MEASURING LOSSES FROM REFLECTED POWER. THE S11 METHOD.
(Jan 29 2013)
This page is a sub-page to LOSSES IN RELAYS, CABLES, ADAPTORS AND CONNECTORS.

The DUT

Four cables and combinations of them is the set of DUTs for which this page is an attempt to evaluate dissipative losses using a HP 8712 C network analyzer.

DUT1 = 0.125 wl 75 ohm, "TV cable"
DUT2 = 0.125 wl 50 ohm, RG223
DUT3 = 0.25 wl 50 ohm, RG223
DUT4 = 0.25 wl 50 ohm, RG223

See figure 1.

Figure 1. The four cables. Three of them RG223, one a 75 ohm TV cable with foam dielectric and a foil screen surrounded by a very thin braided screen. Two layers of braided screen are applied outside the original screen to make the cable RF tight.


The network analyzer is run at minimum bandwidth and 4 times averaging to provida a stable reading with 3 decimals on the dB scale. The instrument is calibrated to show zero when the short or open part from the calibration kit is inserted. Table 1 shows the level of the reflected signal, S11 with open as well as short inserted at the other end of the DUT as well as the losses that is computed as the average of short and open divided by two. (The signal passes the DUT twice.


device            143 MHz                  144 MHz             145 MHz
             Open  Short  loss       Open  Short  loss     Open  Short  loss                
DUT1      -0.317  0.346 -0.007    -0.320  0.351 -0.008   -0.324  0.355 -0.008
DUT2      -0.137 -0.183  0.080    -0.129 -0.192  0.080   -0.121 -0.202  0.081
DUT3      -0.433  0.310  0.031    -0.438  0.312  0.032   -0.440  0.315  0.031
DUT4      -0.170 -0.141  0.078    -0.164 -0.150  0.078   -0.155 -0.158  0.078
DUT13     -0.203  0.111  0.023    -0.195  0.108  0.022   -0.186  0.103  0.021
DUT31     -0.368 -0.064  0.108    -0.366 -0.070  0.109   -0.365 -0.078  0.111      
DUT123     0.079 -0.313  0.059     0.071 -0.291  0.055    0.063 -0.268  0.051
DUT132     0.070 -0.337  0.067     0.062 -0.315  0.063    0.053 -0.293  0.060
DUT312    -0.116 -0.512  0.157    -0.128 -0.505  0.158   -0.140 -0.494  0.159
DUT213    -0.169 -0.410  0.145    -0.187 -0.403  0.148   -0.208 -0.393  0.150
DUT231     0.060 -0.268  0.052     0.050 -0.255  0.051    0.037 -0.240  0.051
DUT42     -0.198 -0.275  0.118    -0.215 -0.260  0.119   -0.233 -0.244  0.119 
DUT24     -0.204 -0.269  0.118    -0.223 -0.255  0.119   -0.241 -0.238  0.120
DUT34      0.213 -0.567  0.089     0.218 -0.572  0.089    0.221 -0.576  0.089
DUT43      0.216 -0.570  0.089     0.221 -0.574  0.088    0.222 -0.577  0.089
DUT32      0.216 -0.575  0.090     0.219 -0.580  0.090    0.222 -0.581  0.090
DUT23      0.215 -0.570  0.089     0.219 -0.573  0.089    0.222 -0.576  0.089

Table 1 Return loss in dB for Open and Short and the loss computed from them.

It is obvious that this method does not work well for DUTs that are not well matched to 50 ohms.