Antenna Definitions

What is Isotropic radiator?
. An isotropic radiator is a hypothetical lossless antenna that radiates its energy equally in all directions. This imaginary antenna would have a spherical radiation pattern



What is GAIN of an antenna?
 The gain of an antenna (in any given direction) is defined as the ratio of the power gain in a given direction to the power gain of a reference antenna in the same direction. It is standard practice to use an isotropic radiator as the reference antenna in this definition. Note that an isotropic radiator would be lossless and that it would radiate its energy equally in all directions. That means that the gain of an isotropic radiator is G = 1 (or 0 dB). It is customary to use the unit dBi (decibels relative to an isotropic radiator) for gain with respect to an isotropic radiator. Gain expressed in dBi is computed using the following formula:
GdBi = 10*Log (GNumeric/GIsotropic) = 10*Log (GNumeric)

Note that when a single number is stated for the gain of an antenna, it is assumed that this is the maximum gain (the gain in the direction of the maximum radiation).
It is important to state that an antenna with gain doesn't create radiated power. The antenna simply directs the way the radiated power is distributed relative to radiating the power equally in all directions and the gain is just a characterization of the way the power is radiated.


What is 3-dB beamwidth?
. The 3-dB beamwidth (or half-power beamwidth) of an antenna is typically defined for each of the principal planes. The 3-dB beamwidth in each plane is defined as the angle between the points in the main lobe that are down from the maximum gain by 3 dB. 
Analogy for 3db beamwidth: Take a torch light and put on and point towards the wall in a dark room. What you see is a circle light on the wall. What is the width of the beam? You cannot easily judge as the intensity slows decreases from the centre towards outside. To decide the beam width,note down the intensity at the centre and find out upto which point, the intesity is half. Just ignore the remaining portion.The width of this beam is 3db beam width.


The 3-dB beamwidth in the plot in this figure is shown as the angle between the two blue lines in the polar plot. In this example, the 3-dB beamwidth in this plane is about 37 degrees. Antennas with wide beamwidths typically have low gain and antennas with narrow beamwidths tend to have higher gain. Remember that gain is a measure of how much of the power is radiated in a given direction. So an antenna that directs most of its energy into a narrow beam (at least in one plane) will have a higher gain.

What is Front-to-back ratio?

 The front-to-back ratio (F/B) is used as a figure of merit that attempts to describe the level of radiation from the back of a directional antenna. Basically, the front-to-back ratio is the ratio of the peak gain in the forward direction to the gain 180-degrees behind the peak. 

What is Antenna Polarization? 

The polarization of an antenna is the orientation of the electric field. 
 An antenna will generate an electromagnetic wave that varies in time as it travels through space. If a wave traveling "outward" varies "up and down" in time with the electric field always in one plane, that wave (or antenna) is said to be linearly polarized (vertically polarized since the variation is up and down rather than side to side). If that wave rotates or "spins" in time as it travels through space, the wave (or antenna) is said to be elliptically polarized. As a special case, if that wave spins out in a circular path, the wave (or antenna) is circularly polarized. This implies that certain antennas are sensitive to particular types of electromagnetic waves. The practical implication of this concept is that antennas with the same polarization provide the best transmission/reception path.
Consider antennas that generate and are sensitive to linearly polarized waves. If a linearly polarized antenna launches a linearly polarized electromagnetic wave traveling "up and down" or vertically, the best possible receiver of that electromagnetic wave will be another antenna that is similarly linearly polarized (vertically polarized). Linear polarization also includes the possibility of the electromagnetic waves traveling "right to left" (horizontally) as well. Often antennas can simply be physically rotated to make them horizontally or vertically polarized, although this may not always be the best choice.
Circularly polarized antennas can radiate electromagnetic waves that spin clockwise or counter-clockwise depending on the structure. So a similarly polarized antenna should be used to receive these signals. This spin direction is typically characterized by left circular polarization (LCP) or right circular polarization (RCP).
Note that the polarization of an antenna doesn't always imply anything about the size or shape of the antenna. A dipole is usually called vertically polarized because of the way a dipole is typically used, that is, because it is mounted vertically, but the antenna is linearly polarized. Likewise, antennas that are circular in their construction do not have to be circularly polarized. Many circular patches are linearly polarized and many rectangular patches are circularly polarized. These examples are simple demonstrations of the fact that the polarization state of an antenna is not related to its shape.

What is E Plane?or Elevated Plane? or Electrical Plane?What is  Magnetic Plane?or H plane? or Aziuthal Plane?

The Elevated plane is the vertical plane which is parallel with the 
antenna . H Plane is perpendicular with the antenna.



What is VSWR?
 The voltage standing wave ratio (VSWR) is defined as the ratio of the maximum voltage to the minimum voltage in a standing wave pattern. A standing wave is developed when power is reflected from a load. So the VSWR is a measure of how much power is delivered to a device as opposed to the amount of power that is reflected from the device. If the source and load impedance are the same, the VSWR is 1:1; there is no reflected power. So the VSWR is also a measure of how closely the source and load impedance are matched. For most antennas in WLAN, it is a measure of how close the antenna is to a perfect 50 Ohms.

In this vwr meter, there are two needles. one needle measures the forward power
and the other needle measures the reflected power. The cross section of these two needles shows the vswr.

What is VSWR bandwidth?

 The VSWR bandwidth is defined as the frequency range over which an antenna has a specified VSWR. Often, the 2:1 VSWR bandwidth is specified, but 1.5:1 is also common.

What is the difference between VSWR and Return Loss?

VSWR is a unitless ratio but Return Loss is in dB. 
If VSWR value is less then system  efficiency is good. But if Return Loss value is more then efficiency is good.

VSWR=1 means the system is 100% efficient.  Return Loss is 46 means, the system is 100% efficient.

VSWR=1.5 means, 96% efficient ( ie 4% enery is lost). Where as Return Loss=14 db means 96% efficient.

Compare VSWR and Return Loss
VSWR       Return Loss
1.01   ....    46.1 dB
1.50  .....    14 dB
2.00   ....    8.72 dB

What are the causes for high VSWR?

Impedance mismatching is main cause of higher VSWR. Proper cable selection, proper connector selection and proper installation will reduce VSWR. VSWR of less than 1.5 is acceptable in most cases.




What is RF Feeder?

It is a cable running between the antenna and the transmitter/receiver.

For example, the cable between your Dish Antenna and your TV is a Feeder Cable.

What are  the different types of Feeder Cables?

1. Twin feeder : It is a pair of wires running parallelly. This is very rarely used because of very high loss. It can be used below 30 MHz

2. Coaxial Cable : this is mostly used for RF frequences below  2GHz.
3. WaveGuide : this is used above 2Ghz. It is just like coaxial cable. But the centre conductor is missing. A wave guide of given size can carry one a certain range of frequences
For example WG10 wave guide can carry 2.6 ghz to 3.95 ghz

What are the characteristics of feeder cable?
1. Loss should be low
2. it should not pick up noise from external sources
3. it should not radiate the rf frequency which is travelling in it.
How the coaxial cable will look like?
There will be inner conductor at the centre.
The outer conductor will be metal tube or braided wire.
In between the two conductors, dielectric material will be sitting.

What is the difference between Feeder cable and Jumper Feeder cable?

Jumper is always of short length and flexible and is present very near to the equipment and antenna.
For the purpose of flexibility, the thickness of the jumper  cable will be small when compared to the main feeder cable.
The main feeder is of long length and should be of less loss (say 4db per 100 meter) and to reduce the loss, this size of the main feeder cable will be bigger than jumper feeder cable. jumper feeder may be of 7db per 100 meter loss.