On Sun, Oct 27, 2019 at 11:40 AM, <subs@...> wrote:
that the shield side is effectively the "other half of the dipole". Usually
the shield of an antenna system would be earthed, but in this case when using
the NanoVNA, it would indeed be floating and subject to the influence of
outside factors such as the hand effect.
This is an important 'Gotcha' that catches out many people when measuring antennas.
They disconnect the feeder and measure the antenna at the feedpoint, then wonder why they get unexpected results.
If you are measuring a coax fed antenna and using the VNA at the feedpoint, make sure you connect the outer of the coax to the outer of the VNA SMA connector so that it's more representative of the actual antenna system.
If you are using a balanced antenna with a balun that is working effectively, you should not see any difference between sweeps with the coax screen connected or disconnected.
The Nano VNA is small enough and sufficiently self contained to appear as a balanced load at low frequencies. However as soon as you connect it to a PC, which may in turn be powered from a grounded mains supply, you have an unbalanced load.
You always need to think, "is the antenna I'm measuring configured as it would be when in normal use or am I adding (or leaving out) something in the system that makes it different".
Regards,
Martin - G8JNJ
