Principles and Characteristics
Radiation Principles: Antenna is radiates by the principle of resonance. Resonance implies maximum standing current wave is observed along the antenna length. A simple dipole antenna has two antenna rods pointing in opposite directions but remains parallel. If an AC voltage generator fed in the middle between two antennas, current flowing on both rods can be seen as in phase when AC generator operates at antenna resonance frequency.
In phase means that current along each antenna rod are traveling in the same direction with equal time varying amplitude. This means the electric field created by two rods aligns in the space and any alternating field due to AC generator can be added together (i.e in phase) to create strong electromagnetic wave presence in the free space.
Note: Current is consisting of flowing electrons, and electron creates inward pointing electric field lines. Moving electron creates disturbance in the inward radiating electric field lines distributed in free space; hence an alternating electron creates sinusoidal like electric field line disturbance/ripples resembling electromagnetic waves propagating through space. Remember the radiation EM wave contains both electric and magnetic field components, the magnetic field is perpendicular in the oscillating electric field.
Principle of Resonance: Resonance creates a standing wave on a open or short ended conductor, which occurs at a minimum of 1/4 electrical wave length or odd multiple of 1/4 such as 3/4, 5/4 wave length etc.
Note: Resonance is the reason that Antenna is always designed with 1/4 wave length or its odd multiple.
Common method to find bandwidth of antenna is using Return loss, a S parameter test, where an antenna is hooked up to a spectrum analyser port. This test measures the ratio of reflected power with respective to incident power (an output by the spectrum analyser) for a range of frequency sweep.
The less reflected power it is of an measurement the more efficient or ideal the antenna is at electromagnetic radiation. Ideally, the return loss (S11) of an Antenna is 0, which is negative infinity in the logarithmic scale.
In practice, one would want to make sure the range of antenna is with a specified range such as Bluetooth (2400 MHz to 2480MHz), where return loss at the start and end frequency is within -10 dB return loss (i.e 10% reflected power and 90% radiated power).
Voltage standing wave ratio (VSWR)
It's ratio of maximum voltage to the minimum voltage for a standing wave on a transmission line. It ranges from 1 to infinity. VRSW of 1 means there is no reflection in the transmission line (i.e load impedance matches transmission line impedance, results in no standing wave, and infinity means full reflection (i.e there is either open or short on the end of the transmission line, results in perfect standing wave .
In practice, VSWR is used to evaluate the level of impedance matching.
In the context of antenna, the antenna is the load of the transmission line, and ideally, we want to minimize any power from reflecting back to the source. Hence, a perfect impedance match is desired (i.e VSWR of 1) is desired for Antenna.
It measures the efficiency of incident power to radiated power.
typical greater than 50% is considered good
it's a measure of peak antenna performance in transmission or reception at a single direction compared to a omni-directional reference antenna (isotropic). It lets the designer the maximum strength of the signal that can be radiated or picked up.
Note: Antenna gain is often the most useful # in viewing antenna design as in line of sight range calculation.
Directivity is the ratio of peak power observed in one direction over average power in all directions.
It measures the power levels transmitted with respect to the angles away from the antenna. in other words, it tells your the directionality or coverage area of the antenna
In practice, you would want to have a omni-directional antenna for your wireless product because the transmitter (e.g. WLAN Access Point) can be anywhere is the building.
Polarization of radiating EM waves describes the direction of electric field with respect to propagation. For an antenna to pick up the EM wave most efficiently, the antenna itself should have the same polarization direction of EM components.
One can think application of polarization as sun glasses, it only allows certain directional component of light EM wave to come in, hence blocking a majority of sunlight from reaching to your eyes.
In a short range design such as house hold, AP is strong enough to overcome this short comings.
Summary and Conclusion
Antenna resonance emits and picks up electromagnetic waves.
Resonance creates standing current waves
1/4 wavelength suffices minimal requirements of resonance of a circuit resonator.
Overview key antenna characteristics, Return loss, VSRW, efficiency, and gain, radiation pattern.
Electromagnetic radiation is based on principle of electrical resonance. Real antenna's performance is characterized by impedance matching VSRW, radiation coverage pattern, antenna best directional gain.
Reference and Further Reading
"Why do antenna radiate", http://www.antenna-theory.com/basics/whyantennasradiate.php
"Mono pole antenna", https://en.wikipedia.org/wiki/Monopole_antenna