Sloping Dipole

The sloping dipole (Fig. 6-8) is popular with those operators who need a low angle of radiation, and are not overburdened with a large amount of land to install the antenna. This antenna is also called the sloper and the slipole in various texts. The author prefers the term “slipole,” in order to distinguish this antenna from a sloping vertical of the same name. Whatever it is called, however, it is a half-wavelength dipole that is built with one end at the top of a support, and the other end close to the

ground, and being fed in the center by coaxial cable. Some of the same comments as obtained for the inverted-vee antenna also apply to the sloping dipole, so please see that section also. Some operators like to arrange four sloping dipoles from the same mast such that they point in different directions around the compass (Fig. 6-9). A single fourposition coaxial cable switch will allow switching a directional beam around the compass to favor various places in the world.

From Practical Antenna Handbook - Joseph P Carr

Inverted-Vee Dipole

The inverted-vee dipoleis a half-wavelength antenna fed in the center like a dipole. By the rigorous definition, the inverted-vee is merely a variation on the dipole theme. But in this form of antenna (Fig. 6-7), the center is elevated as high as possible from the earth’s surface, but the ends droop to very close to the surface. Angle a can be almost anything convenient, provided that a > 90 degrees; typically, most inverted-vee antennas use an angle of about 120 degrees. Although essentially a compensation antenna for use when the dipole is not practical, many operators believe that it is essentially a better performer on 40 and 80 m in cases where the dipole cannot be mounted at a half-wavelength (64 ft or so). By sloping the antenna elements down from the horizontal to an angle (as shown in Fig. 6-7), the resonant frequency is effectively lowered. Thus, the antenna will

By sloping the antenna elements down from the horizontal to an angle (as shown in Fig. 6-7), the resonant frequency is effectively lowered. Thus, the antenna will

need to be shorter for any given frequency than a dipole. There is no absolutely rigorous equation for calculation of the overall length of the antenna elements. Although the concept of “absolute” length does not hold for regular dipoles, it is even less viable for the inverted-vee. There is, however, a rule of thumb that can be followed for a starting point: Make the antenna about 6 percent shorter than a dipole for the same frequency. The initial cut of the antenna element lengths (each quarter wavelength) is L = ft [6.16]
After this length is determined, the actual length is found from the same cutand-try method used to tune the dipole in the previous section. Bending the elements downward also changes the feedpoint impedance of the antenna and narrows its bandwidth. Thus, some adjustment in these departments is in order. You might want to use an impedance matching scheme at the feedpoint, or an antenna tuner at the transmitter.

The dipole feedpoint

The dipole is a half-wavelength antenna fed in the center. Figure 6-2 shows the voltage (V) and current (I) distributions along the length of the half-wavelength radiator element. The feedpoint is at a voltage minimum and a current maximum, so you can assume that the feedpoint is a current antinode.
At resonance, the impedance of the feedpoint is Ro = V/I. 

There are two resistances that make up Ro. The first is the ohmic losses that generate nothing but heat when the transmitter is turned on. These ohmic losses come from the fact that conductors have electrical resistance and electrical connections are not perfect (even when properly soldered). Fortunately, in a well-made dipole these losses are almost negligible. 

The second contributor is the radiation resistance Rr of the antenna. This resistance is a hypothetical concept that accounts for the fact that RF power is radiated by the antenna. The radiation resistance is the fictional resistance that would dissipate the amount of power that is radiated away from the antenna.



For example, suppose we have a large-diameter conductor used as an antenna, and it has negligible ohmic losses. If 1000 W of RF power is applied to the feedpoint, and a current of 3.7 A is measured, what is the radiation resistance?


