Chapter 6 Antennas Antenna Basics
Antennas contain conducting elements (wire or metal poles). Radio waves are electromagnetic waves. An electromagnetic wave contains an electric field component and a magnetic field component. The electric and magnetic fields are at right angles to each other.
The polarization of an antenna is defined as the orientation of the electric field component relative to the Earth’s surface. The electric field component is in the plane of the elements. The feed point impedance is the ratio of the RF voltage to current at the feed point. A resonant antenna will have only a resistive feed point impedance (no reactance)
The radiation pattern of an antenna is a plot of its signal strength in every direction (3D)
An azimuthal pattern is a 2D slice of the radiation pattern in the horizontal plane. An elevation pattern is a 2D slice of the radiation pattern in the vertical plane. Lobes are areas where the antenna is radiating and nulls are the points between the lobes where no, or little, occurs.
An isotropic antenna radiates equally well in all directions
An isotropic antenna radiates equally well in all directions. This is a spherical pattern with one lobe (the sphere) and no nulls. Isotropic antennas are used as a reference. Omnidirectional antennas radiate equally in all horizontal directions. Directional antennas has a main lobe(s) in one, or more directions. Gain, measured in dB, is the ratio of power transmitted in a given direction over the same power from a dipole (dBd) or isotropic radiator (dBi).
A directional antenna has a front-to-back ratio (measured in dB) which is the ratio of the gain of the antenna in the forward direction to the gain in the opposite direction. The front-to-side ratio (measured in dB) is the ratio of the forward power to the power radiated at right angles.
Dipoles Dipoles are simple, but effective HF antenna. They are a half wavelength long with the feed point in the middle.
A dipole in free space has an feed point impedance of 72 Ω, but varies at lower heights. The feed point impedance increases to several thousand ohms as it is moved away from the center to the ends.
The free space length of a dipole in feet is given by 492/f, where f is the frequency in MHz. In practice, the actual length is closer to 468/f, but use 492 for the test. Example: Calculate the free space length of a ½ wave dipole for 3.75 MHz. 492/3.75 = ft Always start with a longer length and then trim to get the desired frequency. Dipoles cut for one frequency can be used at odd harmonics. For example, 7 MHz (40 M) dipole will also work at 21 MHz (15 M).
Ground Planes (Verticals)
Ground plane antennas are essentially vertical dipoles with the lower half replaced by a ground plane.
The pattern is omnidirectional (turn Fig 6-1(B) on its side).
HF ground planes are typically mounted with the vertical element on the ground and radials laid on the ground or just below the surface. The pattern is omnidirectional (turn Fig 6-1(B) on its side). The free space length in feet of a vertical is 246/f, where f is the frequency in MHz. Example: Calculate the length of a ¼ wave vertical at 21.2 MHz. 246/21.2 = 11.6 ft.
Feed point impedance is about ½ that of a free space dipole, or 35 Ω.
The impedance can be increased by increasing the droop angle of the radials.
HF mobile antennas are usually verticals which are much shorter than a full size ¼ wave. Loading is used to increase the elements electrical size. Loading decreases the operating bandwidth without retuning. Loading techniques include; Loading coils at the base or in the middle. Capacitance hats at the top of the antenna. Linear loading is when the antenna is folded back on itself. A corona ball at the tip of the antenna acts to eliminate high voltage discharges from the tip of the antenna.
Effects of Height Above Ground
Random wire antennas are non-resonant antennas that require a antenna tuner. A true random wire antenna connects directly to the antenna tuner and therefore there can be significant RF voltages or currents present. Effects of Height Above Ground An antenna’s feed point impedance varies for different heights. This is due to the interaction between the antenna and its image (a good conducting ground looks like a mirror to RF).
The feed point impedance steadily decreases below a height of ¼ wavelength, and is zero when on the ground.
The dipole’s radiation pattern also changes with height
The dipole’s radiation pattern also changes with height. At heights below ½ wavelength, the pattern is nearly omnidirectional and is maximum straight up.
RF signals that are horizontally polarized will have lower losses when reflecting off the ground.
Dipoles generally have a high angle of radiation which does not favor DX. Verticals have a lower angle of radiation and can work better for DX. Yagi Antennas Yagi antennas are directional antennas with good front-to-side and front-to-back ratios to minimize interference, and good forward gain to hear and be heard better.
A Yagi is a parasitic array antenna made up of two or more elements
A Yagi is a parasitic array antenna made up of two or more elements. The main lobe is in the forward direction of the Yagi. The driven element is an approximately ½ wave dipole. The reflector is a parasitic element behind the driven element (opposite the direction of the main lobe), and is a bit longer than the driven element. The director is a parasitic element in front of the driven element, and is a bit shorter than the driven element. A three element Yagi has a theoretical gain of 9.7 dBi and front-to-back ratio of 30 to 35 dB.
