Minggu, 27 Desember 2009

SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line

SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. This is an item of radio equipment used to check the quality of the match between the antenna and the transmission line.

The VSWR meter should be connected in the line as close as possible to the antenna. This is because all practical transmission lines have a certain amount of loss, causing the reflected power to be attenuated as it travels back along the cable, and producing an artificially low VSWR reading on the meter. If the meter is installed close to the antenna, then this problem is minimized.

Referring to the above diagram, the transmitter (TX) and antenna (ANT) terminals are connected via an internal transmission line. This main line is electromagnetically coupled to two smaller sense lines which are connected to resistors at one end, and diode rectifiers at the other. The resistors are chosen to match the characteristic impedance of the sense lines. One sense line senses the forward wave (connected to FWD), and the other the reflected wave (connected to REV). The diodes convert these to FWD and REV DC voltages respectively, the ratio of which is used to determine the VSWR. In a passive meter, this is indicated on a non-linear meter scale.

To calculate the VSWR, first calculate the reflection coefficient:

Note that an SWR meter does not measure the actual impedance of a load (ie the resistance and reactance), but only the mismatch ratio. To measure the actual impedance, an antenna analyzer or other similar RF measuring device is required. Note also that for accurate readings, the SWR meter must be matched to the line impedance, ie 50 or 75 ohms as applicable. To accommodate both impedances, some SWR meters have switches on the rear, to select the appropriate load resistance for the sense lines.

If a mismatch exists between the transmission line and load, the line will act as an impedance transformer. In this case, the impedance seen at the input to the line will depend on its electrical length, although (for a lossless line) the VSWR will be the same at any point along the line. Mismatched transmission lines are often used for impedance transformation, especially at UHF and microwave frequencies where their dimensions can be very short. For more information on this handy technique, see smith chart.

When not actually measuring VSWR, it is best to remove the more usual type of passive SWR meter from the line. This is because the internal diodes of such meters can generate harmonics when transmitting, and intermodulation products when receiving. Because active SWR meters do not usually suffer from this effect, they can normally be left in without causing such problems.

In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.

The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of current, resulting in the ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.

Practical implications of SWR

The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors including: the antenna's natural resonance at the frequency being transmitted, the antenna's height above the ground, and the size of the conductors used to construct the antenna.

When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna. Energy not transferred to the antenna is reflected back towards the transmitter. It is the interaction of these reflected waves with forward waves which causes standing wave patterns. Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load and damage to the transmitter can occur.

Matching the impedance of the antenna to the impedance of the feed line is typically done using an antenna tuner. The tuner can be installed between the transmitter and the feed line, or between the feed line and the antenna. Both installation methods will allow the transmitter to operate at a low SWR, however if the tuner is installed at the transmitter, the feed line between the tuner and the antenna will still operate with a high SWR, causing additional RF energy to be lost through the feedline.

Many amateur radio operators believe any impedance mismatch is a serious matter. However, this is not the case. Assuming the mismatch is within the operating limits of the transmitter, the radio operator needs only be concerned with the power loss in the transmission line. Power loss will increase as the SWR increases, however the increases are often less than many radio amateurs might assume. For example, a dipole antenna tuned to operate at 3.75MHz—the center of the 80 meter amateur radio band—will exhibit an SWR of about 6:1 at the edges of the band. However, if the antenna is fed with 250 feet of RG-8A coax, the loss due to standing waves is only 2.2dB. Feed line loss typically increases with frequency, so VHF and above antennas must be matched closely to the feedline. The same 6:1 mismatch to 250 feet of RG-8A coax would incur 10.8dB of loss at 146MHz.

A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission. Types of transmission line include wires, coaxial cables, dielectric slabs, striplines, optical fibers, electric power lines, and waveguides.

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