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VSWR  FUNCTIONAL

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VSWR - RETURN LOSS - G  CONVERSIONS

Return Loss Voltage Standing
Wave Ratio
Reflection Coefficient
RL = -20*log|G| dB VSWR = G =
 
RL(dB) VSWR G   RL(dB) VSWR G   RL(dB) VSWR G   RL(dB) VSWR G
46.0 1.01 0.00498   26.0 1.11 0.0521   17.7 1.30 0.130   8.0 2.32 0.398
40.0 1.02 0.00990   25.0 1.12 0.0566   17.0 1.33 0.141   7.0 2.61 0.445
37.0 1.03 0.0148   24.0 1.13 0.0610   16.0 1.38 0.158   6.02 3.01 0.500
34.0 1.04 0.0196   23.5 1.14 0.0654   15.0 1.43 0.178   5.0 3.56 0.562
32.0 1.05 0.0244   23.0 1.15 0.0698   14.0 1.50 0.200   4.0 4.42 0.631
30.4 1.06 0.0291   22.0 1.17 0.0783   13.0 1.58 0.224   3.01 5.85 0.707
29.0 1.07 0.0338   21.5 1.18 0.0826   12.0 1.67 0.250   2.0 8.72 0.794
28.0 1.08 0.0385   20.7 1.20 0.0909   11.0 1.78 0.282   1.0 17.39 0.891
27.0 1.09 0.0431   20.0 1.22 0.100   10.0 1.92 0.316   0.5 34.75 0.944
26.4 1.10 0.0476   19.0 1.25 0.112   9.0 2.10 0.355   0.0 Infinity 1.00

 

VSWR REDUCTION BY MATCHED ATTENUATOR

 

By inserting a matched (nominal system impedance) attenuator in front of a mismatched load impedance, the mismatch "seen" at the input of the attenuator is improved by an amount equal to twice the value of attenuator. The explanation is simple.

Return loss is determined by the portion of the input signal that is reflected at the load (due to impedance mismatch) and returned to the source. A perfect load impedance (complex conjugate of the source impedance) would absorb 100% of the incident signal and therefore reflect 0% of it back to the source (return loss of dB).

For the sake of illustration, assume that the load is an open (or short) circuit, where 0% of the incident signal is absorbed by the load and 100% is reflected back to the source. The reflected signal would therefore have a return loss of 0 dB. Insert a 3 dB  attenuator in front of the load. Now the incident signal is referenced to the input of the attenuator.

As signal at the input of the attenuator will experience a 3 dB reduction in power by the time it reaches the load. That 3 dB less power will be 100% reflected by the load and experience another 3 dB reduction in power by the time is returns back to the input, for a total loss of 6 dB. The same principle applies for a load anywhere() between zero and infinite load impedance (short and open circuits, respectively).

Calculate the improved VSWR as follows:

  1. Convert the load VSWR to load return lossper the following equation:

                          RLLOAD=20*log dB
  2. Add twice the attenuation value to RLLOAD: RLNEW=RLLOAD + 2*ATTEN dB
  3. Convert back to VSWR per the following equation: VSWR=

Of course, the method can be reversed to predict the attenuator required to improve a load VSWR by a predetermined amount. To do so, calculate the desired return loss and subtract the known load return loss. Divide the answer by two to get the attenuator value needed.

  Actually, the attenuator is only rated for its specified attenuation level when it is connected between two nominal impedances. Therefore, the attenuator will either have to be designed to closely match the two impedances at its input and output (source and load, respectively), or an adjustment will need to be made in the specified attenuation value to compensate for the mismatched load impedance.

 

VSWR MISMATCH ERRORS

Both amplitude and phase errors are introduced when mismatched impedances are present at an electrical interface. The result is ripple across the frequency band (since the VSWR of each interface typically varies with frequency), as well as a portion of the incident power being reflected back to the source.

Amplitude Error Phase Error
eA = 20 * log (1 |GA * GB|) dB ef = 180 / p * GA * GB
Resultant MIN and MAX Cascaded VSWR
VSWRMAX = SA * SB
VSWRMIN = SA / SB
 

where

SA = larger of the two VSWRs
SB = smaller of the two VSWRs

Example

VSWRA = 2.5:1  -->  SA = 2.5
VSWRB = 2.0:1  -->  SB = 2.0
VSWRMAX = 2.5 * 2.0 = 5.0  = 5.0:1
VSWRMIN = 2.5 / 2.0 = 1.25  = 1.25:1