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Cigar antennas for 1.2GHz, 2.4GHz and 3.4GHz
Matjaz Vidmar, S53MV
1. Slow-wave structures
In his experiments in the time frame 1886-1889, Heinrich Hertz demonstrated the direct relationship between electromagnetic effects and light as predicted by Maxwell equations two decades earlier. Hertz performed most of his experiments in the frequency range around f ~ 450MHz. As directional antennas Hertz used "optical" devices like converging parabolic mirrors and dielectric lenses.
In the time frame 1890-1900, Nikola Tesla, Guglielmo Marconi and several other experimenters obtained much better results at considerably lower frequencies below f < 100kHz. At these frequencies "optical" devices like mirrors and lenses are unpracitcally large. Many experimenters did not even notice the transition between the reactive near field (with nonzero longitudinal field components) and the (radiative) far field (strictly TEM) of their transmitters.
Thanks to much more sensitive PbS (semiconductor) crystal detectors, Jagadish Chandra Bose reached microwaves in the time frame 1894-1897. At microwave frequencies, Bose built the first hollow metal waveguides and waveguide horn directional antennas. Due to inefficient spark-gap transmitters, the radio range of both Hertz and Bose transmitters was unpractically small.
In his long-distance experiments around 1900, Guglielmo Marconi discovered the directional properties of his wire devices and invented the name "antenna". In 1921, Harold Beverage used the traveling wave in a long-wire directional antenna. Since the latter was large and inefficient, Marconi and other engineers turned their attention to different types of broadside antenna arrays.
In 1926, Shintaro Uda invented artificial dielectrics that allowed cheap and lightweight converging lenses for radio antennas. His invention was published in English by Hidetsugu Yagi in 1928. Although Yagi correctly claimed Uda as the inventor of the new directional antenna, the antenna is seldom called Yagi-Uda. More frequently it is called Yagi or just "beam" antenna.
Lenses are built from materials where the phase velocity of electromagnetic waves differs from free space. Although both dielectric and ferromagnetic materials can be used, dielectric-only materials are easier to obtain for high frequencies like visible light. A dielectric with its relative permittivity larger than unity increases the displacement field D for a given electric field E thus slowing down the electromagnetic wave. The same effect can be obtained by filling free space with unconnected metal spheres. The conducting metal spheres may be made hollow allowing significant material savings:
If the direction of the expected electric field E is known, the hollow spheres may be replaced by metal rods aligned in the direction of the electric field E, allowing further material savings. Finally, if the resonance effects due to the near electromagnetic field around metal rods are used correctly, the effect of the metal rods can be increased further and/or additional material saving can be obtained. The invention of Shintaro Uda is therefore typically able to achieve material savings in the 1000:1 range or even more while replacing natural dielectrics with artificial dielectrics. Finally, resonance effects in artificial dielectrics also allow a relative permittivity less than unity or even negative, enabling further degrees of freedom in antenna design.
Lenses used in optics are typically several orders of magnitude larger than the wavelength. Lenses used in radio antennas are usually much smaller than optical lenses when compared to the wavelength. The cross-section of radio lenses may be comparable to or even smaller than the wavelength. Therefore the shape and orientation of radio lenses may be much different from optical lenses.
Radio antennas typically use dielectric rods oriented in the direction of wave propagation (z axis) thus perpendicular to the electric field E. Such radio lenses may use natural dielectrics and/or artificial dielectrics. Radio lenses built from artificial dielectrics are usually called slow-wave structures:
The simplest slow-wave structure invented by Shintaro Uda uses a number of metal rods oriented in the direction of the electric field E and distributed in the direction of the wave propagation. All metal rods together form a much larger artificial-dielectric rod in the direction z. If circular polarization or both linear polarizations are required, crossed metal rods can be used. Slow-wave structures can also be built from wire loops, metal disks and/or a helical wire. In addition, a number of slow-wave structures of complex shapes of metal sheet were invented for wideband UHF TV antennas.
If a thin metal carrier (boom) can be placed precisely in the axis of slow-wave structures made from rods or disks, its electromagnetic effect is small. Unfortunately the electromagnetic effect of an offset metal carrier (boom) is not small thus limiting the operation of many loop-Yagi antenna designs of different shapes to a single linear polarization. A helix antenna requires some insulating mechanical supports in all cases.
If the cross-section of a metal carrier (boom) is comparable to a significant fraction of the wavelength, its electromagnetic effect can not be neglected even in the axis of a slow-wave structure. The rod lengths, the loop circumferences and the disk diameters have to be slightly increased to account for a thick metal carrier. On the other hand, if the influence of a metal boom in the center of the slow-wave structure is accounted for correctly, the electrical performance of the slow-wave structure actually improves compared to a similar bare structure without a metal carrier.