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Belong to a parts of transmitting or receiving system.  To be designed to radiate or to receive electromagnetic waves.
VSWR : Voltage Standing Wave Ratio
VSWR is a measure of impedance mismatch between the transmission line and the antenna. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity. The combination of the original wave traveling down the coaxial cable (towards the antenna or opposite during receive) and the reflecting wave is called a standing wave. The ratio of the two above described waves is known as the Standing Wave Ratio.

To understand the definition above we must understand what impedance is. Impedance in antenna terms refers to the ratio of the voltage to current (both are present on an antenna) at any particular point of the antenna. This ratio of voltage to current varies on different parts of the antenna, which means that the impedance is different on different spots on the antenna if you could pick any spot and measure it.
As stated before, the impedance for the entire chain from the rf modular to the antenna must be the same, and almost all radio equipment is built for an impedance of 50-ohm. If any part of this chain fails to show a 50-ohm impedance due to e.g. bad connections, incorrect antenna length, etc., the maximum power will not be radiated from the antenna. Instead part (or all) of the wave is reflected back down the line. The amount of the wave reflected back depends on how bad the mismatch is.
The result is presented as a figure describing the power absorption of the antenna. A value of 2.0:1 VSWR, which is equal to 90 % power absorption, is considered very good for a small antenna: 3.0:1 is considered acceptable (-6dB) which is equal to 75 % power absorption.

Impedance matching
An ideal antenna solution has an impedance of 50 ohm all the way from the transceiver to the antenna, to get the best possible impedance match between transceiver, transmission line and antenna. Since ideal conditions do not exist in reality, the impedance in the antenna interface often must be compensated by means of a matching network, i.e. a net built with inductive and/or capacitive components. The VSWR result is optimized by choosing the proper layout and component values for the matching net and the maximum potential of the antenna is shown.

Return Loss
This is basically the same thing as VSWR. If the antenna absorbs 50 % of the signal and 50 % is reflected back, we say that the Return Loss is -3dB. A very good antenna might have a value of -10dB (90 % absorbed & 10 % reflected).
When studying a graph showing Return Loss/VSWR, a deep and wide dip of the curve is good since this shows an antenna with good bandwidth (spreadband). Consequently, the narrower the dip is, the bigger risk that also desired channels will be reflected away (narrow band).

Efficiency (ß) is a figure showing the ratio of the total radiated power (Pr) to the total input power (Pi). Efficiency has no unit and the ideal figure is 1.
= Pr / Pi
In real life, a figure of 75% is very good while 50% is acceptable. It is essential to know how the measurement was performed before comparing figures from different manufacturers: was a matching network used? Was the measuring point as close to the antenna as possible or was the transmission line included? Often, the figure for efficiency will dramatically decrease when the antenna is built into a device.

This is similar to gain but the heat losses (i.e. the effectivity) are disregarded. We will then get a pattern as the dotted line shown in the figure. The value for directivity is then calculated just as for gain, the only difference being that the relation is now c/b.
D = P(peak) / P(av)

Radio waves are built by the electric and the magnetic fields. These two fields are perpendicular to each other. The sum of the fields is the electromagnetic field. Energy flows back and forth from one field to the other - This is what is known as "oscillation". The position and direction of the electric field with reference to the earth surface (the ground) determines wave polarization. In general, the electric field is the same plane as the antenna's radiator.
Horizontal polarization -- the electric field is parallel to the ground.
Vertical polarization -- the electric field is perpendicular to the ground.
There is one special polarization known as Circular polarization. As the wave travels it spins, covering every possible angle. It can either be right-handed or left-handed circular polarization depending on which way its spinning.
Note: Small antennas have no clear polarization.

This is the directiveness of a directional antenna, defined as the angle between two half-power (-3 dB) points on either side of the main lobe of radiation.

dB Units
Decibel (dB) is a mathematical expression showing the relationship between two values. The RF power level at either transmitter output or receiver input is expressed in Watts, but it can also be expressed in dBm. The relation between dBm and Watts can be expressed as follows:
P dBm = 10 x Log P mW

Antenna polarization:
In a specified direction from an antenna and at a point in its far field, is the polarization of the (locally) plane wave which is used to represent the radiated wave at that point.

Directional antenna:
An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others.

For a linearly polarized antenna, the plane containing the electric field vector and the direction of maximum radiation.

Far-field region:
That region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region.

Frequency bandwidth:
The range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard.

Front-to-back ratio:
The ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction.

Half-power beamwidth:
In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value.

Half-wave dipole:
A wire antenna consisting of two straight collinear conductors of equal length, separated by a small feeding gap, with each conductor approximately a quarter-wave length long.

