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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".

You are correct. A coaxial cable is really a 3-conductor topology for RF
energy.

1) The inner portion of the center conductor which only supplies
"bulk" to the conductor
and really does nothing for conducting RF energy.

2) The outer surface of the center conductor which conducts RF energy
(skin depth).

3) The inner surface of the shield which conducts RF energy (skin
effect).

4) The outer surface of the shield which only supplies "bulk" to the
conductor and really
does nothing for conducting internal energy. However, if not
correctly decoupled, may
carry common mode currents which distort antenna patterns and
introduce noise to the
receiving system (degrade receiver noise floor).

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.

All correct.

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.

For your application I'd recommend a current choke consisting of 2 to 3
turns loosely spaced of your RG-316 on a small (1" OD or so) 61 material
ferrite toroid. Place this immediately at the connection of the coax braid
to the PCB with absolutely minimal pigtail.

Dave - W?LEV


On Sat, Mar 15, 2025 at 6:34?PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

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.

--

*Dave - W?LEV*


--
Dave - W?LEV