Where does the word “antenna” come from? As related by Dr. Ulrich Rohde, N1UL, the term originated with Guglielmo
Marconi during early radio tests in 1895 during which he used wire “aerials” attached to a vertical tent pole. The aerial
wire then ran down the pole to the transmitter. In Italian, a tent pole is known as “l’antenna central” and so the pole with
the wire became simply, “l’antenna.” In the beginning of radio, antennas were attached directly to generators and
transmitters and were considered part of a common assembly. It wasn’t until after 1900 that antennas began to be
regarded as separate elements of the system, independent of the transmitter or receiver. While there are an enormous
variety of antennas, they share basic characteristics and all are designed to radiate and receive electromagnetic waves.
In 1820 Hans Oerstad discovered that a current flowing in a wire would deflect the needle of a nearby compass. We
attribute this effect to a magnetic or H-field, which at any given location is denoted by the letter H. The magnetic field’s
amplitude is expressed in A/m (Amperes/meter) along with a direction. (Direction can also be expressed as some value
of phase with respect to a reference.) Because a magnetic field has both amplitude and direction, it is a vector. A
compass needle (a small magnet itself) will try to align itself parallel to H. As the compass is moved around the
conductor, the orientation of the needle changes accordingly. The orientation of the needle gives the direction of H. If
you attempt to turn the needle away from alignment you will discover a torque trying to restore the needle to its original
position. The torque is proportional to the strength of the magnetic field at that point. This strength is called the field
intensity or amplitude of H at that point. If a larger current flows in the conductor the amplitude of H will increase in
proportion. Currents flowing in an antenna also generate an H-field. An antenna will also have an electric or E-field. The
magnitude of vector E is expressed in V/m (volts per meter), so for a potential of V volts and a spacing of d meters, E =
V/d V/m. The amplitude of E will increase with voltage and/or a smaller separation distance (d). In an antenna, there will
be ac potential differences between different parts of the antenna and from the antenna to ground. These ac potential
differences establish the electric field associated with the antenna.
An electromagnetic wave, as the name implies, is composed of both an electric field and a magnetic field that vary with
time. Electric and magnetic fields that do not change with time, such as those created by a dc current or voltage, are
called electrostatic fields. The fields of a radio wave are created by an ac current in an antenna, usually having the form
of a sine wave. As a result, the fields in a radio wave vary in the same sinusoidal pattern, increasing and decreasing in
strength and reversing direction with the same frequency, f, as the ac current. It is the movement of electrons —
specifically the acceleration and deceleration as the ac current moves back and forth — that creates the electromagnetic
wave. The two fields of the electromagnetic wave are oriented at right angles to each other. The term “lines of force”
means the direction in which a force would be felt by an electron (from the electric field) or by a magnet (from the
magnetic field). The direction of the right angle from the electric field to the magnetic field, clockwise or
counterclockwise, determines the direction the wave travels. This is called a propagating wave. To an observer staying
in one place, such as a stationary receiving antenna, the electric and magnetic fields of the wave appear to oscillate as
the wave passes. That is, the fields create forces on electrons in the antenna that increase and decrease in a sine wave
pattern. Some of the energy in the propagating wave is transferred to the electrons as the forces from the changing
fields cause them to move. This creates a sine wave current in the antenna with a frequency determined by the rate at
which the field strength changes as the wave passes.
If the observer is moving in the same direction as the wave and at the same speed, however, the strength of the fields
will not change. To that observer, the electric and magnetic field strengths are fixed, as in a photograph. This is a
wavefront of the electromagnetic wave; a flat surface or plane moving through space on which the electric and magnetic
fields have a constant value. Just as an ac voltage is made up of an infinite sequence of instantaneous voltages, each
slightly larger or smaller than the next, an infinite number of wavefronts make up a propagating electromagnetic wave,
one behind another like a deck of cards. The direction of the wave is the direction in which the wavefronts move. The
fields on each successive wavefront have a slightly different strength so as they pass a fixed location, the detected field
strength changes as well. The fixed observer “sees” fields with strengths that vary as a sine wave.
Because the velocity of wave propagation is so great, we tend to ignore it. Only 1⁄7 of a second is needed for a radio
wave to travel around the world — but in working with antennas the time factor is extremely important. The wave
concept
evolved because an alternating current flowing in a wire (antenna) creates propagating electric and magnetic fields. We
can hardly discuss antenna theory or performance at all without involving travel time, consciously or otherwise.
