Balanced vs differential signals

analogantennacircuit analysisRFsignal

I would like to verify my understanding of the following scenario with regards to why coax is 'unbalanced' and how a choke interacts with this unbalance:

  • Consider a half-wave resonant dipole, if one leg develops a +1V referenced to earth, the other leg will be at -1V, not 0V, referenced to earth. Additionally, if the two drawn capacitors are equal in capacitance, both legs have an equal impedance to earth. This is what balanced means.
  • Pretend I solder the blue leg of the dipole to the shielding of some coax cable. The black vertical line emphasizes the thickness of the shielding.
  • Coax: the shield sits at earth, while the core voltage fluctuates above and below earth voltage. It is unbalanced because as drawn, the core is shielded, and its path to earth is only through the red leg and its capacitor. Meanwhile, the shield has infinite capacitors from the outside of the shield to earth all in parallel. This presents a different impedance to earth for the shield vs the core.
  • If the red antenna leg is at +1V, the core is at +1V, the blue antenna leg will be at -1V while the shield is at 0V. Thus, a current Icm flows across the outside of the coax shield, to the blue antenna leg back to earth (coupling etc). This is the "common mode" current, and it is in the same direction as the core current.
  • I now put a ferrite ring around the coax cable.
  • I1 and I2 are differential but not balanced (equal and opposite voltages with different impedances to earth). Icm and the core current are common mode and their flux's add. They are impeded by the choke. Since I2 is equal and opposite I1, its magnitude is also reduced.

Diagram

Two questions:

  1. Is that an accurate intuition as to why coax is considered unbalanced in addition to the effects of adding a choke around the coax? I am self teaching and would like to know which resources I can rely on.

  2. Adding a ferrite ring choke decreased all currents; since V=IR, does that mean that the voltages on the ends of the dipole would decrease as well? Can a dipole antenna stop receiving if a large enough choke was placed around its coax cable?

Best Answer

This is my view of the problem

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According to the electromagnetic theory of guided TEM waves: the current flowing at the surface of the central conductor and that at the inner surface of the shielding must be equal but flowing in opposite direction. A coaxial cable is an current balun circuit. But that is true inside a coax, not outside.

The blue current is the differential current of the RF generator. Ideally, it is balanced.

In reality, the red current has found a new path outside the coax because of the junction with the antenna's right arm (the inner and outer surfaces of the shield are both connected to each other). According to Kirchoff's law, the red current must return to the RF generator because the red current is part of the blue current.

The green current is a common-mode current induced by the antenna's radiation on the feed line. This current couples with the environment, creating a loop with parasitic capacitances.

Green current disturbs the antenna by unbalancing its arms. Red current reduces system performance. Both contribute to the shield's radiance.

One solution is to use a ferrite to attenuate currents on the shield's outer surface.

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