"Mini-Beam Geometry Basics"
By Dick Stevens, N1RCT
There is a natural progression of the
the "mini-beams" deriving from the basic yagi, which in itself owes a lot to the common dipole. As an aid to seeing how several types of beam relate to each other, I have drawn them all following the outline of the simple square box shown here and all views are from directly above. . I have colored the driven element Red and the parasitic element Green. Structural booms are shown as dark gray, tubing as wide and wire as narrow. While the beams may get proportionately much smaller, I have shown them in the same size box. I have not included any of the hundreds of variations on a theme such as loading coils, top hats, and the like. All the examples are horizontal mono-beams of two elements. Some arrangements are much easier to make multi-band and/or multi-element. The examples here are also all horizontal beams but if you visualize for a moment, arrangements like the Quad (square or diamond) will appear to you. I would like to be able to include the radiation pattern polar diagrams and the 3-D pictures of the antennas but space on my ISP does not permit. In all these pictures, the length of an element is about 1/2 wavelength.
Here is the simple dipole,
which is the most basic of all antennas. It has a maximum gain in
two directions, which reduces the power that can beamed in one
desired direction and increases the noise that will be picked up
from behind when listening. On the other hand, the dipole can
"hear" a lot of stations and not just ones it is
pointed at, which can be an advantage in some situations. The
quest for an enormous front-to-back ( F/B ) ratio can
hurt in some situations we will talk about.
It is important to remember that electrons exchange energy only via photons. An electric current (electrons) can generate photons which can be absorbed by other electrons in the wire or released into space, where we call the photons radio waves, visible light, X-Rays, etc depending on their frequency / energy. A parasitic element is picking up photons from the driven element and the resulting current in the parasitic elements causes the electrons to give off photons again if we have designed everything just right. This new stream of photons interacts with the original stream from the driven element to cause the peaks and valleys of the resulting radiation pattern. Radio waves / photons do not physically reflect from a reflector but are first attracted to and absorbed by free electrons in the tubing and then re-emitted. They join the stream of other photons but are at a different point in the sine wave (phase) which can cause them to add constructively or destructively to other photons, resulting in the observed radiation pattern.
The big evolutionary jump from the dipole is
parasitic element addition. This makes a major increase in the forward gain and a very high F/B ratio. The classic arrangement is on a boom which is about 1/4 wavelength long. (The popular short boom version is abt. 1/8 wavelength). The dipole elements are also 1/4 wavelength, each side. Arranging this in our box basically adds a parasitic element on the left hand side of the box, a reflector. The two arms form an electronic circuit and energy is coupled or induced from the driven element to parasitic reflector. This coupling is key to any parasitic element and is the only way it can get the current from the driven element that it immediately re-radiates but just enough later in time to cause constructive interference in the desired direction and destructive interference in the opposite direction. All parasitic element must capture current and then re-radiate with just the right phase difference, caused by the distance the radio waves had to travel, which is the element spacing.
The next step in the evolution of the mini-beam is to increase the coupling between the elements by bending in the elements toward each other. This is a " U " shaped element and I call it a Moxon beam as I first read about it in Les Moxon G6XN 's book. To digress a moment, I have modeled all these shapes using Antenna Optimizer by K6STI which permits modeling the geometry as algebraic equations rather than constants like most other antenna programs. AO will then attempt to optimize various antenna parameters by adjusting the element dimensions. I developed a general set of equations which covers all the antennas mentioned here and it is near magic to see program slowly change the shape and angles of an antenna as it searches for the best arrangement. Thanks, Brian. Those little tips on the beam ( shown narrow to simulate wire instead of tubing ) make the antenna a little shorter and generally improve the F/B, Gain, and SWR over the plain two-element yagi.
Once we start bending elements willy-nilly, there are many ways to do it. The ends could have pivoted in to make a shallow or deep "v" shapes; the main element lengths do not have to be equal which gives rise to a trapezoid shape; each element can be "folded" like a "folded dipole", etc. We can also make the design so that the tips are a major part of the total length and not just coupling reactance, which results in a much reduced turning radius of the beam. This one is named after Fred Caton, VK2ABQ and has excellent characteristics. It is also easily designed as a tri-bander with additional wire elements in the center. As a very general statement, wire reduces the bandwidth over tubing. This design uses four insulated rods to hold the wires in position. The design calls for the element tips to be tied together using a coat button as an insulator, so the tips come very close together. In fact, AO shows that they can be overlapped (but insulated) for best F/B ratio.
If we look at the VK2ABQ beam and say .. "lets make those insulated arms out of aluminum and put them to work" ... We get the " X " beam antenna, which is really more a " W " than an " X ". A continuous light cord connecting the ends of the arms provides considerable rigidity. This beam in general has a very poor front-to-back ratio, typically 10 dB but up to about 17 dB can be achieved at the expense of forward gain if desired. With this antenna, there is additional strong coupling achieved at the center in the form of capacitive coupling while the end wires contribute inductive coupling. Small adjustments to the outer wires make large changes in the antenna complex resistance. This permits a wide variation of electrical designs from a very wide bandwidth design at moderate gain to very high gain at narrow bandwidth. The X Beam is my choice for RTTY contesting and I will be writing articles on details of my designs, electrical and mechanical. For the moment, I will point out that the poor front-to-back ratio of the X-Beam is an advantage for me in contesting as I can work the USA off the back of the beam while targeting Europe with the maximum gain direction.
The last variation we will show is the " Hex " beam which is a re-angling of the X-Beam arms from 45 degrees to 30 degrees and the addition of two insulating arms to move the ends out at an angle. Actually, drawing this in a box is not a pure hexagon; a circle with the arms as radii is the correct shape, in order that the insulated rod be the same length as the other arms. This arrangement solves the F/B problem of the X-Beam and further reduces the turning radius (to about 10 feet for a 20M beam). Is this the ultimate? Probably, in a practical sense, but we can visualize something like a Pfeiffer Quad element cut up a bit. Les Moxon, G6XN has designs that get into the three-dimensional realm by bending the insulated arms of the VK2ABQ beam up in the air and using them as a 3-D multi-band antenna (which look much like the sail on a Polynesian canoe).
Starting with the VK2ABQ design, these antennas are low windage and low weight and are quite at home on a TV rotor and light-weight tubing pole/towers. Often the gain of the resulting antenna structure can be higher if we can get a light-weight, low windage antenna up higher than we could a larger and heavier antenna. Height is all-important and the mini-beams are much easier to get up high. The light weight of the pole and antenna permit small ropes for guy wires .. I use 3/16" Dacron every 10' and the guy anchors are ordinary screw hooks in trees and buildings. The resulting 52 foot structure for 20 meters I use has quite a low visibility. The cost is a small fraction of the conventional tower arrangements and the low angle gain at 52 feet is better than a THREE element monoband Yagi at 37 feet.
With inexpensive poles and rotors, monobanders for 15 and 10 meters on their own arrangements become practical and permit a second rig for monitoring other bands while calling CQ on the main rig. Multi-band antennas are not good for this.