Understanding VSWR

vswr bandwidth sweep

Understanding VSWR & Return Loss

One of the key parameters of any radio transmission component is VSWR or Return Loss (RL), which both measure how well the component is matched to other system components. Both are used interchangeably, with VSWR providing the measure as a ratio, and Return Loss providing the measure as a decibel figure. This short article we aim to provide an introductory understanding of these concepts.

Definition of VSWR

The Voltage Standing Wave Ratio (VSWR) is defined as the ratio of the maximum amplitude to the minimum magnitude of the voltage standing wave at an input port of an antenna. The VSWR is a measure of the matching of the antenna to the source and feeder cables. A low VSWR will minimize reflections from the antenna.

VSWR Specification Definition

When disclosing VSWR on test reports and datasheets, Halberd Bastion follow the BASTA reporting recommendations. BASTA define the reporting of antenna VSWR per the following specification.

  • An absolute parameter.
  • Positive value without unit. The range is between 1 (no reflection) and infinite (total reflection).
  • Specification shall reference the full frequency range, full electrical downtilt range, and associated ports of the antenna unless specifically detailed otherwise (example: 1710-2180 MHz; 0°-10°; +45 and -45 ports).

Basic Example

In any wave system, whether it be physical (percussion/sound, water) waves, or electromagnetic, the interaction between any wave and a boundary will result in a reflection. The simplest example might be a sound wave hitting a thin wall, whereby some of the wave's energy is reflected off the wall back towards the source, with the remainder passing through the wall to be heard in the next room. The amount of energy being reflected vs amount transmitted is measured with VSWR (when expressed as a ratio), or Return Loss (when measured in decibels).

The simplest way to understand the parameter is considering these parameters as describing how well the transmission is 'absorbed' by the component. Return Loss is a little more intuitive, which expresses the absorption (or transfer) as a number of decibels - to provide a simple example, an antenna with a Return Loss of 10 dB can be thought of saying that the reflected energy is 10 dB lower than the input. Putting some numbers behind this example, a 20 W input would have a 2 W reflection and as such 18 W is absorbed by the antenna.

VSWR is slightly less intuitive but refers to the same measurement. In our example above, the power (dB) of the reflected sound versus the power (dB) of the sound present in the other room can be expressed as a ratio. Should almost all of the sound's energy pass through the wall we would have a ratio approaching 1:1, conversely if almost all sound was reflected back as an echo we would have an extremely high ratio, perhaps 10:1. These ratios can be directly converted into Return Loss, for example a perfectly matched system with a VSWR of 1.1:1 would have a Return Loss of 26 dB. Conversely a poorly matched system with a VSWR of 10:1 would have an RL of only 1.7 dB.

Electromagnetic Waves & Practical Consequences

Understanding these concepts in our more familiar world of RF is not a particularly big mental leap. In the case of to antennas; our transmission is an electromagnetic wave rather than a percussion wave, our medium now copper wire, and our interface boundary is the metal-on-metal contact between connection points whether that be between two coaxial connectors or a solder contact on a PCB. The physics however remains the same.

Intuitively metal contacts are substantially more efficient at transferring energy, but less intuitive is the susceptibility of the much higher frequencies we are now operating at. To stress how important VSWR and RL are to your network, consider a reduction in performance from VSWR of 1.3:1 to 1.5:1 - this is a change in Return Loss of 16 dB to 13 dB. This 3 dB reduction in radiating/receiving performance means that to compensate your radio must now work harder, and consequently in the case of IoT systems reducing battery life (by having to broadcast louder) and reducing network capacity and performance (due to the lower modulation and coding scheme).

Accordingly matching of the antenna to its transmission line is absolutely critical to the performance of not just the individual device, but the entire network.

