The following information has been summarized from various sections of the ARRL Antenna Book, along with some additional notes. The ARRL Antenna Book gives many further details and applications. This summary has been prepared to assist those who may be looking for a basic understanding of antenna fundamentals, and can be useful information for those considering building their own antennas.

Any piece of conducting material will work as an antenna on any frequency. This comes from the fact that there is some small inductance in any wire, strip of metal, or resistor which will allow some radiation to escape into space. However, the main objective should be to build an antenna that will function primarily as a radiator of RF rather than as a generator of heat.

The only reason for building a sophisticated antenna is to control the radiation pattern

The radiation pattern is controlled by focusing the radiated energy. The geometry of the antenna and the proximity of nearby objects are the main controlling factors.

The total amount of energy radiated from an antenna remains constant for a given transmitter output power. When this energy is focused, the energy radiated in one or more directions will be increased, and the energy radiated in other directions will decrease. This is what gives an antenna "gain".

Anything that enters into the aperture of an aantenna (1/8 wavelength on each side of a dipole antenna) will affect the operation of the antenna. The effects are pattern distortion, skewing of balance, change of feed point impedance and resonant frequency shift.

Sometimes it is desirable to cause intentional aperture interference. Placing other conductors into the aperture will cause severe pattern distortion. This can be beneficial when this distortion takes place in such a manner as to focus the radiated energy into a tight beam. This is the basic operating principle of parasitic beam antennas.

A 1:1 SWR does not indicate that an antenna system is resonant. SWR is only the ratio between the impedances of the feed line and the antenna load. For example, we can connect a 50 ohm pure resistor across a 50 ohm transmission line and there will be a 1:1 SWR, but there will be no radiation from the resistor.

It is possible to cut a piece of feed line to just the right length, and measure a 1:1 SWR at the transmitter end of that feed line.

A high SWR does not cause feed line radiation. Most radiation from coaxial cable is caused by terminating the unbalanced feed line with a balanced load. The rest of the radiation from a feed line is due to other problems such as: discontinuities in the outer conductor (corrosion in the braid), improperly installed conductors, objects coming within the "field space" of the antenna (trees, buildings, etc), and routing the feed line too close to and parallel to the antenna.

Antenna performance depends on the antenna's feed-point impedance, directivity, gain, efficiency, and polarization.

Impedance is a mathematical combination of pure resistance and reactance. Reactance can be capacitive or inductive.

The feed point is the location where the transmission line meets the antenna. Feed-point impedance is composed of self impedance and mutual impedance.

Self impedance is the voltage applied at the feed point divided by the current flowing into the feed point.

Mutual impedance is due to the effect of nearby conductors (or objects such as trees or ground).

A thicker diameter antenna wire lowers the frequency of resonance (the point where the SWR = 1.0) and lowers the range of reactance at any given frequency. The impedance at the resonant frequency increases slightly for larger diameter antenna wires.

The Q of a thin wire antenna is higher than that of a thick wire antenna.

A low Q antenna is broadbanded (little change in impedance with change in frequency). Therefore, thick wire antennas are more broadbanded. This means that there will be less change in SWR from one end of the band to the other.

Larger diameter antenna wire lowers the wavelength at resonance. An antenna cut for ½ wave is actually less than ½ wave in electrical length due to using larger diameter wire. Therefore, for thicker wires the length of the antenna must be increased slightly.

For simple wire antennas, the reactive near field (or induction field) extends to about ½ wavelength from the radiating center of the antenna.

Beyond the reactive near field exists the radiating near field and the radiating far field.

Traveling electromagnetic waves exist in the radiating far field, where the intensity of the wave is inversely proportional to the distance and where the electric and magnetic components of the wave are perpendicular to each other and are in time phase. The total energy of the wave is equally divided between the electric and magnetic fields.

To make accurate antenna measurements, the measuring instruments should be several wavelengths away from the antenna under test.

Directivity is the ratio of the power density at its maximum point on the surface to the average power density.

Gain of an antenna is the efficiency of the antenna multiplied by its directivity.

An antenna’s efficiency is the ratio of the power radiated to the power input at the antenna.

Beamwidth is the width of the major lobe (or lobes) between the half-power or –3 dB points.

Polarization is defined as the polarization of the antenna's electric field in the direction where the field strength is maximum. A vertical antenna is polarized vertically, and a horizontal antenna is polarized horizontally if its elements are oriented parallel to the earth surface.

When more than one ionospheric layer is involved in the wave travel, it is sometimes possible for reception to be good in one direction and poor in the other, over the same path.

Wave polarization usally shifts in the ionosphere. The tendency is for the arriving wave to be elliptically polarized, regardless of the polarization of the transmitting antenna.

The effect of tapering an antenna element is to alter its electrical length. A tapered element must be made longer than a cylindrical element to obtain the same resonant frequency.

The feed-point impedance of a monopole (ground plane) antenna is half of that of a dipole antenna, or roughly 36 ohms in theory.

For a ground mounted monopole antenna without a fairly elaborate grounding system, the efficiency is not likely to exceed 50%, and may be much less, particularly at monopole heights below ¼ wavelength.

The feed-point impedance of an antenna is affected by the height of the antenna above ground because of mutual coupling between the antenna and the ground.

Different feed-point impedances may be found when an antenna is erected at identical heights, but over different types of earth. Ground is not the same all over.

If a horizontally polarized ½ wave antenna (dipole) is greater than 0.2 wavelengths above ground, the feed-point impedance will not be noticeably affected. At 0.1 wavelength above ground the feed-point impedance of such an antenna will be closer to 20 ohms, rather than the 72 ohms expected at optimum height. For example, an 80 meter dipole needs to be about 50 feet above ground and away from other objects for best results, and then a 75 ohm transmission line cable would be used. If a 50 ohm cable is desired (to better match with the radio) the antenna height could be more like 35 feet above ground. Experimentation is the best means of determining the best height because one cannot be sure where the true ground level is located (it may be under the earth level).

A ground mounted monopole will approach a feed-point impedance of 36 ohms only when it has a wire mesh extending in all directions at least ½ wavelength in distance.

A monopole mounted at least 1/8 wavelength above ground and having four ¼ wavelength radials will approximate the ideal antenna and can also have a feed-point impedance of about 36 ohms.

The use of plastic-insulated wire will typically lower the resonant frequency of a halfwave dipole about 3%.

The length of a practical ½ wave antenna, including the effect of diameter and end effect, is in the order of 5% less than the length of a halfwave in space. To calculate the length of wire required for a half wave antenna use the following formula:

L = 468 / f

This formula gives the wavelength (L) in feet, where f is the center frequency of operation in megahertz.

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