It is always important to match the feedpoint impedance of an antenna to the transmission-line impedance. Maximum power transfer always occurs (in any system) when the source and load impedances are matched. In addition, if some applied power is not absorbed by the antenna (as happens in a mismatched system), then the unabsorbed portion is reflected back down the transmission line toward the transmitter. This fact gives rise to standing waves, and the so-called standing wave ratio (SWR or VSWR) discussed in Chap. 3. This is a problem to overcome. Matching antenna feedpoint impedance seems to be simplicity itself because the free-space feedpoint impedance of a simple dipole is about 73 Ω, seemingly a good match to 75-Ω coaxial cable. Unfortunately, the 73-Ω feedpoint impedance is almost a myth. Figure 6-3 shows a plot of approximate radiation resistance (Rr) versus height above ground (as measured in wavelengths). As before, we deal in approximations in Fig. 6-3; in this case, the ambiguity is introduced by ground losses. Despite the fact that Fig. 6-3 is based on approximations, you can see that radiation resistance varies from less than 10 Ω, to around 100 Ω, as a function of height. At heights of many wavelengths, this oscillation of the curve settles down to the freespace impedance (72 Ω). At the higher frequencies, it might be possible to install a dipole at a height of many wavelengths. In the 2-m amateur radio band (144 to 148 MHz), one wavelength is around 6.5 ft (i.e., 2 m ×3.28 ft/m), so “many wavelengths” is relatively easy to achieve at reasonably attainable heights. In the 80-m band (3.5 to 4.0 MHz), however, one wavelength is on the order of 262 ft, so “many wavelengths” is a practical impossibility. There are three tactics that can be followed. First, ignore the problem altogether. In many installations, the height above ground will be such that the radiation resistance will be close enough to present only a slight impedance mismatch to a standard coaxial cable. The VSWR is calculated (among other ways) as the ratio:

good engineering practice (there are sometimes practical reasons) it is nonetheless necessary to install a dipole at less than optimum height. So, if that becomes necessary, what are the implications of feeding a 60-Ω antenna with either 52- or 75-Ω standard coaxial cable? Some calculations are revealing: For 75-Ω coaxial cable:

In neither case is the VSWR created by the mismatch too terribly upsetting. The second approach is to mount the antenna at a convenient height, and use an impedance matching scheme to reduce the VSWR. In Chap. 23, you will find information on various suitable (relatively) broadbanded impedance matching methods including Q-sections, coaxial impedance transformers, and broadband RF transformers.“Homebrew” and commercially available transformers are available to cover most impedance transformation tasks. The third approach is to mount the antenna at a height (Fig. 6-3) at which the expected radiation resistance crosses a standard coaxial cable characteristic impedance. The best candidate seems to be a height of a half-wavelength because the radiation resistance is close to the free-space value of 72 Ω, and is thus a good match for 75-Ω coaxial cable (such as RG-11/U or RG-59/U).

From Practical Antenna Handbook : Joseph J. Carr

Simple Halfwave Dipole Antennas

The simple dipole, or doublet, is a case in point. This antenna is also sometimes called the Hertz, or hertzian, antenna because radio pioneer Heinrich Hertz reportedly used this form in his experiments. The half-wavelength dipole is a balanced antenna consisting of two radiators (Fig. 6-1) that are each a quarter-wavelength, making a total of a half-wavelength. The antenna is usually installed horizontally with respect to the earth’s surface, so it produces a horizontally polarized signal.  

Let say you have L long dipole antenna .

for halfwave dipole antenna L = 1/2L = 1/4 L + 1/4L

In its most common configuration (Fig. 6-1), the dipole is supported at each end by rope and end insulators. The rope supports are tied to trees, buildings, masts, or some combination of such structures. The length of the antenna is a half-wavelength. Keep in mind that the physical length of the antenna, and the theoretical electrical length, are often different by about 5 percent. A free-space half-wavelength is found from
 
                                                         L = 492/Fmhz   feet   [6.1]





In a perfect antenna, that is self-supported many wavelengths away from any object, Eq. 6.1 will yield the physical length. But in real antennas, the length calculated above is too long. The average physical length is shortened by up to about 5 percent because of the velocity factor of the wire and capacitive effects of the end insulators. A more nearly correct approximation (remember that word, it's important) of a half-wavelength antenna is
 
