An antenna, in its simplest form, is just a length of wire. Its function being to convert 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:

where wavelength is in meters, and frequency in MHz.
There are many types of antenna:- directional, whip, multi-element arrays, wire, loop to name just a few. All have different properties and characteristics. Some antennas will be suitable for high frequency, some only work at low frequency, a ferrite loop antenna for example being suitable for use on long and medium waves.

The antennas used for FM transmitters on my RF Circuits are just a short vertical section of wire, or you can use a telescopic whip antenna. The whip antenna, when positioned vertically will transmit and receive in all directions (omnidirectional) and known also as an isotropic antenna.

A 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 and Far Field
Antenna characteristics differ with design and operating frequency and many factors that affect performance. All transmitting antennas have regions called the near and far field. These regions are shown in the diagram below.

The centre vertical line represents the length of the antenna, L, in meters. R represents the radius of the near field.

near far diagram
The "near field" is the area closest to the antenna, also known as the induction field. Field strengths in the near field are different from the far field and can be difficult to predict exact field strengths.

When measuring a transmitter output, the measurements must be made in the "far field". In the far field, the power received per unit area from an isotropic antenna is calculated from the following equation:

where Pr = received power, Pt = transmitted power and d is distance from transmitter in metres.

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, Pt = transmitted power and d is distance from transmitter in metres.

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 :

This formula assumes that all energy leaving the transmitter is converted to RF energy, there are no losses in the transmitting or receiving antenna and no loss in the path from transmitter to receiver. In all cases these conditions are never met. There are losses in the receiving and transmitting antenna, and the signal is attenuated on its way to the receiver. Trees, buildings hills all reduce radio waves before they reach the receiver.

Propagation of Signals
Propagation is the term that describes how well an EM wave travels. There are many factors affecting propagation; the frequency of the electromagnetic wave , the ionosphere, the height of transmitting and receiving antennas to name just a few. I have made a few practical measurements using the FM transmitters circuits on this site. The results are presented in the analysis section.

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