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Antennas come in a wide range of sizes and shapes, depending on their intended use and the frequency and wavelength of the signals they're designed to handle:
Size
Antennas can be as small as a piece of wire or rod for a car radio, or as large as those that capture satellite signals. The size of an antenna is determined by the frequency and wavelength of the signals it's designed to handle. For example, antennas for higher frequencies need to have smaller elements so that more can fit within a given physical height.
Shape
Antennas can be simple or complex in shape, depending on their intended use. For example, directional antennas have a narrow beamwidth in the shape of a lobe, while omnidirectional antennas have a more spherical propagation.
Antenna Size
Frequency Dependency:
The size of an antenna is related to the wavelength, λ of the signal.
Ideal antenna size is typically a fraction of the wavelength, often λ/2 or λ/4.
Example: c=f λ: A signal at 1 GHz (wavelength ~30 cm) would have an ideal dipole antenna length of λ/2=15 cm.
Large Antennas:
Used for low-frequency (long wavelengths), such as in AM radio.
Often impractical for very low frequencies due to their large physical size.
Can collect more power and provide better signal quality.
Small Antennas:
Used for high-frequency signals (short wavelengths), such as Wi-Fi or 5G.
Easier to integrate into compact devices like smartphones.
The dipole antenna consists of two linear conductors end-to-end with a length of one half wavelength (λ/2).
Dipole construction consists of two λ/4 elements end-to-end with a few in the center by a transmission line. At resonance, the antenna appears to be a 73Ω resistor. A dipole’s horizontal radiation pattern looks like a figure 8. In 3D, the pattern is shaped like a doughnut with maximum radiation perpendicular to the length of the antenna.
A popular variation of the dipole is the ground plane or Marconi antenna. It consists of a single λ/4 element that’s mounted vertically and it works with the earth or a metallic base called the ground plane. The ground plane antenna is just a half of a dipole, with the other element of the dipole represented by the ground plane. The polarization is vertical and the radiation pattern is circular or omnidirectional.
The patch or microstrip antenna is common at microwave frequencies (greater than 1 GHz). It’s a square or circular patch of conductive material about one half-wavelength across. Creating one is easy because it’s usually implemented on a printed-circuit board (PCB). The loop antenna is also popular in some noncritical applications. It’s just a continuous loop of conductor, wire, or PCB trace.
Antenna Common Shape and Their Applications:
Dipole Antenna:
Straight wire, often λ/2 long.
Used in FM radios and some Wi-Fi routers.
Loop Antenna:
Circular or rectangular loops of wire.
Used in RFID systems and AM radio receivers.
Yagi-Uda Antenna:
Linear arrangement of elements.
Used in TV antennas and ham radio.
Parabolic (Dish) Antenna:
Reflective surface shaped like a parabola.
Used in satellite communications and radar.
Patch (Microstrip) Antenna:
Flat, rectangular or circular.
Common in mobile devices and IoT applications.
Helical Antenna:
Spiral-shaped.
Used in space communications.
Shape Impact on Antenna Performance:
Beamwidth and Directionality:
Narrower shapes like dishes focus energy into a beam, providing high directionality.
Broad shapes like dipoles radiate more uniformly in multiple directions.
Polarization:
Shape affects the electric field's orientation, influencing compatibility with the signal source.
Efficiency and Gain:
Complex shapes like fractal antennas maximize surface area, improving bandwidth and efficiency in compact designs.
Practical Considerations
Space Constraints:
Portable devices require small, compact antennas like chip or patch antennas.
Environmental Conditions:
Rugged shapes like loops can be more durable in harsh conditions.
Multi-Band Use:
Antennas like fractals or log-periodic support multiple frequencies, shaped to resonate at various wavelengths.
By balancing size and shape with the intended operating frequency, environment and application, designers optimize antenna performance for specific needs.
