lunes, 7 de marzo de 2011

Scheme of Standing Wave Ratio (SWR)

Scheme of Standing Wave Ratio (SWR)

In telecommunications for example Communication and WiFi HT 2.4 MHz or 5.8 MHz, standing wave ratio (SWR) is the partial wave amplitude ratio stood at antinode (maximum) for amplitude in an adjacent node (minimum), in electrical transmission lines.
SWR is usually defined as the ratio of voltage is called the VSWR, for voltage standing wave ratio. For example, the value of standing wave VSWR 1.2:1 shows the maximum amplitude is 1.2 times larger than the minimum value of standing waves. It is also possible to define the SWR in terms of currents, yield ISWR, which has the same numerical value. The power standing wave ratio (PSWR) VSWR is defined as the square.
Practical implications of the SWR.
The most common one to measure and check the SWR is when installing and tuning the transmission antenna. When the transmitter is connected to the antenna feed line, the impedance of the antenna and feed lines must be identical to the maximum energy transfer from the feed line to the antenna becomes possible. Impedance of the antenna varies based on many factors, including: the natural resonance frequency of the antenna on that is being sent, the antenna height above ground, and the size of conductor used to make antenna.
When the antenna and feedline has no impedance matching, several electrical energy can not be transferred from the feedline to the antenna. Energy is not transferred to the antenna is reflected back to receiver. It is the interaction of these waves are reflected by the forward wave which causes the standing wave pattern. Reflected power has three major implications on radio transmitters: Radio Frequency (RF) energy loss increases, the distortion in the transmitter because the reflected power from the transmitter load and damage can occur

SWR-Matching Impedance

Matching the impedance of the antenna to the impedance of the feed line is usually done by using an antenna tuner. tuner can be plugged in between the transmitter and the feed line, or between the feed lines and antennas. Both methods of installation will allow the transmitter to operate at a low SWR, but if the tuner is mounted on the transmitter, feed line between the tuner and antenna will still operate with a high SWR, causing additional RF energy to be lost through the feedline.
Many amateur radio operators to consider the problem serious impedance mismatch. However, this does not happen. Assuming the discrepancy is within the limits of operation of these transmitters, radio operators only need to worry about the loss of power on the transmission line. Power loss will increase with increasing SWR, but this increase is often less than might consider many amateur radio. For example, a dipole antenna tuned to operate at 3.75MHz center of the 80-meter-band radio amateurs will show SWR around 6:01 on the outskirts of the band. However, if the antenna was given with 250 feet of coax RG-8A, losses due to standing waves of only 2.2dB. Feed line loss typically increases with frequency, so antenna VHF and above must match closely with the feedline. 6:01 mismatch equal to 250 feet of RG-8A coax will cause harm to 10.8dB 146MHz.



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