MEASUREMENT OF GAIN BANDWIDTH OF DIPOLE ANTENNAS
1. To identify whether an antenna is resonating or non-resonating type.
2. To measure basic antenna’s Gain Bandwidth using log
Antennas are a fundamental component of modern communications systems. By
definition, an antenna acts as a transducer between a guided wave in a transmission line and an
electromagnetic wave in free space.
An antenna is a device that transmits and/or receives electromagnetic waves.
Electromagnetic waves are often referred to as radio waves. Most antennas are resonant
devices, which operate efficiently over a relatively narrow frequency band. An antenna must
be tuned to the same frequency band that the radio system to which it is connected
operates in, otherwise reception and/or transmission will be impaired.
Here the antenna we are using is a dipole antenna as basic antenna and log periodic antenna as transmitting antenna
b.1) Antenna resonance
An RF antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency. This is the frequency where the capacitive and inductive reactances cancel each other out. At this point the RF antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.
Impedance of an RF antenna with frequency
The capacitance and inductance of an RF antenna are determined by its physical properties and the environment where it is located. The major feature of the RF antenna design is its dimensions. It is found that the larger the antenna or more strictly the antenna elements, the lower the resonant frequency. For example antennas for UHF terrestrial television have relatively small elements, while those for VHF broadcast sound FM have larger elements indicating a lower frequency. Antennas for short wave applications are larger still.
Every antenna is at resonance according to length to which it is cut ! They are usually cut to resonate at a particular band of frequencies, i.e., Vhf Television band etc.
If no resonance then no signal is present and in such a case antenna is merely a random chunk of metal.
FIGURE 1: DIAGRAM SHOWING IMPEDENCE OF AN RF ANTENNA WITH FREQUENCY
b.2) what is difference between resonant and non-resonant antenna?
The magnetic field that an antenna puts out will produce an electric current on any conducting surface that it strikes, however if that surface has a characteristic length the induced current will be much stronger on the object. For example, when a Citizens Band signal travels through the air, it completes a cycle in approximately 36 feet. If the object that the magnetic wave strikes is 18 feet long (1/2 wave length), 9 feet long (1/4 wavelength) or 36 feet long (1 full wavelength), then the induced current will be much higher than if the signal struck a metal object that was not some appreciable fraction of the wavelength of the signal.
A resonant antenna is so much more efficient at converting (receiving or transmitting) current between the field and the antennas feed-point than a non-resonant antenna that much effort is put into configuring resonance. A non-resonant antenna still works as an antenna but simply requires a more sensitive receiver or more powerful transmitter.
If you have ever heard people say they want to "tune" their antenna, they usually mean they mechanically change lengths in relation to the frequency / wavelength they are trying to match. It is also possible to change electrical properties to match frequency, which is handier for matching multiple frequencies with a single antenna.