                                                                L = 468/Fmhz  ft [6.2]
 
where L is the length of a half-wavelength radiator, in feet F MHz is the operating frequency, in megahertz
 
Example Calculate the approximate physical length for a half-wavelength dipole operating on a frequency of 7.25 MHz. Solution:
                                                                L = 468/Fmhz  ft

                                                                   = 468/7.25 ft 

                                                                   = 64.55 ft
 
or, restated another way:
                                                                 L = 64 ft 6.6 in
 
It is unfortunate that a lot of people accept Eq. 6.2 as a universal truth, a kind of immutable law of The Universe. Perhaps abetted by books and articles on antennas that fail to reveal the full story, too many people install dipoles without regard for reality. The issue is resonance. An antenna is a complex RLC network. At some frequency, it will appear like an inductive reactance (X = +jXL), and at others it will appear like a capacitive reactance (X =–jXC). At a specific frequency, the reactances are equal in magnitude, but opposite in sense, so they cancel each other out: XL – XC = 0. At this frequency, the impedance is purely resistive, and the antenna is said to be resonant. The goal in erecting a dipole is to make the antenna resonant at a frequency that is inside the band of interest, and preferably in the portion of the band most often used by the particular station. Some of the implications of this goal are covered later on, but for the present, assume that the builder will have to custom-tailor the length of the antenna. Depending on several local factors (among them, nearby objects, the shape of the antenna conductor, and the length/diameter ratio of the conductor) it might prove necessary to add, or trim, the length a small amount to reach resonance.

Fractal Antenna Elements and Arrays

In today wireless communication, there has been an increasing need for more compact, portable and wideband radiators. There is a need to evolve antenna designs to minimum size which can be used in many practical applications in modern 2G, 3G, LTW, WiFi and WiMax wireless communications systems. Fractal antenna is one such antenna which is irregular in shape and it is mainly used for wireless applications. Thus, the objective is to design a novel fractal geometry which exhibits self similarity property and can be confined to space. The new proposed fractal antenna is designed in such a way that it can be operating at a frequency of 2.4GHz. This structure is built up through replication of a base shape, improving antenna performance. The purpose of this project is to explore fractal elements antennas through simulation and design experimentation. In the proposed approach, simulators are carried out using FEKO simulator 6.1 and the results are compared with the existing structures of monopole and Koch fractal. The design is implemented in planar structure also to improve its characteristics when compared to the wire monopole.

 With the advance of wireless communication systems and increasing importance of other wire-less applications, wide-band and low profile antennas are in great demand for both commercial and military applications. Multi-band and wideband antennas are desirable in personal communication systems,small satellite communication terminals, and other wireless applications. Wideband antennas.
Also find applications in Unmanned Aerial Vehicles (UAVs), Counter Camouflage, Concealment and Deception(CC&D), Synthetic Aperture Radar (SAR), and Ground Moving Target Indicators (GMTI).Some of these applications also require that an antenna be embedded into the air frame structure  Traditionally, a wideband antenna in the low frequency wireless bands can only be achieved with heavily loaded wire antennas, which usually means different antennas are needed for different frequency bands. Recent progress in the study of fractal antennas suggests some attractive solutions for using a single small antenna operating in several frequency bands.The purpose of this article is to introduce the concept of the fractal, review the progress in fractal antenna study and implementation, com-pare different types of fractal antenna elements
and arrays and discuss the challenge and future of this new type of antenna.

Fractals

Benoit b.Mandelbrot investigated relationship between fractals and nature using the discoveries made by Gaston Julia, Pierre Fatou and Felix Hausdorff. He showed that many fractals existed in nature and that fractals could accurately model certain phenomena.He introduced new types of fractals to model more complex  structures, such as trees or mountains. as shown at figure 1.

By furthering the idea of a fractional dimension, he coined the term fractal.
  
