Hello Roger,
Thank you very much for your quick answer.
My answers are in your text:
On Wed, May 27, 2020 at 08:30 PM, Roger Need wrote:
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.
I was my understanding, that the calibration will give at the end of the 50 cm RG-316 cable
nominal 50 Ohm impedance (reference point).
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
With that 50 Ohm resistor in the picture I got a good S11 Return Loss measurement.
So, this was not my point.
The problem arised when I exchanged this 50 Ohm resistor with a 50 cm RG-315 cable,
connected to the CH1 port of the nanoVNA.
The calibration with that 50 cm RG-316 cable was made before.
So, how can I improve the situation?
73, Rudi DL5FA
