over a 2:1 range of frequencies or more. Their impedance is constant over a frequency range 4:1 or more, with the forward gain increasing at 6 dB per octave.
The resultant pattern is the cumulative effect of the radiation at all four legs of the antenna. This pattern is uni-directional, it can be made bi-directional by removing the terminating resistance.
The maximum gain from a Rhombic is along the direction of the main axis, which passes through the feed point to terminate in free space. The polarization obtained from a horizontal Rhombic is in the plane of rhombus, which is horizontal. But portions of the radiation, which do not combine with the main lobe, result in considerable side lobes having both horizontal and vertical polarization which can be good for ham radio use.
It can be truthfully be said, that "a Rhombic antenna occupies more space per db of gain than any other antenna"! The Rhombic has one of the poorest gain-per-acre rankings of any high gain HF antenna array. On the other hand, a Rhombic antenna does have the distinct advantage of working over very wide frequency ranges with flat SWR and high gain, something a basic monoband yagi can never do. The Rhombic is also a very simple antenna, requiring only four supports, three supports for the Vee beam, and one support for inverted Vee derivatives. If you have a large rural property, you may want to design, build, and use a Rhombic Antenna.
The Rhombic is excellent for point-to-point communications and exhibits a very low takeoff angle--a definite plus for DX it also so has some higher angle which is great for ham use, it fill in the gaps.
Easy to construct
L should be long enough (2 to 4l at the lowest frequency)
The values of q and a determining the shape of the main lobe
Symmetry of the total antenna system, including balun, feed lines and resistor. This seems simple to do, but really is not! First you need a very good 12:1 current balun that works properly over the entire frequency range of the rhombic.
You need a high power, non inductive load that is electrically symmetrical around a central ground tab (see info else where on this blog about resistors easy to find)
Then you have a huge antenna with pinpoint accuracy.
One of the remote relay control boxes used at K0UO
Above: Note for amateur radio use the minor higher loops are very useful for making closer in contacts. The higher and split forward lobes were considered useless and a waste of RF power for the Point to Point stations in the past. The ARRL did put it to good use with their antenna for good stateside coverage (1930 to 1980s). See the ARRL coverage chart in below, and since going to other antennas, they have never been able to have the same consistent RF field strength throughout the lower 48 States.
A Vee Beam is just 1/2 of a Rhombic
Above is a control box used at K0UO, with a 12 to 1 current balun with lightning protection
Patents for Rhombic: US2517238A - Radiating termination for a rhombic antenna - Google Patents