Re: How do receiving antennas work

Greetings:

Reference: F. E. Terman, Electronic and Radio Engineering,
McGraw-Hill, 1955

Electrical energy that has escaped into free space exists in the form of
electromagnetic waves. These waves, some of which are called radio waves,
travel at the velocity of light and consist of magnetic and electric fields that
are at right angles to the direction of wave travel.

Right Hand Rule: Point the index finger on your right hand straight ahead.
This is the direction of wave travel.
Raise your thumb up to form a right angle with your index finger. This is the
direction of the magnetic (H) field.
Bend your middle finger to the left to form a right angle with the index finger.
This is the direction of the electric (E) field.
One half cycle later the E and H fields will point in the opposite direction.

One half of the electrical energy contained in the
wave exists in the form of electrostatic energy , while the remaining half is in
the form of magnetic energy. The essential properties of a radio wave are the
frequency, intensity, direction of travel, and the plane of polarization. The
polarization is defined as the direction of the electric field. Radio antennas can
be analyzed by using the electric field or by using the magnetic field. Radio
waves are produced by an alternating current and they will vary in intensity
with the frequency of the current in a circuit

The space around every electrical circuit carrying alternating current is filled
with electric and magnetic fields and the circuit radiates a certain amount of
electrical energy in the form of electromagnetic waves. However, the amount
of energy radiated is extremely small unless the dimensions of the circuit
approach the size of a wavelength. Thus a power line carrying alternating
current at 60 cycles per second (60 Hertz or 60 Hz) will radiate
practically no energy because the wave length is more that 4800 km (3000
miles) long. On the other hand a circuit carrying alternating current at 100
million cycles per second (100 Megahertz or 100 MHz), in the FM radio
band, will radiate a significant amount of energy because the wave length is
only 3 meters (9 ft) long.

Before I answer your specific questions I will review some basic physical
principles that are involved. In 1820 Hans Christian Oersted (1777-1851)
demonstrated that an electric current in a wire would deflect a compass
needle and that the direction of the needle’s deflection was determined by the
direction of the current flow in the wire. These experiments showed that
electricity and magnetism were a related phenomena. Oersted’s publication of
his experiments were quickly repeated by other experimenters and it was
quickly demonstrated that a moving magnetic field could generate a current
in a wire and that the maximum current was generated when the magnetic
field was at right angles to the wire (cutting the wire). It was also
demonstrated that a coil with many loops of wire carrying current could form an
electromagnet. In 1831 Michael Faraday demonstrated that energy could be
transferred from one electrical circuit to another, in a process he called
induction, by coupling their magnetic fields. Once again, the maximum energy
transfer occured when the magnetic field was a right angles to both circuits.
This process led to the development of transformers used in alternating
current circuits to this day . In 1864 James Clerk Maxwell (1831-1879)
developed a set of equations (Maxwell’s Equations) that showed
that electromagnetic energy could travel through space as a wave composed
of alternating electrical fields propagating at right angles to alternating
magnetic fields. Maxwell’s equations showed that these
electromagnetic waves traveled at the speed of light and that light itself must
be an electromagnetic wave. In 1894 Heinrich Hertz demonstrated that radio
waves travel at speed of light and can be refracted and polarized
demonstrating what Maxwell’s equations predicted. In1901 Guglielmo
Marconi (1847-1937) demonstrated the transmission of
Morse code signals across the Atlantic Ocean with a wireless (radio)
telegraph. The telegraph and Morse’s code, which were commercialized in the
mid eighteen hundreds, where used all over the world by that time. This
technology also resulted from Oersted’s pioneering experiments.

Now to answer your question. I will discuss the magnetic field approach to
antenna operation instead of the electric field approach, because I feel that it
relates more directly to the pioneering experiments that I discussed above. If a
ring of wire (now called a loop antenna) is set up in a location where the
alternating magnetic fields of a radio wave pass through the wires of the loop,
an alternating current will be generated in the loop with the direction of the
current being determined by the direction of the magnetic field as it
passes through the loop. This is similar to the transformer except that the
input coils (the radio transmitter) and the output coils (the receiving antenna)
are separated by a large distance and are coupled by electromagnetic waves
not induction fields. The strength of the current in the coil will be determined
by the strength of the magnetic field passing through it. After the radio energy
leaves the transmitting antenna the wave expands in a spherical wave front
filling space with electromagnetic energy. The strength of the magnetic field
diminishes as the square of the distance from the transmitter, just as light from
a flash light or a laser beam diminishes as the square of the distance from the
source.