Mandelbrot defined a fractal as a rough or fragmented geometric shape that can be subdivided in parts, each of which is (at least approximately)
a reduced-size copy of the whole. (A strict mathe-matical definition is that fractal is an object whose Hausdorff-Besicovitch dimension strictly exceeds its topological dimension).Most fractal objects have self-similar shapes although there some fractal objects exist that are hardly self-sim-ilar at all. Most fractals also have infinite complexity and detail, that is, the complexity and detail of the fractals remain no matter how far you “zoom-in,” as long as you are zooming in on the right location. Also, most fractals have fractional dimensions.Fractals can model nature very well. They can be used to generate realistic landscapes or sunsets, wire-frames of mountains, rough terrain, ripples on lakes,coastline, seafloor topography, plants, and ionospheric layers. Fractals can be divided into many types. Figures1 (a, b, c) show some examples.Many theories and innovative applications for fractals are being developed. For instance, fractals have been applied in image compression, in the creation of music from pink noise, and in the analysis of high altitude lightning phenomena [2].

Fractal concepts applied to antennas 

There are many ongoing efforts to develop low profileand wideband antennas such as frequency independent antennas, as reviewed in [3]. One fundamental propertyof classical frequency independent antennas is the irability to retain the same shape under certain scaling transformations, which is the self-similar property shared by many fractals. Several frequency-independent antennas can be generalized as fractal antennas [4],although they had nothing to do with fractals when first developed.

 Figure 2 shows a log-periodic antenna and aspiral antenna, both of which can be categorized as fractal antennas.The general concepts of fractals can be applied to develop various antenna elements and arrays. Applying fractals to antenna elements allows for smaller, resonant antennas that are multiband/broadband and may be optimized for gain. Applying fractals to antenna arrays develops multiband/broadband arrays. The fact that most fractals have infinite complexity and detail can be used to reduce antenna size and develop low profile antennas. When antenna elements or arrays are designed with the concept of self-similarity for most fractals, they can achieve multiple frequency bands because different parts of the antenna are similar toeach other at different scales. Application of the fractional dimension of fractal structure leads to the gain optimization of wire antennas. The combination of the infinite complexity and detail and the self-similarity makes it possible to design antennas with very wideband performance. The first application of frac-tals to antenna design was thinned fractal linear and planar arrays [5]-[9], i.e., arranging the elements in a fractal pattern to reduce the number of elements in the array and obtain wideband arrays or multiband performance. An-other advantage of these fractal arrays is that the self-similarity in their geometric structure may be exploited in order to develop algorithms for rapid computation of their radiation patterns. These algorithms are based on convenient product representations for the array factors and are much quicker to calculate than the discrete Fourier transform approach.Cohen [10] was the first to develop an antenna ele-ment using the concept of fractals. He demonstrated that the concept of fractal could be used to significantly reduce the antenna size without degenerating the performance. Puente et al. [11] demonstrated the multi-band capability of fractals by studying the behavior of the Sierpinski monopole and dipole. The Sierpinski monopole displayed a similar behavior at several bands for both the input return loss and radiation pattern.Other fractals [12]-[13] have also been explored to obtain multi-band or ultra-wideband antennas.

 Fractal antenna elements

 The fractal concept can be used to reduce antenna size, such as the Koch dipole, Koch monopole, Koch loop,and Minkowski loop. Or, it can be used to achieve multiple bandwidth and increase bandwidth of each single band due to the self-similarity in the geometry, such as the Sierpinski dipole, Cantor slot patch, and fractal treedipole. In other designs, fractal structures are used to achieve a single very wideband response, e.g., the printed circuit fractal loop antenna. Several fractal antenna elements are presented in Figure 3.

 Koch monopole and dipole— The Koch curve has been used to construct a monopole and a dipole in order to reduce antenna size. 