VSWR to Return Loss Conversion Table

VSWR RL (dB)   VSWR RL (dB)   VSWR RL (dB)   VSWR RL (dB)   VSWR RL (dB)
1.001 66.025   1.060 30.714   1.138 23.803   1.480 14.264   5.400 3.255
1.002 60.009   1.061 30.575   1.140 23.686   1.490 14.120   5.600 3.136
1.003 56.491   1.062 30.438   1.142 23.571   1.500 13.979   5.800 3.025
1.004 53.997   1.063 30.303   1.144 23.457   1.520 13.708   6.000 2.923
1.005 50.484   1.064 30.171   1.146 23.346   1.540 13.449   6.200 2.827
1.006 50.484   1.065 30.040   1.148 23.235   1.560 13.201   6.400 2.737
1.007 49.149   1.066 29.912   1.150 23.127   1.580 12.964   6.600 2.653
1.008 47.993   1.067 29.785   1.152 23.020   1.600 12.736   6.800 2.573
1.009 46.975   1.068 29.661   1.154 22.914   1.620 12.518   7.000 2.499
1.01 46.064   1.069 29.538   1.156 22.810   1.640 12.308   7.200 2.428
1.011 45.24   1.070 29.417   1.158 22.708   1.660 12.107   7.400 2.362
1.012 44.489   1.071 29.298   1.160 22.607   1.680 11.913   7.600 2.299
1.013 43.798   1.072 29.181   1.162 22.507   1.700 11.725   7.800 2.239
1.014 43.159   1.073 29.066   1.164 22.408   1.720 11.545   8.000 2.183
1.015 42.564   1.074 28.952   1.166 22.311   1.740 11.370   8.200 2.129
1.016 42.007   1.075 28.839   1.168 22.215   1.760 11.202   8.400 2.078
1.017 41.485   1.076 28.728   1.170 22.120   1.780 11.039   8.600 2.029
1.018 40.993   1.077 28.619   1.172 22.027   1.800 10.881   8.800 1.983
1.019 40.528   1.078 28.511   1.174 21.934   1.820 10.279   9.000 1.938
1.02 40.086   1.079 28.405   1.176 21.843   1.840 10.581   9.200 1.896
1.021 39.667   1.080 28.299   1.178 21.753   1.860 10.437   9.400 1.855
1.022 39.867   1.081 28.196   1.180 21.664   1.880 10.298   9.600 1.816
1.023 38.885   1.082 28.093   1.182 21.576   1.900 10.163   9.800 1.779
1.024 38.52   1.083 27.992   1.184 21.489   1.920 10.032   10.000 1.743
1.025 38.7   1.084 27.892   1.186 21.403   1.940 9.904   11.000 1.584
1.026 37.833   1.085 27.794   1.188 21.318   1.960 9.780   12.000 1.451
1.027 37.51   1.086 27.696   1.190 21.234   1.980 6.660   13.000 1.339
1.028 37.198   1.087 27.600   1.192 21.151   2.000 9.542   14.000 1.243
1.029 36.895   1.088 27.505   1.194 21.069   2.100 8.999   15.000 1.160
1.03 36.607   1.089 27.411   1.196 20.988   2.200 8.519   16.000 1.087
1.031 36.327   1.090 27.318   1.198 20.907   2.300 8.091   17.000 1.023
1.032 36.055   1.091 27.266   1.200 20.828   2.400 7.707   18.000 0.966
1.033 35.792   1.092 27.135   1.210 20.443   2.500 7.360   19.000 0.915
1.034 35.537   1.093 27.046   1.220 20.079   2.600 7.044   20.000 0.869
1.035 35.29   1.094 26.957   1.230 19.732   2.700 6.755   22.000 0.790
1.036 35.049   1.095 26.869   1.240 19.401   2.800 6.490   24.000 0.724
1.037 34.816   1.096 26.782   1.250 19.085   2.900 6.246   26.000 0.668
1.038 34.588   1.097 26.697   1.260 18.783   3.000 6.021   28.000 0.621
1.039 34.367   1.098 26.612   1.270 18.493   3.100 5.811   30.000 0.579
1.04 34.151   1.099 26.528   1.280 18.216   3.200 5.617   32.000 0.543
1.041 33.941   1.100 26.444   1.290 17.949   3.300 5.435   34.000 0.511
1.042 33.763   1.102 26.281   1.300 17.692   3.400 5.265   36.000 0.483
1.043 33.536   1.104 26.120   1.310 17.445   3.500 5.105   38.000 0.457
1.044 33.341   1.106 25.963   1.320 17.207   3.600 4.956   40.000 0.434
1.045 33.15   1.108 25.809   1.330 16.977   3.700 4.815   42.000 0.414
1.046 32.963   1.110 25.658   1.340 16.755   3.800 4.682   44.000 0.395
1.047 32.78   1.112 25.510   1.350 16.540   3.900 4.556   46.000 0.038
1.048 32.602   1.114 25.364   1.360 16.322   4.000 4.437   48.000 0.362
1.049 32.427   1.116 25.221   1.370 16.131   4.100 4.324   50.000 0.347
1.05 32.256   1.118 25.081   1.380 15.936   4.200 4.217   55.000 0.316
1.051 32.088   1.120 24.943   1.390 15.747   4.300 4.115   60.000 0.290
1.052 31.923   1.122 24.808   1.400 15.563   4.400 4.018   65.000 0.267
1.053 31.762   1.124 24.675   1.410 15.385   4.500 3.926   70.000 0.248
1.054 31.604   1.126 24.544   1.420 15.211   4.600 3.838   75.000 0.232
1.055 31.449   1.128 24.415   1.430 15.043   4.700 3.753   80.000 0.217
1.056 31.297   1.130 24.289   1.440 14.879   4.800 3.673   85.000 0.204
1.057 31.147   1.132 24.164   1.450 14.719   4.900 3.596   90.000 0.193
1.058 31   1.134 24.042   1.460 14.564   5.000 3.522   95.000 0.183
1.059 30.856   1.136 23.921   1.470 14.412   5.200 3.383   100.000 0.174

