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Antenna basics

射频方法与案例   http://rfcc.bokee.com/

VSWR

VSWR is a measure of impedance mismatch between the transmission   line and its load. The higher the VSWR, the greater the mismatch. The   minimum VSWR, i.e., that which corresponds to a perfect impedance    match, is unity.

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 radio 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 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.

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. 

Smith Chart  

One common way of visualizing the VSWR is a polar plot called Smith   chart. From this plot the VSWR value, the return loss and the impedance   for the different frequencies can be derived. Therefore it is an important   instrument for understanding antennas. To learn more about the SMITH   chart, see e.g. http://sss-mag.com/smith.html

Retrun Loss

This is basically the same thing as VSWR.

If 50 % of the signal is absorbed by the antenna 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).

Return Loss Chart  

Note: To be able to compare figures from different manufacturers, you   must be aware of the conditions under which the measurement was   made. Was impedance matching used or not?

  
Conversion table VSWR / Return Loss

Bandwidth

Normally a radio needs to work on multiple frequencies. For example, the   2.4 GHz ISM band used by Bluetooth/Wi-Fi/Zigbee/WiMedia devices   has a range from 2400-2483 MHz. In this band PAN   communication uses 78 channels for its frequency hopping technique, 1 MHz between each channel.

 This means that the antenna must perform well over a range of   frequencies. So, the goal must be to make it resonant in the middle of   that band. The term that is important here is bandwidth or how much   band your antenna works well over. One method of judging how well   (efficiently) your antenna is working is by measuring VSWR. 

Typically, bandwidth is measured by looking at SWR, i.e., by finding the   frequency range over which the SWR is less than 2.

Efficiency

Efficiency is a figure showing the ratio of the total radiated power to the   total input power . Efficiency has no unit and the ideal figure is 1.

Efficiency =radiated power /input power

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.

Note: This is a good figure of merit, especially for small antennas.  

Efficiency

Gain & 3D Pattern

Antenna gain is a measure of directivity. In order to explain this better,   we must first have a look at the different antenna types and their radiation patterns.

Basically there are only two types of antennas: The dipole antenna   (Hertzian) and the vertical antenna (Marconi). All antennas can be   broken down to one of these types (although some say that there is only   one - the dipole). In addition to this we have a theoretical perfect   antenna (non-existent) that radiates equally in all directions with 100%   efficiency. This antenna is called an isotropic radiator.

Basic Antenna types 
 
  
Antenna Radiation Patterns  

This is similar to gain but the heat losses (i.e. the efficiency) are  disregarded. We will then get a pattern as the dotted line shown in the   figure. Point "d" refers to directivity,point "a" to gain and point "b" to the   isotropic reference. 

Gain presented as 3D gain

 

The gain can also be presented as a 3D gain. The radius of the spheriod is   proportional to the antenna gain.

Gain in theory Since all real antennas will radiate more in some directions   than in others, you can say that gain is the amount of power   you can reach in one direction at the expense of the power lost in the   others. When talking about gain it is always the main lobe that is   discussed.

Gain may be expressed as dBi or dBd. The first is gain compared to the   isotropic radiator and the second gain is compared to a half-wave dipole   in free space (0 dBd="2".15 dBi).

It may be worthwhile considering the fact that instead of doubling your   amplifier output, you could alternatively use an antenna that has 3db   more gain than your current antenna and achieve exactly the same  effect.

Note: Small antennas usually have low gain, often between 0 and 2dBi.

Note: Regarding efficiency and radiation patterns - what is  true for  transmission is generall also true for  reception.

Directivity

This is similar to gain but the heat losses (i.e. the   efficienc