What is an Antenna?
An antenna is a circuit element that provides a transition from a guided wave on a transmission line to a free space wave, and vice versa.
It is made of a thin electrical conductor to efficiently radiate electromagnetic (EM) energy. Antenna characteristics are the same, whether a transmitting antenna or a receiving antenna. This phenomenon is called antenna reciprocity, reciprocity, which implies that an antenna captures EM energy with the same efficiency that it radiates EM energy.
Antennas are designed and constructed to fit a specific frequency or frequency band. If you use an antenna designed for a different frequency, it will radiate only a small portion of the transmitter’s generated RF power and only absorb a small portion of the receiver’s RF signal power. It is very important to use a proper frequency antenna.
Antennas are compared to an isotropic antenna that is theoretical. In a three-dimensional spherical pattern, this antenna radiates all the transmitter power.
An isotropic antenna is theoretical only because the pattern of radiation is distorted in certain directions in the construction of antennas.
Omnidirectional antennas transmit the horizontal plane equally in all directions. Omnidirectional antennas are normally used in intraplant applications where the radio path lies entirely within an industrial site or factory.
The radio path often consists of strong reflected signals coming from any direction over relatively short distances (meters versus miles), and this may not always be apparent.
Directional antennas further distort the patterns of radiation and in a “forward” or specific direction have higher gains. These antennas are often used in intraplant applications for longer line – of – sight radio paths.
Gains and Losses:
Using a high-gain antenna has the following effects at the transmitter:
- Increases efficient transmission power in some directions and decreases power in others.
- Gain compensates for the loss in the coaxial cable.
- Makes the antenna more directional at the transmitter — a good effect in unnecessary directions to reduce unwanted RF radiation.
At the receiver, high-gain antennas have the following effects:
- Increases the signal received from some directions and decreases the signal from others. The gain will also increase the external noise received.
- If the increased noise exceeds the sensitivity of the receiver, the gain improvement has been negated—that is, in noisy environments antenna gain can have little effect at the receiver, but in no-noise environments, it can have an effect.
- Gain compensates for the loss in coaxial cable
- Makes the antenna more directional
Line of sight & Fresnel zones:
The openness of the signal path between the transmitting and receiving antennas is one of the many factors affecting RF power transfer from transmitter to the receiver.
Line-of-sight, or LoS, is a common expression in microwave radio communication. This means signal is able to see a straight path between transmitting antennas and receiving them.
If one does not have a clear line-of-sight path between antennas, the signal path loss will definitely be more severe than through open space
The free space required for the transmission of energy in the form of electromagnetic waves takes the form of football – shaped zones: the first solid zones followed by the annular (hollow) zones surrounding the first. This elliptical volume is called Fresnel zones
The precise shapes of these Fresnel zones are a function of wavelength and distance between antennas, not the antenna size or configuration itself.
Specific antennas and cables will be tested, and the test results will indicate whether higher – gain antennas or lower – loss cables are needed. For example:
- If a test yields a 5 dB fade margin, you know that the fade margin will increase to 10 dB by inserting an extra 5 dB antenna gain or removing 5 dB coaxial loss. You can insert the gain either at the end of the transmitter or receiver, or both.
- If a test shows that a net antenna / cable gain of 6 dB is required for reliable operation on a line – of – sight path and a cable length of 10 meters is required at both ends of the link to give line – of – sight, you can select which cable to use and then select the antennas to give a net gain of 6 dB.