Rhombic and V Beam Design
- skylarkcolo
- Nov 19, 2021
- 6 min read
Updated: Oct 22
The rhombic is the largest and most refined of the long-wire antennas, consisting of two Vs, open-end to open-end. The result is 4 wires contributing aligned lobes for higher gain and narrower beamwidth. And the rhombic suppresses unwanted sidelobes better than the V antenna
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 of the side-lobes can be useful for making QSOs.
When an antenna is good at what it does, we can count on efforts to make the good even better. For narrow-beamwidth point-to-point communications, the rhombic is very good. One very old technique to improve performance somewhat is the use of multiple wires in each side of the rhombic. They come together at the feedpoint and at the terminating resistor end, but spread vertically where the facing Vs are widest. Some claims about the technique will prove correct, such as the addition of a small increment of gain. However, other claims may turn out to have other foundations than the use of multiple wires.
I only use single wire designs at my site, less work. If I want more gain, I just make the antenna longer, however some of you don't have the space.. a 3 wire arrays only gives about 1 dB gain. But you can enlarge the average wire diameter, the gain does increase by a numerically noticeable amount. Also the array of front-to-back values are better..
Laport developed a scheme for using closely spaced rhomboid structures in parallel. The centerlines for each of the independent rhomboids fed in parallel are offset from each other. The technique will offer a small gain advantage over the single-wire rhombic, but will reduce sidelobes by a very significant amount.
Fig. 10 provides a 3-dimensional pattern for the rhombic with 10 wavelength legs. It reveals that the terminated rhombic exerts the most control over the morass of small lobes that populate the overall radiation pattern Also the.relationships between the value of the terminating resistor and the feedpoint impedance that bear on the smoothness of SWR curves that cover a 2:1 frequency range. The termination provides considerable more bandwidth up to 4 to 1.
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 type slide rules. It allowed you to calculate tilt angles for antenna wavelength (in frequency), dimensions, etc. I have one but can't find it, if you have one please sent me a photo.
SEE Above: Now you need a non inductive terminating resistor
Initial and later studies in rhombic antennas provide more complex equations to calculate compromises where the elevation and the V'ing angle do not match. Some of the equations appear in nomographic form. For example, one such nomograph appears in the ARRL chapter on long-wire and traveling-wave antennas, as well as in articles and text devoted specifically to the design of rhombic antennas. (See the Harper volume in the reference list.) Such nomographs are capable of guiding the rhombic designer to excellent results.
Some programs are limited to 500 segments. Tthe longer rhombics require up to 1000 segments, if we adhere to our 20-segment per wavelength standard. However, a full 6 wavelength-per-leg rhombic comes in at under the 500 segment mark.
We may use the selected height and the associated values of angle A to design any number of rhombic antennas. In fact, we can use a simple long-wire as the starting point. NEC allows us, via the GM command, to rotate the wire by the required number of degrees dictated by the value of angle A for a given wire length. (Programs like EZNEC use a different but equally effective method of rotating wires.) Hence, we can easily create a V and find its coordinates. From those coordinates, we can complete the rhombic by doubling the overall length and bringing 2 new wires back together--or almost together.
The use of angle A assures us of lobe direction coincidence and gain addition along the centerline of the antenna. We may then let NEC calculate the gain and actual elevation angle for the selected antenna height over any selected soil. Before we close this series, we shall find that NEC's handling of rhombic design and at least one nomographically based design turn out to be virtually identical. Traditional methods are quite accurate, but in the present age of computerized antenna design, the modeling process is often simpler. As we have seen from our experience with single long-wire and V antennas, the modeling method also provides ready supplementary information, for example about sidelobes, feedpoint impedances, and power dissipation in the load resistance of terminated antennas.
I am now using the biggest HF Re-entrant Rhombic arrays, and they are the highest forward gain HF antennas with its 90% efficiency, now with very high gain, and low noise receive characteristics. So, the Rhombic Arrays can beat the massive stacked HF beam arrays that I had up before.
How to build a 20 meter Rhombic
e 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
https://www.translatorscafe.com/unit-converter/en-US/calculator/effective-antenna-aperture/
73
General Steve Walz, V31KW/K0UO