HF Antennas For All Locationspdf
HF Antennas for All Locationspdf
HF Antennas for All Locationspdf
HF antennas are devices that radiate or receive radio waves in the high frequency (HF) band, which ranges from 3 to 30 MHz. HF antennas are used for various purposes, such as amateur radio, shortwave broadcasting, military communication, and international broadcasting. HF antennas can be classified into two main types: directional and omnidirectional. Directional antennas focus the radio waves in a specific direction, while omnidirectional antennas radiate or receive radio waves equally in all directions.
HF Antennas For All Locationspdf
The performance of HF antennas depends on several factors, such as the antenna design, the height above ground, the ground conductivity, the surrounding environment, and the propagation conditions. Therefore, there is no single antenna that works best for all locations and situations. However, some general principles and guidelines can help amateur radio operators choose and install HF antennas that suit their needs and preferences.
Basic Antenna Concepts
Before discussing the types and characteristics of HF antennas, it is useful to review some basic antenna concepts that are relevant for HF communication. These concepts include wavelength, impedance, radiation pattern, gain, polarization, and bandwidth.
Wavelength: The wavelength of a radio wave is the distance it travels in one cycle of oscillation. It is inversely proportional to the frequency of the wave. For example, a radio wave with a frequency of 10 MHz has a wavelength of 30 meters. The wavelength of a radio wave determines the physical size of an antenna. Generally, an antenna should be at least a quarter-wavelength long at the operating frequency to be efficient.
Impedance: The impedance of an antenna is the ratio of voltage to current at the feed point, where the antenna is connected to the transmitter or receiver. Impedance is measured in ohms and can be complex, meaning it has a real part (resistance) and an imaginary part (reactance). The impedance of an antenna depends on its design, length, height, and environment. Ideally, the impedance of an antenna should match the impedance of the feed line and the transmitter or receiver to minimize power loss and reflection.
Radiation pattern: The radiation pattern of an antenna is a graphical representation of how it distributes energy in space. The radiation pattern can be plotted in two dimensions: azimuthal (horizontal) and elevation (vertical). The radiation pattern shows the direction and strength of the main lobe (the area where most of the energy is radiated or received) and the side lobes (the areas where less energy is radiated or received). The radiation pattern also shows the front-to-back ratio (the ratio of power in the main lobe to the power in the opposite direction), the front-to-side ratio (the ratio of power in the main lobe to the power in the perpendicular direction), and the beamwidth (the angular width of the main lobe).
Gain: The gain of an antenna is a measure of how well it concentrates energy in a desired direction. Gain is expressed in decibels (dB) relative to a reference antenna. The most common reference antennas are isotropic (a theoretical antenna that radiates equally in all directions) and dipole (a simple wire antenna that is half-wavelength long). Gain is always with respect to a reference. For example, an antenna with a gain of 5 dBi has 5 dB more gain than an isotropic antenna, while an antenna with a gain of 5 dBd has 5 dB more gain than a dipole antenna.
Polarization: The polarization of a radio wave is the orientation of its electric field. The polarization of a radio wave depends on the orientation of the antenna that transmits or receives it. The most common types of polarization are horizontal (the electric field is parallel to the ground), vertical (the electric field is perpendicular to the ground), and circular (the electric field rotates around the axis of propagation). For optimal signal reception, the polarization of the transmitting and receiving antennas should match.
Bandwidth: The bandwidth of an antenna is the range of frequencies over which it operates efficiently. Bandwidth is measured in hertz (Hz) or as a percentage of the center frequency. For example, an antenna with a bandwidth of 300 kHz at 10 MHz has a bandwidth of 3%. The bandwidth of an antenna depends on its design, length, diameter, and environment. Generally, wider antennas have wider bandwidths.
Types of HF Antennas
There are many types of HF antennas, each with its own advantages and disadvantages. Some of the most common types are described below.
