On Wed, May 27, 2020 at 11:02 AM, <reuterr@...> wrote:
I replace now the 50 Ohm resistor at the SMA plug with a coax cable RG316
to the nanoVNA CH1.
To my surprise the S11 Return loss looks much different, than with the 50 Ohm
resistor.
See: NooElelc_Balun_9-1_S21_Loss.png.
What In am doing wrong?
73, Rudi DL5FA
When you replace a 50 ohm resistor load with the RG-316 cable connected to the nanoVNA you no longer have a 50 ohm resistive load. This is due to two factors:
- RG-316 cable is only specified as a "nominal" 50 ohm characteristic impedance. Its characteristic impedance varies considerably with frequency as shown below.
- The NanoVNA CH1 port is also not 50 ohms as reported by others on this site. You can measure it yourself and see what I mean.
The net effect of these two effects is that you will see a complex impedance at the end of the RG316 that differs from 50 ohms so the balun will not be seeing 450 ohms and the return loss will be different.
One other point. The style of 50 ohm SMA load you are using is not a good 50 ohm load. They are usually made with a 51 ohm resistor with a long lead (inductive) and have a poor contact with the load case. So this will affect your initial Return Loss measurement
Roger
Roger Need
