On Fri, Mar 14, 2025 at 08:39 PM, Nico wrote:
It took me a few reading of your two last replies but I think I got it. If I
understand correctly, the shield of my coax is now integral part of the
antenna system as it is connected to the ground pad which is the antenna and
system ground and also the ground plane. In this case then, how will I ever
achieve an accurate reading ?
When you use a coaxial cable the RF current goes up and down the center conductor and the inner surface of the coax shield. The reason I say inner shield is that due to the "skin effect" the RF current only travels on the surface of the inner surface of the shield and slightly below the surface. The outer surface is effectively isolated and acts like a "third wire".
Now in your case you have the following. You have a microstrip transmission line and only one side is connected to your helical radiator. So what is the return path of the RF current? It is the ground side of the microstrip which is connected to the ground plane. So your antenna consists of the helical radiator and the other half is the transmission line ground side and PCB ground plane. Both are radiating RF.
When you connect your RG316 cable and nanoVNA the RF current flows on the inner conductor and inner surface of the shield. The outer surface is attached to the ground plane so it will radiate as well and is part of the antenna system. When you wrap the coax around a ferrite you do not affect the current flowing inside the coax - only the current on the outer shield is reduced. The current flowing on the outer surface is known as "common mode current" The common mode current reduction is due to the complex impedance (R + X) of the inductor which is effectively in series with the outer shield surface. If you google "braid breaker", "current balun" or "RF Choke" you will find more info on this subject.
Roger Need