Forward to Transmitting
Basic Antenna Theory
Antenna theory is a large subject, so this is just a brief introduction, highlighting some important antenna topics which are relevant
to circuits on this site.
In its simplest form, an antenna is just a length of wire. Its function being to convert an electromagnetic energy into voltage for reception, or to
transduce a varying voltage into electromagnetic energy for transmission. An electromagnetic wave (EM) consists of changing magnetic and
electric fields and travels through space at 300,000,000 meters per second. The wavelength of an EM wave or radio wave can be determined from
the following equation :
where wavelength is in meters, and frequency in MHz. Similarly the frequency can be derived from:
There are many types of antenna, :- directional, whip, multi- element arrays, to name just a few, and all have different characteristics. The antennas
used for many of the circuits on this site, is generally just a straight piece of wire or a telescopic whip antenna may be used. The whip antenna,
when positioned vertically will transmit and receive in all directions (omnidirectional) and known also as an isotropic antenna. The dipole antenna is
made from two lengths of straight wire and can be arranged horizontally or vertically. Depending upon its orientation, a transmitted wave will either
be horizontally or vertically polarized. When using a dipole it is important to make sure that both receiving and transmitting antenna have the same
orientation.
Near / Far Field
Antenna characteristics differ with design and operating frequency and there are many factors that affect performance. When measuring the output
from a transmitting antenna , the measurements must be made in an area known as the "far field". The "near field" is the area closest to the antenna,
also known as the induction field, and field strengths in this area are different to the far field. The near far boundary is shown in the diagram below:
The centre vertical line represents the length of the antenna, L in meters, R is the radius of the near field. Outside the near field, is the region
known as the far field. This is where all measurements of field strength should take place. In the far field, the power received per unit area from an
isotropic antenna is calculated from the following equation:
This equation is also referred to as the inverse square law, since doubling the distance gives a four fold reduction in signal power. Many radio
receivers have a manual which includes a specification sheet. One of the properties quoted is sensitivity, this indicates the "weakest" signal that
can be received. Sensitivity is usually quoted in units of V/m or volts per meter. The equation below is used to calculate field strength in V/m :
Where E is field strength in V/m, d is distance in meters and Pt is the power of the transmitter in watts. It should be noted that this equation
assumes 100% energy is transferred from the transmitter to the antenna, and that the antenna has unity gain (a dipole). In practise, there are losses
in coupling the signal from transmitter to antenna, and also losses in the antenna itself. In the next section there are examples of transmitted and
received signals.
Transmitting
How far can an electromagnetic wave travel? In theory and travelling through space, once started a radio wave will carry on forever. However, in
the real world, an EM wave is attenuated by the atmosphere and other factors. Whatever power is used for transmission, the received signal gets
weaker the further the distance from transmitter. After a certain point, the signal is so weak that natural and atmospheric noise are greater than the
original signal.
Sensitivity
If a radio receiver is quoted as having a sensitivity of 20uV/m then this is the weakest signal that the particular receiver can "hear". To receive
signals or stations weaker than this, then a high gain antenna or preamplifier is required. The problem with amplifying weak signals, is that you also
amplify the inherent background noise that is present.
Example:
If a radio station broadcasts a signal with a 100W transmitter then what is the field strength at a distance of 10 Km. Using the above
equations, this is easily determined :