Electromagnetic waves propagate at the speed of light for the medium through which they travel. The speed of light is
highest in the vacuum of free space, approximately 300 million or 3 × 10⁸ meters per second. It is often more convenient
to remember the speed as 300 m/ms. (A more exact value is 299.7925 m/μs). This is called the wave’s velocity of
propagation and is represented by the familiar “speed of light” symbol, c. It is also useful to know a radio wave’s
wavelength — the distance traveled during one complete cycle of a wave. Since one complete cycle takes 1/f the
velocity of a wave is the speed of light, c, the wavelength, λ, is thus: λ = c / f (1) In free-space λ = 299.7925 × 10⁶ / f
where λ is the free-space wavelength in meters. More convenient approximate formulas for use at radio frequencies are:
λ in meters = 300 / f in MHz, and (2a) λ in feet = 983.6 / f in MHz (2b) The ratio between the wave’s velocity in a specific
Medium and that of free space is called the medium’s velocity factor (VF) and is a value between 0 and 1. If the medium
is air, the reduction in velocity of propagation can be ignored in most discussions of propagation at frequencies below 30
MHz. In the VHF range and higher, temperature and moisture content of the medium have increasing effects on the
communication range. In materials such as glass or plastic the wave’s velocity can be quite a bit lower than that
of free space. For example, in polyethylene (commonly used as a center insulator in coaxial cable), the velocity of
propagation is about 2⁄3 that in free space. In distilled water (a good insulator) the speed is about 1⁄9 that of free space.
A wave is said to be polarized in the direction of the electric lines of force. Polarization is vertical if the electric lines are
perpendicular to the surface of the Earth. If the electric lines of force are horizontal, the wave is said to be horizontally
polarized. Horizontally and vertically polarized waves may be classified generally under linear polarization. Linear
polarization can be anything between horizontal and vertical. In free space, “horizontal” and “vertical” have no meaning,
since the reference of the seemingly horizontal surface of the Earth has been lost. In many cases the polarization of
waves is not fixed, but rotates continually, sometimes at random. When this occurs the wave is said to be elliptically
polarized. A gradual shift in polarization in a medium is known as Faraday rotation. For space communication, circular
polarization is commonly used to overcome the effects of Faraday rotation. A circularly polarized wave rotates its
polarization through 360° as it travels a distance of one wavelength in the propagation medium. The direction of rotation
as viewed from the transmitting antenna defines the direction of circularity — righthand
(clockwise) or left-hand (counterclockwise). Linear and circular polarization may be considered as special cases of
elliptical polarization.
The energy from a propagated wave decreases with distance from the source. This decrease in strength is caused by
the spreading of the wave energy over ever-larger spherical surfaces as the distance from the source increases. A
measurement of the strength of the wave at a distance from the transmitting antenna is its field intensity, which is
synonymous with field strength. The strength of a wave is measured as the voltage between two points lying on an
electric line of force in the plane of the wave front. The standard of measure for field intensity is the voltage developed
in a wire that is 1 meter long, expressed as volts per meter. (If the wire were 2 meters long, the voltage developed
would
be divided by two to determine the field strength in volts per meter.) The voltage in a wave is usually low so the
measurement is made in millivolts or microvolts per meter. The voltage goes through time variations like those of the
current that caused the wave. It is measured like any other ac voltage — in terms of the RMS value or, sometimes, the
peak value. It is fortunate that in amateur work it is not necessary to measure actual field strength as the equipment
required is elaborate. We need to know only if an adjustment has been beneficial, so relative measurements are
satisfactory. These can be made easily with home-built equipment.
In free space, the field intensity of the wave varies inversely with the distance from the source, once in the radiating
far field of the antenna. If the field strength at 1 mile from the source is 100 millivolts per meter, it will be 50 millivolts per
meter at 2 miles, and so on. The relationship between field intensity and power density is similar to that for voltage and
power in ordinary circuits. They are related by the impedance of free space, which is approximately 377 Ω. A field
intensity of 1 volt per meter is therefore equivalent to a power density of P= E²/Z= 1 (volt / m)²/377Ω= 2.65mW/ m2
Because of the relationship between voltage and power, the power density varies with the square of the field intensity,
or inversely with the square of the distance. If the power density at 1 mile is 4 mW per square meter, then at a distance
of 2 miles it will be 1 mW per square meter. It is important to remember this so-called spreading loss when antenna
performance is being considered. Gain can come only from narrowing the radiation pattern of an antenna, which
concentrates the radiated energy in the desired direction. There is no “antenna magic” by which the total energy
radiated can be increased. In practice, attenuation of the wave energy may be much greater than the inverse-distance
law would indicate. The wave does not travel in a vacuum and the receiving antenna seldom is situated so there is a
clear line of sight. The Earth is spherical and the waves do not penetrate its surface appreciably, so communication
beyond visual distances must be by some means that will bend the waves around the curvature of the Earth. These
means involve additional energy losses that increase the path attenuation with distance, above that for the theoretical
spreading loss in a vacuum.