A Yagi can have additional directors to increase gain, but the gain is limited.
Larger diameter elements improves the bandwidth of at Yagi. This is also true for other antennas. Element spacing, boom length, and the number of elements all effect the SWR and performance of a Yagi. Yagi antennas have a feed point impedance of around 20 to 25 ohms.
A gamma match is used to match the Yagi’s feed point impedance to 50 Ω
A gamma match is used to match the Yagi’s feed point impedance to 50 Ω. The driven element can be electrically connected to the boom which makes construction easier.
Loop Antennas Loops can be 4 sided (quad loop) or three sided (delta loop). Radiation is maximum broadside to the plane of the loop. A directional antenna can be made from much like a Yagi with a driven loop and parasitic loops. A quad antenna uses a full wave driven element ¼ wave on a side. A delta antenna uses a full wave driven element 1/3 wavelength on a side.
Director elements for a quad or delta are about 5% shorter than the driven element. Reflectors for a quad or delta are about 5% longer than the driven element. The gain of a two element quad or delta is about the same as a 3 element Yagi, but the front-to-back ratio is better for the 3 element Yagi.
The polarization of the quad antenna can be changed by moving the feed point.
Specialized Antennas Near vertical incidence sky-wave (NVIS) is an antenna that is designed to radiate mostly straight up (high vertical angle). NVIS is useful for short skip to cover a regional area. Useful for emergency communications A dipole mounted λ/10 to λ/4 above the ground is an NVIS antenna. A horizontal loop is an NVIS antenna.
Stacked Antennas Vertically stacking antennas narrows the elevation beamwidth of the antenna and increases the gain. Spacing of about λ/2 provides about 3 dB gain. For another 3 dB gain, the array must be doubled to four.
Log Periodic Log periodic antennas are a driven array. They can be designed to be very broad band (10:1), but have less gain and lower front-to-back ratio than a Yagi. TV antennas are usually log-periodic. Length and spacing of the elements varies logarithmically.
Travelling Wave Antenna
The beverage antenna is an example of a travelling wave antenna. The beverage antenna is used for directional receiving, but is not a very efficient radiator.
Multiband Antennas A trap dipole uses LC traps so that the dipole looks resonant for more than one band. At their resonant frequency the trap looks like an open circuit and cuts off the rest of the antenna. At lower frequencies the trap adds inductance making the antenna look electrically longer At higher frequencies the trap adds capacitance making the antenna look electrically shorter.
All feed lines have two conductors
All feed lines have a characteristic impedance (Z0). The characteristic impedance of parallel feed lines (balanced feed lines) is determined by the radius of the conductors and the distance between them. TV type twin lead has an impedance of 300 Ω. Open wire or ladder line has impedances from 300 to 600 Ω.
The characteristic impedance of coaxial transmission lines is determined by the diameter of the inner and outer conductors, and their spacing. The insulating material between the inner and outer conductors affects the feed line loss and velocity factor (how fast a wave travels down the cable). 50 Ω and 75 Ω cables are the most common.
Forward and Reflected Power
A feed line delivers all the power to the antenna when the antenna’s feed point impedance and the feed line’s characteristic impedance are the same. A reflection occurs when the impedances don’t match. Forward power is power traveling toward the antenna and reflected power is power reflected back due to the impedance mismatch. The forward and reflected power create standing waves on the transmission line.
A standing wave ratio (SWR) of 1:1 represents a perfect match (no reflected power). A higher SWR means more reflected power. SWR is the ratio of the antenna impedance to the feed line characteristic impedance, and is always greater than 1. Example: What is the SWR in a 50 Ω transmission line when connected to an antenna with a feed point impedance of 25 Ω? SWR = 50/25 = 2:1
Example: What is the SWR in a 50 Ω transmission line when connected to an antenna with a feed point impedance of 250 Ω? SWR = 250/50 = 5:1 A high SWR can damage a transmitter because of the reflected power returning the transistors (or tubes) in the final power amplifier. Matching the antenna to the feed line will maximize the power delivered from the transmitter to the antenna.
A device to match the feed line to the antenna is called an impedance matcher, transmatch, antenna coupler, or antenna tuner.
A section of transmission line connected in parallel, called a stub, can be used to match impedances. Impedance matching does not change the SWR in the feed line from the matching device to the antenna. Only the SWR between the transmitter and the impedance matching device will be low.
Air insulated transmission lines tend to have the lowest loss.
Feed Line Loss All feed lines will dissipate some energy as heat. This loss effects both receive and transmit. Air insulated transmission lines tend to have the lowest loss. Loss is measured in dB/100 feet. Loss increases with frequency for all transmission lines. Example: RG-8 has a loss of 1.08 dB/100 ft at 30 MHz and 2.53 dB/100 ft at 150 MHz.
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