RF ground confusion

Question

I'm a bit confused by RF grounding. Let's take a HF 1/4 wave vertical antenna for example.

When current flows through the centre of the coax to the antenna, my understanding is that the current then needs a return path, which is back through the outer shield of the coax back to ground.

The antenna is what converts the electrical current into RF radiation.

Therefore, what travels back through the outer coax (or should)? Is this the actual electrical current or the RF signal? Feel free to criticize this question if it doesn't make much sense. I'm still trying to grasp some things. Thanks.

Answer 1

If the coax is operating without common-mode current (a typically desirable property of a properly designed antenna system), then it's the case that at any point, at any time, the current in the shield is equal in magnitude but opposite in direction to the current in the center conductor.

When someone says "the coax shield is ground" what they mean is the electric potential of the shield is zero. Or in other words, the electric potential difference between the shield and a point infinitely away from any charged particle in the universe is zero. Since the Earth is pretty big, fairly conductive, and mostly not charged, its electric potential is also mostly zero. So zero electric potential also means the electric potential difference between the shield and a copper rod driven in the ground is zero. This is desirable because it means the coax doesn't radiate.

The reason the electric potential of the shield is zero is that when the electric and magnetic fields associated with the voltage and current of the shield and the center conductor are equal in magnitude but opposite in direction, the sum of these fields is zero for all points outside the shield. So, the wave is contained entirely inside the coax.

However, no common-mode current isn't a given: it's the result of properly designing the antenna system. This is why feeding a dipole with coax requires a balun, for example.

Answer 2

RF energy is AC, not DC. Return path has less meaning here.

Don't think of coax as a center conductor and a shield, think of it as a single transmission line with two halves. The current in each half is (or should be) opposite of the current in the other half. Each current forms an equal and opposite magnetic field from the other half and (ideally) the two fields cancel, preventing the coax from radiating.

The antenna could be considered as a transducer, converting electrical energy into light (at an RF frequency). Or you could consider it as an impedance match to free space. In either case, it takes the AC power from the feed line and radiates a portion of that as radio waves (as radiation resistance). What isn't radiated might be converted to heat (as loss resistance) or reflected back down the feed line (both sides, causing SWR).

There are three "grounds" in radio:

• RF ground
• Electrical ground
• Lightning ground

These three can be connected but are functionally different.

RF ground is not really ground -- it's just the other half of a monopole antenna. Not all antennas need one. It's called ground, because frequently the other half of a monopole vertical is the actual ground, or touching the ground.

Electrical ground, sometimes called safety ground, is part of the electrical code for commercial power. All electrical grounds should be connected in part to reduce noise, and in part to prevent stray voltage in places people can touch.

Lightning ground should be positioned to conduct the majority of a lightning strike around sensitive things and into the physical ground. It should have as few sharp bends as possible and minimize resistance; all lightning grounds should connect to a single point so you don't get voltage differentials between points in the ground system. Typically electrical ground should be connected to lightning ground at a single point.

Answer 3

The antenna requires a ground reference such as radial spokes for a vertical antenna or elevated sufficiently to use the coupling to earth or rear reflector to create radiation patterns.

Impedance matching determines how much of the signal is reflected back in the current loop of RF , called RL, return loss or s11 from scattering parameters.

While the coax is tightly coupled by distributed capacitance and inductance. With controlled impedance $$Z_o=\sqrt{L/C}$$ and the shield reducing radiation of the centre conductor, the conduction loss/100m and transfer loss from the common mode impedance of the braid style, and ground resistance, the emissions in the cable can be minimized. The best being rigid coax, then semi-rigid is determined by the transfer impedance ratio over this spectrum, often done in CATV requirements.