On Sun, Feb 23, 2025 at 12:36 AM, Nico wrote:
My board is hooked up to the vna through an RG-174 I had laying around. It is
2 meters exactly. I have added an E-Delay of 10.1ns as per a calculator I
found on the Times microwave website. I don't know if there is a bug in the
vna's firmware but I had to put 0.66 as the velocity factor in order to make
it display 2.00 meters as the cable length. If I put 66 as in 66%, it shows
200 meters. Anyways, I'm not sure if that part is that relevant.
When you use the edelay feature in a NanoVNA to move the reference plane from the SMA connector to a distant point you really need to have a short cable so the attenuation is minimal. This is not the case with 2M of RG174 at 900 MHz as others have pointed out. You really need to establish the reference plane at the end of the cable by "de-embedding" and doing the SOL calibration with the cable open, shorted and using a 50 ohm SMD load.
The other problem you have is what you are attempting to measure. From the pictures it looks like a helical antenna with a short connecting PCB trace that will be a loading inductor. The "other half" of the antenna looks like the ground plane of the PCB. When you connect your RG174 cable to the board the outer surface of the coax shield and the NanoVNA will now be part of the antenna, will radiate and affect your measurement results. You need to slide many ferrite beads of the correct mix over the cable near where it attaches to the board to act as an RF choke. If you shorten the RG174 and see the results change (after calibrating at the end again) then this indicates this kind of problem.
Another thing to consider is that the manufacturer does not specify an antenna with a 50 ohm feedpoint impedance. Maybe the antenna suggested is an end fed half wave and they have a matching circuit. I am only guessing here.
Roger
Roger Need