Return Loss to Insertion Loss

It's important to understand the practical consequences of VSWR and Return Loss on system performance, in which the effect is realised through Insertion Loss. For a given power input PI, VSWR will cause a reflection, and accordingly a loss of power PR, resulting in the effective transfer of power PO. For poorly tuned antennas PR represents a significant loss of system efficiency, resulting in reduced communications range.

VSWR insertion loss diagram
Courtesy of Anritsu
VSWR (x:1) Return loss (dB) Insertion loss (dB) Power lost (%)
0 100.00%
17.4 1 6.868 79.40%
8.7 2 4.329 63.10%
5.8 3 3.021 50.10%
4.4 4 2.205 39.80%
3.6 5 1.651 31.60%
3.0 6 1.256 25.10%
2.6 7 0.967 20.00%
2.3 8 0.749 15.80%
2.1 9 0.584 12.60%
1.92 10 0.458 10.00%
1.78 11 0.359 7.90%
1.67 12 0.283 6.30%
1.58 13 0.223 5.00%
1.50 14 0.176 4.00%
1.43 15 0.14 3.20%
1.38 16 0.11 2.50%
1.33 17 0.088 2.00%
1.29 18 0.069 1.60%
1.25 19 0.055 1.30%
1.22 20 0.044 1.00%
1.19 21 0.035 0.80%
1.17 22 0.027 0.60%
1.15 23 0.022 0.50%
1.13 24 0.017 0.40%
1.119 25 0.014 0.30%
1.106 26 0.011 0.30%
1.094 27 0.009 0.20%
1.083 28 0.007 0.20%
1.074 29 0.005 0.10%
1.065 30 0.004 0.10%

References

  1. NGMN, "Recommendations on Base Station Antenna Standards", NGMN Alliance, N-P-BASTA v9.6, Jan. 2013.