Section 7.8 · Problems · 5 of 5
Problem 7.8.5: A coupler measurement
A coupler measurement
#A -dB directional coupler is inserted in a - line. The line power is .
- Find the coupled-port power and the power that continues to the load.
- If the coupler directivity is , find the reverse leakage at the coupled port and the isolation.
- The load has VSWR , and a second, reversed coupler samples the reflected wave. Find the reflected power and its sample.
- The forward wave leaks into that sample through the finite . Between which limits can the measured and VSWR lie?
Work in dB and dBm (). The coupled port is below the line, and a wave travelling the wrong way reaches it below. In (d) add the leak to the wanted sample as amplitudes with an unknown phase.
The load receives , a through loss of only .
This is what the coupled port receives from a wave travelling backwards, and what port 4 receives from the forward wave.
, so (), and the reversed coupler delivers ().
In the reversed coupler the forward wave leaks down, . Relative to the forward sample () its amplitude is , and the reflected sample has the amplitude on the same scale. With the phase unknown,
The reflected sample () stands above the leak (), an amplitude ratio , which is the relative error of (d).
The reflected sample stands only above the forward leak (Figure 7.8.26), an amplitude ratio of : this is the uncertainty of in (d).
A reflectometer cannot see reflections smaller than its own leak: with nothing below (VSWR ) can be measured at all. Good couplers reach , and vector network analysers remove the rest by calibration.