To get useful power from the current circulating in the loop antenna we can
place a small resistor (the receiver input terminals) in series with the loop and
the current generated by the magnetic part of the wave crossing (cutting) the
wire will produce a voltage across the resistor. This voltage can then be
amplified by the radio receiver circuitry up to about a one volt level, a level
that headphones or speakers require for operation. Today’s receivers typically
require about 10 microvolts to be developed across the resistor
for high quality sound reproduction. This means the various circuits in a
radio receiver must amplify the antenna voltage by a factor between one
hundred thousand and one million times to get up to one volt.
In actual practice the antenna resistance is a complex impedance, which
means that it is composed of is a combination of resistors, coils, capacitors.
This impedance is usually 300 ohms for flat twin lead transmission lines
between the receiver and the antenna and between 50 to 70 ohms for coaxial
cable antenna hookups.
Using Ohms Law (current = voltage divided by resistance) gives
0.03 microamps (0.03 millionths of an amp) of current for 300 ohm antennas
and 0.2 microamps for 50 ohm antennas with a 10 microvolt signal

Within certain limits, which are beyond the scope of this discussion, the more
turns (loops ) in the loop antenna the more current (and voltage) will be
generated for the receiver circuit. Also, the larger the diameter of the loops,
the more current will be generated in the loop antenna. It is well known that
when an electrical coil, such as a loop antenna, is placed in series with a
variable capacitor, a resonant circuit is formed. If this resonant circuit is
“tuned” to the frequency of the radio wave (the station), the energy builds up
and is stored within the loop antenna and will be greatly increased by each
passing waves magnetic field, just as a tuning fork can be made to build up
vibrations from a distant sound source that is tuned to the same exact
frequency. In this manner a tuned loop antenna can deliver much more current
to the radio receiver. However, the loop must be retuned for each change in
radio station. The loop antenna for my Soney AM radio receiver is about 13
cm (6 inches) in diameter with about 10 turns in the coil. It is not tuned so
that it can covers the whole AM band from 500 kilohertz to 1500
kilohertz. Because they are so sensitive, most modern receivers do not require
a tuned loop antenna.

Antenna Radiation PatternsThe orientation of the loop antenna in the electromagnetic field is also
important. The loop antenna has the advantage that it’s orientation is not
sensitive to the polarization of the electric field as most rod and wire antennas
are. Consider a commercial AM radio transmitter in the USA (called medium
wave radio in Europe) that by law and for practical purposes uses vertical
polarization. AM transmitter antenna towers are vertical, requiring that the
radiated electric field to be vertically polarized relative to the surface of the
earth. The magnetic fields will then be horizontal to the surface of the earth.
Case1:
First we place the plane of the loop antenna at right angles to the direction
toward the radio transmitter and the direction of travel of the electromagnetic
wave so that all points on the loop are equally distant from the radio
station’s transmitter. Then the horizontal magnetic fields of the
electromagnetic wave will cut through both sides of the loop at the same
exact time. This will generate equal vertical currents in each half of the loop.
These currents travel in opposite directions in the loop and cancel each other
out producing no voltage signal in the resistor (receiver).
Case 2:
Now if we place the plane of the loop toward the direction of the radio
transmitter, the magnetic fields of the electromagnetic wave will first pass
through the leading edge of the loop facing the transmitter and then will pass
through the back side of the loop slightly later in time. This slight difference in
time, called a phase shift, causes a maximum amount
of current to flow in one direction in the loop during one half of the cycle and
a maximum amount of current to flow in the other direction in the loop during
the second half of the cycle when the magnetic field changes direction.
However, loop antennas are not very efficient and cancellation still occurs
between the currents from both sides of
the loop, but
not total cancellation. You will have to take my word about this result for
it requires vector algebra to prove this result . Thus we have a maximum
of AC current generated in the loop and an AC voltage produced in the
resistor for the receiver to amplify.

If we rotate the angle of the loop on a vertical axis from the position of
Case 2 toward the position of Case 1, the current and voltage generated for
the receiver will decrease as the angle moves toward the position of Case 1.
This effect also occurs if we rotate the loop on a horizontal axis. If we could
picture the sensitivity of the loop antenna in 3D we have a picture of the loops
radiation pattern. This three dimensional shape looks like a fat
doughnut with a very small hole in the middle. If the loop was a tire the
maximum signals would be received if we point any part of the rim of the
donut toward the transmitter. The signal would go to zero if we point
either hub of the wheel (the hole in the donut) toward the transmitter. This
characteristic of loop antennas has made them very useful as radio direction
finders on aircraft and ships. By rotating the loop in on a vertical axis
containing a compass indicator, the compas angle where the signal zeros out,
gives you a compass line on a map passing through the radio transmitter on
which you are located. Of course you do not know which hub is facing
toward the transmitter and which hub is facing away from the transmitter.
You must find the compass direction to a second radio station and plot it on
the map. The point where the two lines cross is your location.

A Radio Direction finding Experiment
Take a small AM transistor radio out in an open field away from buildings and
metal structures. Locate a strong station, preferably one that you know
where the transmitting antenna is located. Rotate the radio horizontally and
slighty vertically vertically until the radio station is canceled out (nulled out).
This means that the hub of the internal loop antenna in the radio is facing
toward or away from the transmitter location. Most
transistor radios place the loop antenna on a ferrite rod (a special black
ceramic magnetic material that enhances loop operation) that is usually, but
not always, placed in the longest dimension of the radio case. Usually this
means that so that you end up pointing the end of the radio case toward
the station. I have had Boy Scout groups find their (unknown) location on a
map by finding the compass directions required to null out AM radio stations
by this method. Most aeronautical maps show the location of high power AM
radio transmitters for this purpose.
Antennas made from straight wires and rods are usually analyzed using the
electric field; however, these antennas are polarization sensitive and would be
difficult to address in this forum with out diagrams to help. However, the
process would be similar to the one used for the loop antenna.

Best regards, Your Mad Scientist
Adrian Popa