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Rhombic Math & Design

Updated: Feb 7

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Good symmetry is of vital importance for the performance of this antenna. The width of the antenna's main lobe is determined by the angles q and a, often referred to as tilt and apex angle. In general, the wider the rhombic (greater a and smaller q) the wider the beam and vice versa. Of coarse q and a are linked since the sum of half the tilt angle and apex angles is always 90 degrees: q/2 + a/2 = 90

Effective Antenna Aperture Calculator below is useful. Antenna gain G is directly proportional to the antenna aperture A and is increased by means of focusing radiation in only one direction while reducing radiation in all other directions. So, the narrower the width of the beam, the higher the antenna gain.

a 14dBd antenna on 7 MHz is Ae 6,014.99624 m²

a 14dBd antenna on 14 MHz is Ae 1,503.74976 m²

Capture area or Effective Aperture is determined by antenna gain and the wavelength, not by antenna physical size.

The big antennas use wire rope, not copper wire to carry the weight and to keep wire sag lower. The most suitable metal is relevant in relation to the mechanical properties of the install. For rhombic antennas the concern should be using maximum size wire/cable rather than the conductivity of the material. Larger is also better for RF skin effect, and a traveling wave antenna will be terminated anyway.

 A rhombic with 4 wavelength legs is, of course, twice as long overall as a V with 4 wavelength legs, but the width is about the same, since the same angle-based construction is involved. The terminating impedance of a rhombic (600 to 900 Ohms) is in series with the collinear array wires. Therefore, we see far less difference between the gain of the unterminated and the terminated versions. However, we can achieve very high front-to-back ratios.

For commercial service, the major failing of all long-wire technology was the high level of the sidelobes, clearly evident on all of the patterns. The correct V or rhombic angle might combine two or more long-wire lobes, but it did little to suppress the other lobes in the long-wire pattern. For amateur use the side-lobes can be useful.

A V Beam is just 1/2 of a rhombic. The V array derives directly from the single long-wire antenna. In fact, a V array is nothing more than two single long-wires connected at a feedpoint junction and fed in series. The V array makes use of one of the problems for a single wire: the two main lobes do not come completely together to form a single lobe. The V array turns the problem into an advantage. If we angle each leg of the V beam in just the right way, we can get two of the lobes--one from each leg--to point in the same direction and let their gain levels add. Fig. 1 shows the outline of how we obtain a true bi-directional unterminated array from 2 long-wire antennas.

The U.S. Army/Navy made an Rhombic antenna calculators that I have, were developed during the 40's for Army use in field design of antennas. They are large and made of plastic (plexiglass) and are circular slide rule style. It alloew you to calculate tilt angles for antenna wavelength (in frequency), dimensions, etc. I have one but can find it, if you have one sent me a photo.

When Simple Geometries Become Complex A Rhombic Case Study

L. B. Cebik, W4RNL/SK

Also See

Models vs. Prototypes: Why Field Adjustment Will Always be Necessary

L. B. Cebik, W4RNL/SK


General Steve Walz, V31KW/K0UO

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