One example is shown in Figure 4. In Figure 4(a), the first resonance of a Koch dipole is at 961 MHz while that of a regular dipole with the same length is at 1851 MHz. Therefore, the length of the antenna is reduced by a fact of the 1.9. The current distribution and radiation patterns for both the Koch dipole and the regular dipole at the resonant frequencies are shown in Figure 4(b)-(f). It is worth mentioning that the radiation pattern of a Koch dipole is slightly different from that of a regular dipole because its fractal dimension is greater than 1.Koch loop and Minkowski loop— The Koch curve canalso be used to form a loop of reduced size. Another example is the Minkowski loop formed with a 90-degreebend. Both types of fractal loops can reduce the diame-ter of the loop and achieve approximately the same performance as a regular single wire loop.Sierpinski monopole and dipole— The Sierpinski gasket is a self-similar structure. In an ideal Sierpinski gasket, each of its three main parts is exactly a scaled version of the object (scaled by a factor of two). The self-similarity properties of the fractal shape are translated into its electromagnetic behavior and results in a multi-band antenna. The variation on the antenna’s flare angle shifts the operating bands, changes the impedance
level, and alters the radiation patterns.Cantor slot patch— The Cantor slot patch is another example of multiband fractal structure. This type of patch has been applied in multiband microstrip antennas and multiband frequency selective surfaces.Fractal Tree— Various fractal tree structures have been explored as antenna elements and has been found that the fractal tree usually can achieve multiple wide-band performance and reduce antenna size.Printed circuit fractal loops— The printed circuit fractal loop antenna is designed to achieve ultra wide-band or multiple wideband performance and significant-ly reduce the antenna dimensions. The antenna has a constant phase center, can be manufactured using print-ed circuit techniques, and is readily conformable to an airframe or other structure. 

Fractal antenna arrays 

The concept of the fractal can be applied in design and analysis of arrays by either analyzing the array using fractal theory, or placing elements in fractal arrangement, or both. Fractal arrangement of array elements can produce a thinned array and achieve multi-band performance. Examples are the Cantor linear array, Cantor ring array and Sierpinski carpet planar array, as shown in Figure 5.

 Cantor linear array— Cantor linear array is based on a Cantor set with a number of design variables. When thinned, these arrays have a performance that is superior to their periodic counterparts and appear similar to or better than their random counterparts for a moderate number of elements. Figure 6 shows the calculated array factor of a five level Cantor linear array. 


 The largest distance as shown in Figure 6(a) is d5= 180 cm.Figure 6(b)-(g) shows the plot of array factor at different frequency bands. Itis interesting to note that there are no grating lobes at these frequencies although the distance between array elements is quite large at higher frequency bands.Cantor ring array—Similar to the Cantor linear array, Cantor ring arrays have also been explored to achieve a thinned array and achieve multiple operating frequency bands.Sierpinski carpet planar array— Sierpinski carpet planar array can be considered to be a two dimensional Cantor linear array, alsohaving multiband performance.

 Conclusions 

The concepts of fractals can be applied to the design of antenna elements and arrays. Publications about the electromagnetic theory of fractal structures, and various fractal elements and arrays are redily available. The fact that most fractals have infinite complexity and detail makes it possible to use fractal structure to design smallsize, low profile, and low weight antennas. In addition,most fractals have self-similarity, so fractal antenna elements or arrays also can achieve multiple frequency bands due to the self-similarity between different parts of the antenna. Application of the fractional dimension of fractal structures can lead to gain optimization of wire antennas. The combination of the infinite complexity and detail and the self-similarity makes it possible to design antennas with very wideband performance.There are many interesting issues remaining to be addressed in the application of fractal structures in the design and analysis of antenna elements and arrays. For instance, when fractals are applied to image compression, adjacent parts are independent of one another.This is not the case when fractals are applied to antenna element and array design. Although they are geometrically similar, mutual coupling exists between different parts of a fractal antenna and the nature of that coupling depends on the distance and geometry of the fractal structure.

From Tehnical Feature :
X. Yang, J. Chiochetti, D. Papadopoulos and L. SusmanAPTI, Inc