WE-030A Reflection Measurement Error Estimate
For the internal directional bridge after tracking errors are removed with open circuit calibration:
ra = (rm-D)/(1+S(rm-D))
where ra is the actual refection coefficient of the antenna, rm is the measured reflection coefficient, D is the bridge directivity and S is the source match. All variables are complex numbers.
From this:
The maximum value |ra| can be is: (|rm|+|D|)/(1-|S|(|rm|+|D|)) and the minimum value |ra| can be is: (|rm|-|D|)/(1+|S|(|rm|-|D|)) if the phases of rm, S and D happen to combine worst case or best case respectively.
i.e. for a measured return loss of 20dB i.e. |rm| = 0.1 with bridge directivity 36dB i.e. |D|=0.016 and source match 22 dB i.e. |S|= 0.08
|ra| is at most: 0.12 i.e. return loss 18.6 dB
and at least: 0.083 i.e. return loss 21.6 dB
(The |D| , |S| values used are the worst case values for WE-030A).
So the maximum bridge measurement return loss error is +1.4dB and -1.6dB for this case.
This corresponds approximately to a bridge measurement error in the SWR off approximately +- 4% for this example. (obtained from table below)
Return loss SWR
18.6 1.27
20 1.22
21.6 1.18
For the WE-030A there is an additional uncertainty of 1 dB in the measured return loss due to the limited display/marker resolution. This adds an additional error of about 3% in the SWR for the above example, i.e. total error about +- 7%.
For the adjustment of most HF antenna's this level of accuracy should be satisfactory.