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Balun is not necessarily a characteristic of an antenna. It is more of an additional component in some antenna. We suggest that you read about various types of antennas before reading about balun as it is used only for designing an antenna. Balun is derived from the expression ‘'''bal'''anced to '''un'''balanced’. It simply converts an unbalanced signal to a balanced signal. To put it very simply, a coaxial cable which has outer coating as ground (zero potential) and inner core as signal (some voltage V) is an unbalanced signal. While a twisted cable pair in which one wire has potential V and other has potential -V is a balanced signal. Some antennas such as monopole work when unbalanced signal are fed to it. On the other hand dipole antenna require unbalanced signal (+V, -V). Transmitting circuits usually generate unbalanced signal. Hence to convert unbalanced signal to balanced signal baluns are required. <br \>
There are numerous types of baluns. Some use transformers and autotransformers, some acts as RF choke. Ham enthusiasts often use ‘delay line type’ baluns. They are nothing but introducing delay using transmission lines and using it to balance a signal.
=== Decibel (dB) ===
Again, it's not a characteristic of antenna but there is always a lot of confusion regarding this. Power in decibel is defined as,
[[File:PowerDB.JPG|center|frame]]
Here the reference power is 1W when power is denoted in Watts. Hence the resultant decibel is known as dBW. When power is denoted in milliwatts, the resultant decibel is denoted by dBm. Here the reference power is 1 mW. <br \>
When decibels are used to describe gain, apart from dBW and dBm sometimes dBi and dBd are also used. dBi denotes gain of the antenna with respect to that of isotropic antenna. As the gain of an isotropic antenna is 1, nothing changes numerically. dBd denotes gain of antenna with respect to that of a standard half wave dipole antenna (gain is 2.15). Numerically, dBd = dB - 2.15. <br \>
Decibel is widely used as it easier to perceive quantities when they are represented in decibels.
== Antenna Types and their Characteristics ==
=== Half wave dipole ===
[[File:HWD.png|thumb|frame]]
* Dipole antennas are one of the earliest used antennas. The antenna used by Hertz in his experiment to transmit electromagnetic waves was also similar to dipole antenna. it's geometry is linear and it's structure is, to simply speak, two wires in a line.
* As the name suggests, the length of this antenna is half of its wavelength at the desired frequency of operation. The signal is feeded to this antenna midway. It has an omnidirectional radiation pattern and linear polarization.
* It has low directivity (2.15 dB) and input impedance of 73 + j42.5 ohm. The antenna can be made resonant (input impedance purely real) by reducing length of antenna to 0.48 fraction of wavelength
=== Folded dipole ===
[[File:FD.png|thumb|frame]]
* Folded dipole antenna is resonant and radiates well when it's length is odd integer multiple of half wavelength (eg. 0.5λ, 1.5λ, 2.5λ...etc).
* Input impedance of half wavelength folded dipole is four times that of normal half wavelength dipole i.e approx. 280 ohms.
* It is linearly polarised.
* Radiation pattern similar to half wavelength dipole.
=== Monopole ===
[[File:Monopole.png|thumb|frame]]
* It is the simplest kind of antenna. It's just a wire and a ground plane with the wire perpendicular to ground plane. From electromagnetic theory, the ground plane acts as a mirror and the monopole antenna acts as a dipole antenna of twice it's length. Hence most of its properties are same as a dipole antenna of twice length
* The main points of difference between a monopole and dipole are it's resonant length, input impedance and directivity. A dipole resonates at 0.48 fraction of wavelength while the monopole resonates at half this length as monopole acts as a dipole twice it's length. The input impedance of monopole is half of a dipole of twice length i.e. 36.5 + j21.25 ohm (for length 0.25*wavelength). The directivity of monopole is twice (+3 in dB) of dipole antenna i.e. 5.15 dB.
* In this discussion we have assumed the ground plane to be infinite. It is not so in any practical application, so the characteristics of this antenna can change slightly due to finite ground plane. The characteristics can be simulated using various simulation softwares.
* They are a very popular choice due to their small size simplicity and are used in almost all common antenna applications.
=== Yagi ===
[[File:Yagi.jpg|thumb|frame]]
* While monopole and dipole are omnidirectional antennas, yagi is a directional antenna. it's made up of only wires and has only one feed (where you apply signal). It has three elements - feed, directors and reflector. All the three elements are attached to a single long rod which is called “boom”. The feed, directors and reflector are perpendicular to the boom and in the same plane. Their order is - reflector then feed then directors. Feed is the element where signal to be transmitted is applied. The feed is typically a dipole or a folded dipole. The feed is made resonant in the presence of other elements. This length is different from the resonant length of isolated antenna. It is mostly calculated using simulations.
* The other two elements are also called parasitic elements as they are not excited by any signal. The directors are used to increase the directivity of the antenna. Increasing the number of directors increases directivity of the antenna. Their length is slightly less than the feed element and progressively decreases when we move further away from feed. The number of directors can be as large as 20-30. The reflector, as the name suggests, seem to reflect radiation thus decreasing its radiation in unwanted direction. Usually only one reflector is used but antennas having more than one reflectors have been designed. They are physically longer than feed element. Its theory of operation is quite interesting, so do give it a [https://en.wikipedia.org/wiki/Yagi%E2%80%93Uda_antenna#Theory_of_operation read].
* About it's characteristics, it is a highly directive antenna whose gain is usually more than 10 dB. Its bandwidth is typically small. The input impedance is that of the feed which can be simulated.
* To design this antenna, quite a lot of variables are needed to be decided. These include length of all elements and their spacing. All these variable belong to a small range of lengths and are tweaked using simulations before actually making the antenna.
* Most of you may have seen these kind on antennas on your roof. They were widely used as television antennas and even today are very popular amongst ham enthusiasts. They are quite easy to make and it's directivity can be increased by just increasing the number of directors. They are regarded as IEEE Milestone. Their importance just can’t be overstated.
=== Crossed Yagi ===
[[File:CY.jpg|thumb|frame]]
*All the antennas we have looked at so far were linearly polarised antennas. But in actuality circular polarised can be made quite easily from all the linearly polarised antennas. As we mentioned in polarisation, circular polarization is constructed simply from two equal magnitude linearly polarized orthogonal signals which are 90° out of phase. Antennas with same geometry will have same output radiation for same input. Hence two linearly polarized antennas with same geometry placed in such a way such that their output is orthogonal to one another and a 90° phase shifted signal given to one of them will result in circular polarisation antenna. The antennas have to placed in near field region of each other.
* In a crossed Yagi antenna, two yagi antennas are used to achieve circular polarization. Both the antennas are placed geometrically orthogonal to one another and 90° phase shifted signals are given to their feed. Hence it acts as a circularly polarised antenna. Usually this 90° phase shift is added by introducing an additional cable of length about 0.25*wavelength (actually the length also depend upon the kind of wire used. The phase difference due to transmission cable is it's path difference times velocity factor which depends on cable). Weather it's RHCP or LHCP can actually be changed by changing its connections.
[[File:CY1.gif|left|frame]]
[[File:CY2.gif|right|frame]]