For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation.

Input impedance:
The impedance presented by an antenna at its terminals.

A measure of power transfer from one antenna to another.

Isotropic radiator:
A hypothetical and loss less antenna, which has equal radiation intensity in all directions.

Major/main lobe:
The radiation lobe containing the direction of maximum radiation.

Microstrip antenna:
An antenna that consists of a thin metallic conductor bonded to a thin-grounded dielectric substrate.

Omnidirectional antenna:
An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane.

Radiation efficiency:
The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.

Side lobe suppression:
Any process, action or adjustment to reduce the level of the side lobes or to reduce the degradation of the intended antenna system performance resulting from the presence of side lobes.

We manufacture wireless handheld and fixed device. What kind of antenna that Procell recommend us?
You can choose our rubber, stubby or whip antenna for your device using. There are many connector types in our antenna to fit your handheld device, and so many different brackets in our antenna to cooperate with different way to fix antenna for your device, like screw, magnetic, sticker --- way for your fixed device choice.
We are a distributor of wireless equipment and our customers use standard Access point or wireless card cards products in their equipment. They are concerned about the signal strength and would like t
You can suggest settling the Access point into our antenna building enclosure that is able to external transmission distance. We can make connector like UFL, MMCX, or SMA, --) to work with your device.
Also you can suggest using our high gain antenna to cooperate with our adapter cable with your device to external distance well.
If antenna back side is metal material, how to avoid reflection problem?
When antenna connector is outside of AP, you can use our external antennas with adapt cable to find best place to settle antenna and get the maximum signal strength, Or use RS-H/P, PA-S/N, SAL or whip antenna- (with cable one) series antennas to directly settle in best place. Please kindly note, that will be bad place if some metal to close antenna, it will be effort antenna electronic function.
We are trying to build an extensive wireless environment for our office. The existing antenna that is in used is a rubber antenna (around 2-3dBi). What can Procell offer us to improve the efficiency o
Depend on house size & building style. 

In a single floor, you might be able to use smaller Sector or panel (Like SA or PA SOHO series.) for single direction way using. Or just change higher gain rubber antenna, like RA or RS series antenna in 5, 7, or 9dBi for 360degree external distance.

For some floors building: You also can settle CA series(Dome) antenna in highest Ceiling to connect with your device, and wireless signal(data) will be transmission from highest Ceiling forward to down way(like next floor or after next floors.).
We are using outdoor wireless equipment and we need to increase the transmission range. We need to transmission data in point to point. What kind of antenna can you offer us.
For outdoor antenna and you want to use point to point transmission. We make YAGI, PANEL, Grid antennas that are perfect for point to point application. Please refer to information available in our website, you can choose the different gain to according to the extend distance difference. Basically, the directional distance will be longer to follow the gain getting to be big .but the eradiation angel will be smaller to follow the gain is getting to bigger. Most of our antennas are hi gain & outdoor using you can select their features based upon your requirements.
We are using outdoor wireless equipment and we need to increase the transmission range. We need to transfer data in point to multi point or multi point to multipoint. What kind of antenna can you offe
For outdoor antenna and you want to use point to Multi point transmission, we make PANEL, OMNI & Sector antennas that are perfect for multi point for your application.
We suggest OMNI or SECTOR antenna can settle in the middle to collect signal, and Panel or Yagi or Grid can be in different place to send signal to middle device.
Please refer to information available in our website, you can choose the different gain to according to the extend distance & eradiation angle difference. Basically, the directional distance will be longer to follow the gain getting to be big, but the eradiation angel will be smaller to follow the gain is getting to bigger in panel antenna, But the Sector & OMNI will not change eradiation angle in Horizontal, only change in vertical angle if to based on Vertical antenna . Most of our antennas are hi gain & out door using you can select their features based upon your requirements.
We could not find the type of antenna we need from your website. What do you suggest?
We will be pleased to be your OEM/ODM supplier. We have the technological know-how to design and manufacture antennas according the specifications of your wireless equipment, be it rubber antennas, panel antennas, OMNI antennas, PC board flat antennas or any other types of antennas that can be used for you.

At the end, we have to say all of wireless system will need very good quality in every single component, from active radio to adapt cable to antenna, even splitter, lightning protector. If you are try to reduce cost, and to use lower quality component, like to use bad cable or not good function antenna, then your wireless system must be bad, however you use very good quality active radio, because single can be lose by there, and interference will be came into system by there, besides, if VSWR is not good, it will cause the transmission not stable, even cause to burn the active radio. To cherish your wireless system, please choose the qualified component carefully.