Dipole: A dipole antenna is a simple wire antenna that consists of two equal-length conductors connected to a feed line at the center. A dipole antenna is usually half-wavelength long at the operating frequency and is fed with a balanced feed line, such as a ladder line or a twin-lead. A dipole antenna radiates or receives radio waves equally in all directions perpendicular to its axis, forming a figure-eight pattern. A dipole antenna has an impedance of about 73 ohms in free space, but it varies depending on the height above ground and the surrounding environment. A dipole antenna can be installed horizontally, vertically, or at an angle. A horizontal dipole antenna has horizontal polarization, while a vertical dipole antenna has vertical polarization. A dipole antenna can be modified to operate on multiple bands by adding traps, coils, or stubs.
Inverted V: An inverted V antenna is a variation of the dipole antenna that is shaped like an inverted V. The feed point is at the center of the horizontal wire, while the ends of the wire are sloped downward and attached to supports at a lower height. An inverted V antenna has a lower impedance than a dipole antenna, typically around 50 ohms, making it easier to match with a coaxial feed line. An inverted V antenna also has a wider bandwidth than a dipole antenna, as it is less affected by the ground. An inverted V antenna has a similar radiation pattern to a dipole antenna, but with slightly less gain and slightly more omnidirectional coverage.
End-fed wire: An end-fed wire antenna is a long wire antenna that is fed at one end and terminated at the other end with an insulator or a resistor. An end-fed wire antenna can be any length, but it is usually a multiple of a quarter-wavelength long at the operating frequency. An end-fed wire antenna can be installed horizontally, vertically, or as a sloper (sloping from a high point to a low point). An end-fed wire antenna has a high impedance, typically several thousand ohms, making it difficult to match with a feed line. Therefore, an end-fed wire antenna usually requires an impedance transformer, such as an unun (unbalanced to unbalanced) or a balun (balanced to unbalanced), and an antenna tuner to operate efficiently. An end-fed wire antenna has a complex radiation pattern that depends on its length, height, orientation, and environment. Generally, an end-fed wire antenna radiates or receives radio waves more strongly in the direction of its length.
Vertical: A vertical antenna is a wire or rod antenna that is perpendicular to the ground. A vertical antenna can be any length, but it is usually a quarter-wavelength long at the operating frequency and is fed with an unbalanced feed line, such as a coaxial cable. A vertical antenna requires a ground system to function properly. The ground system consists of radials (wires that extend from the base of the vertical antenna in different directions) or a counterpoise (a wire or metal plate that acts as an artificial ground). The ground system provides the return path for the current and reduces the ground losses. A vertical antenna radiates or receives radio waves equally in all directions in the horizontal plane, forming a circular pattern. A vertical antenna has vertical polarization and low-angle radiation, making it suitable for long-distance communication.
Yagi: A Yagi antenna is a directional beam antenna that consists of one or more driven elements (dipoles that are fed with a feed line) and one or more parasitic elements (dipoles that are not fed but are coupled to the driven elements). The parasitic elements can be reflectors (longer than the driven elements and placed behind them) or directors (shorter than the driven elements and placed in front of them). The reflectors increase the gain and directivity of the Yagi antenna in the forward direction, while the directors increase the gain and directivity of the Yagi antenna in both forward and backward directions. The number and spacing of the elements determine the performance of the Yagi antenna. Generally, more elements result in higher gain and narrower beamwidth. A Yagi antenna can be designed to operate on one band or multiple bands by using traps or other techniques. A Yagi antenna is usually mounted on a mast or tower and rotated by a rotator to point in different directions.
Choosing and Installing HF Antennas
Choosing and installing HF antennas is a challenging but rewarding task for amateur radio operators. There are many factors to consider, such as the available space, the budget, the operating bands, the propagation conditions, the desired coverage, and the personal preferences. Here are some tips and suggestions to help you choose and install HF antennas that suit your needs and preferences. - Do some research before buying or building an HF antenna. Read books, magazines, websites, and forums about