Frequency independent Antenna Networks and Communication Department Dr. Marwah Ahmed

History of Frequency-Independent Antenna Frequency independent antennas had their beginning in research conducted in the late 1950s An early nonmilitary application of frequency- independent antennas was the logperiodic dipole TV antenna that was used on houses around the world starting in the 1960s. 1-Jan-19 Networks and Communication Department

Principle of Frequency-Independent Antenna it is desirable to have the pattern of an antenna remain constant over a very wide range of frequencies. An antenna with a bandwidth of about 10:1 or more is referred to as a frequency- independent antenna. The purest form of a frequency-independent antenna has constant pattern, impedance, polarization, and phase center with frequency. Few antennas meet all these criteria. 1-Jan-19 Networks and Communication Department

Characteristics that Produce Broadband Behavior The characteristics that yield broadband behavior are: Emphasis on angles rather than lengths: avoid fixed physical length elements and produce wide bandwidth. Rumsey [H.8.3: Rumsey] noted that, in practice, frequency- independent antennas are designed to minimize finite lengths and maximize angular dependence. The concept of angle emphasis has been exploited to produce a family of frequency-independent antennas. 1-Jan-19 Networks and Communication Department

Characteristics that Produce Broadband Behavior Self-complementary structures: self-complementarity: Consider a metal antenna with input impedance 𝑍 𝑚𝑒𝑡𝑎𝑙 . A dual structure can be formed by replacing the metal with air and replacing air with metal. The resulting complementary antenna has input impedance 𝑍 𝑎𝑖𝑟 . An example is a ribbon dipole and its complement, the slot antenna, shown in Fig. 7-27. 1-Jan-19 Networks and Communication Department

Characteristics that Produce Broadband Behavior Thick metal—“fatter is better.” Increasing the wire diameter of even resonant antennas such as a dipole widens its bandwidth. Ideally, frequency-independent antennas should display all three of these properties. It is found in practice that successful wide bandwidth designs emphasize these properties, but in many cases strict adherence is not required. 1-Jan-19 Networks and Communication Department

Feature of Frequency-Independent A distinguishing feature of frequency-independent antennas is their self-scaling behavior. Most radiation takes place from that portion of the frequency-independent antenna where its width is a half-wavelength or the circumference is one wavelength—the so-called active region. Radiation is maximum perpendicular to the plane of the structure and can be explained in a fashion similar to the one-wavelength loop 1-Jan-19 Networks and Communication Department

SPIRAL ANTENNAS 1-Jan-19 Networks and Communication Department

SPIRAL ANTENNAS Spiral antennas and their variations are usually constructed to be either exactly or nearly self- complementary. This yields extremely wide bandwidths of up to 40:1. 1-Jan-19 Networks and Communication Department

Equiangular Spiral Antennas The equiangular spiral curve shown in Fig. 7-28 is represented by the generating equation: 𝑟= 𝑟 0 𝑒 𝛼∅ where 𝑟 0 is the radius for ∅=0 and 𝛼 is a constant controlling the flare rate of the spiral. 1-Jan-19 Networks and Communication Department

Equiangular Spiral Antennas 1-Jan-19 Networks and Communication Department

Equiangular Spiral Antennas The spiral of Fig. 7-28 is right-handed. Left-handed spirals can be generated using negative values of 𝛼, or by simply turning over the spiral of Fig. 7-28. 1-Jan-19 Networks and Communication Department

Planar Equiangular Spiral Antenna The equiangular spiral curve is used to create the antenna of Fig. 7- 29, which is referred to as the planar equiangular spiral antenna. The structure of Fig. 7-29 is self-complementary, so 𝛿= 𝜋 2 . It does not have to be constructed this way, but pattern symmetry is best for the self-complementary case. 1-Jan-19 Networks and Communication Department

Planar Equiangular Spiral Antenna The impedance, pattern, and polarization of the planar equiangular spiral antenna remain nearly constant over a wide range of frequencies. The feed point at the center, the overall radius, and the flare rate affect the performance. The radiation pattern of the self-complementary planar equiangular spiral antenna is bidirectional with two wide beams broadside to the plane of the spiral. 1-Jan-19 Networks and Communication Department