Slotted measuring line
why in measuring the reflection coefficient using slotted measuring line we take the swr as the one measured at the first minimum in front of the load? if this is not right, how to measure the reflection coefficient using slotted measuring line?
regards
Nice 1960 technology.
A) be sure the line excursion is longer enough (ie >λ/4).
B) Be sure the in what region the detector is working (square law or linear?)
C) By moving the pick up along the line, find the minumum, record it, find the maximum, record it, then calculate the ratio, here you have VSWR (see note)
D) The slotted line is able to measure the actual VSWR of the load plus an eventual Line. As it's placed near the load, it measure the VSWR of the load alone.
(note) if the detector work on square law, a discussion is needed. In this case, perhaps you shoud square the ratio. Find the user manual.
You can also measure the load impedance. First put a short instead of the load. Find the location of the first null. Then put the load on and find the position of the null (or lowest signal level point) that is closest and toward the generator. From this position, the wavelength of the signal in the slotted line, and the SWR you can use a smith chart to find the load impedance.
Do you mean that the VSWR gets "better" as you move further away from the load on the slotted line, so wich "VSWR" do you believe? You take the VSWR measurement of the closest to the load as the most accurate. That is becuase as you get further away from the load the signals attenuate (the slotted line has some loss per unit length). If you had a very long slotted line and tried to measure the VSWR at the far end from the load, the incident signal will be strongest (since it has not been attenuated by the slotted line loss), and the reflected signal will be weakest (because the incident wave was further attenuated by the slotted line loss, and the reflected wave was also attenuated by the slotted line loss, giving an erroneous VSWR reading that is "too good" to be true).
I think you gave the answer yourself.
If the attenuation of the slotted line is so high to be observed, and if you are interested to read VSWR of the "load", take measurement as close as possible to the load.