VSWR相关资料,希望能帮到你

ronever

VSWR 相关资料,希望能给一些刚学天线的人或者想懂天线的人一些参考,比较好的一份东西

【文件名】:0767@52RD_VOLTAGE STANDING WAVE RATIO (VSWR) REFLECTION COEFFICIENT.pdf
【格　式】:pdf
【大　小】:39K
【简　介】:
【目　录】:

john019

VOLTAGE STANDING WAVE RATIO (VSWR) / REFLECTION COEFFICIENT
RETURN LOSS / MISMATCH LOSS
When a transmission line is terminated with an impedance, Z , that is not equal to the characteristic impedance of L
the transmission line, Z , not all of the incident power is absorbed by the termination. Part of the power is reflected back O
so that phase addition and subtraction of the incident and reflected waves creates a voltage standing wave pattern on the
transmission line. The ratio of the maximum to minimum voltage is known as the Voltage Standing Wave Ratio (VSWR)
and successive maxima and minima are spaced by 180E (8/2).
where E = maximum voltage on the standing wave max
E = minimum voltage on the standing wave min
E = incident voltage wave amplitude i
E = reflected voltage wave amplitude r
The reflection coefficient, D, is defined as E /E and in general, the termination is complex in value, so that D will r i
be a complex number.
Additionally we define: The refection coefficient, D, is the absolute value of the magnitude of ’.
If the equation for VSWR is solved for the reflection coefficient, it is found that:
Consequently,
The return loss is related through the following equations:
Return loss is a measure in dB of the ratio of power in the incident
wave to that in the reflected wave, and as defined above always has a
positive value. For example if a load has a Return Loss of 10 dB, then
1/10 of the incident power is reflected. The higher the return loss, the
less power is actually lost.
Also of considerable interest is the Mismatch Loss. This is a measure
of how much the transmitted power is attenuated due to reflection. It
is given by the following equation:
Mismatch Loss = -10 log ( 1 -D ) 2
For example, an antenna with a VSWR of 2:1 would have a reflection coefficient of 0.333, a mismatch loss of 0.51 dB, and
a return loss of 9.54 dB (11% of your transmitter power is reflected back). In some systems this is not a trivial amount and
points to the need for components with low VSWR.
If 1000 watts (60 dBm/30 dBW) is applied to this antenna, the return loss would be 9.54 dB. Therefore, 111.1 watts would
be reflected and 888.9 watts (59.488 dBm/29.488 dBW) would be transmitted, so the mismatch loss would be 0.512 dB.
1.01 1.02 1.04 1.06 1.1 1.08 1.2 1.4 1.6 1.8 2.0 1.3
20
10
8654
2
3
1.2
1.5
1.02
1.05
1.1
1.3
1.7
1.03
1.08
Input VSWR
Input
VSWR
Attenuator
X dB
Load
VSWR
Load
Example
1.5:1 (Example)
6-2.2
Figure 1. Reduction of VSWR by Attenuation
Transmission line
attenuation improves the
VSWR of a load or
antenna. For example, a
transmitting antenna with a
VSWR of 10:1 (poor) and a
line loss of 6 dB would
measure 1.5:1 (okay) if
measured at the transmitter.
Figure 1 shows this effect.
Therefore, if you
are interested in
determining t h e
performance of antennas,
the VSWR should always
be measured at the antenna
connector itself rather than
at the output of the
transmitter. Transmit
cabling will load the line
and create an illusion of
having a better antenna
VSWR. Transmission lines should have their insertion loss (attenuation) measured in lieu of VSWR, but VSWR
measurements of transmission lines are still important because connection problems usually show up as VSWR spikes.
Historically VSWR was measured by probing the transmission line. From the ratio of the maximum to minimum
voltage, the reflection coefficient and terminating impedance could be calculated. This was a time consuming process since
the measurement was at a single frequency and mechanical adjustments had to be made to minimize coupling into circuits.
Problems with detector characteristics also made the process less accurate. The modern network analyzer system sweeps
very large frequency bandwidths and measures the incident power, P , and the reflected power, P . Because of the i r
considerable computing power in the network analyzer, the return loss is calculated from the equation given previously, and
displayed in real time. Optionally, the VSWR can also be calculated from the return loss and displayed real time.
If a filter is needed on the output of a jammer, it is desirable to place it approximately half way between the jammer
and antenna. This may allow the use of a less expensive filter, or a reflective filter vs an absorptive filter.
Special cases exist when comparing open and shorted circuits. These two conditions result in the same 4 VSWR
and zero dB return loss even though there is a 180E phase difference between the reflection coefficients. These two
conditions are used to calibrate a network analyzer.[em03]

xdouliu

就这样子梦钱[em01]