Hello Forum,

I always thought, naively, that a circuit powered by a DC or RF generator with a transmission line connecting it to a load, needed two conductors to close the circuit. One conductor being the signal conductor while the other the return conductor.

Is the return conductor always necessary in a circuit? If so, why?
The voltage is a concept that needs to be applied at two points (conductors).

Does current in a circuit always need to form a closed circuit? Why?
With two conductors we can then have two currents, equal and opposite. But what do we do with a transmission line made of only one conductor? Both equal and opposite currents exist on conductor: zero net current...maybe?

If we connect a AC battery terminal to a single wire, that wire becomes an antenna.

Has anyone heard about single-wre transmission lines (Goubau line)?
How does a single-conductor transmission line like the Goubau line work?

Is the longitudinal current inside it equal to zero? If it is zero, why so?
How about the voltage on this line?
All I know the field propagates as a surface wave wrapped outside the single metal wire. I tried to find simple conceptual explanation but not luck so far.

(Microwave waveguide are composed of only one conductor too. But the field (modes) are trapped inside the guide and propagate by reflecting off the guide walls. There is then propagation without any conductors, free space propagation (free space can be viewed as an infinite spherical waveguide).

Thanks
antennaboy

 

You are confusing electricity with RF. While related, they aren't the same things.

As usual, Wikipedia is your friend.

http://en.wikipedia.org/wiki/Single-wire_transmission_line

Electricity in a circuit always requires a return line. A circuit is a loop, it can apply to auto racing as well as electricity.

RF, on the other hand, is a mix of fundamental forces. It is electric and magnetic fields 90° coupled together, and it radiates. Light is an example of this too. With RF, when an antenna converts it back to electricity a dual conductor is again needed, look at cables and 300Ω twin lead. An antenna is a transducer (look it up on Wikipedia), it converts one thing into another, and in many cases back again. A speaker is another example of a transducer, it converts electricity into sound, and sound into electricity.

 

Thanks Mr. Bill.

I kind of think that an RF circuit is simply a AC circuit working at RF frequencies.
The theory of transmission lines shows up and things get more complicated.

The simple hydraulic analogy that current flows in one conductor and out of another works well only for DC.

But AC, DC, RF are all, fundamentally, working based on Maxwell equations....
Clearly it is a stretch to use those equations for a DC circuit.....

 

Depends on how you define DC. If you turn a circuit on and off in anything resembling a cyclic manner it has a frequency, but no antenna.

Also, when you turn something on or off it has a squarish edge, which are chock full of frequencies. A square wave has many harmonics, it isn't just one frequency. This may be a bit advanced for you yet, but eventually you'll cover it if you are going to school.

Before digital really took over everything RF was king. Think about it, it is almost like magic. You can talk to someone or control something over a long distance. It was what I cut my teeth on way back when.

 

Hello Forum,

I always thought, naively, that a circuit powered by a DC or RF generator with a transmission line connecting it to a load, needed two conductors to close the circuit. One conductor being the signal conductor while the other the return conductor.

Is the return conductor always necessary in a circuit? If so, why?
The voltage is a concept that needs to be applied at two points (conductors).

Does current in a circuit always need to form a closed circuit? Why?
With two conductors we can then have two currents, equal and opposite. But what do we do with a transmission line made of only one conductor? Both equal and opposite currents exist on conductor: zero net current...maybe?

If we connect a AC battery terminal to a single wire, that wire becomes an antenna.

Has anyone heard about single-wre transmission lines (Goubau line)?
How does a single-conductor transmission line like the Goubau line work?

Is the longitudinal current inside it equal to zero? If it is zero, why so?
How about the voltage on this line?
All I know the field propagates as a surface wave wrapped outside the single metal wire. I tried to find simple conceptual explanation but not luck so far.

(Microwave waveguide are composed of only one conductor too. But the field (modes) are trapped inside the guide and propagate by reflecting off the guide walls. There is then propagation without any conductors, free space propagation (free space can be viewed as an infinite spherical waveguide).

Thanks
antennaboy

If not sure what the name is for passing current through the earth is exactly, but if my very limited knowledge of Nikola Tesla / Colorado Springs serves me correctly no return wire was employed.

 

If not sure what the name is for passing current through the earth is exactly, but if my very limited knowledge of Nikola Tesla / Colorado Springs serves me correctly no return wire was employed.

In single wire telegraph transmissions, the return path is called the "earth return".
https://en.wikipedia.org/wiki/Earth-return_telegraph

 

Does current in a circuit always need to form a closed circuit? Why?

Because the electrons need a return path.
You can't have electrons going down a wire without some way for them to return to the source.

But what do we do with a transmission line made of only one conductor?

Transmission lines do have two conductors.
For example a coax line has a center conductor and a conducting shield.

Has anyone heard about single-wre transmission lines

That carries electro-magnetic wave signals not current.

 

An interesting concept, " If we connect a AC battery terminal to a single wire, that wire becomes an antenna. "
If you locate a source of AC batteries please let us know.

Now consider the word "circuit", which inherently comes from circle. So there always needs to be a return path. And with all of those special case transmission line types, you really need to know the background to even have a clue.

 

The classic single-wire transmission lines would be this: https://en.wikipedia.org/wiki/Goubau_line

https://accelconf.web.cern.ch/BIW2012/papers/tupg007.pdf

There is a current on the surface of the wire but it's localized (conductor electrons moving slightly to keep each section of conductor neutral in response to the external EM potential) to the EM field that surrounds the conductor was it moves.

You need a return path/circuit (that can be a loop of current on a conductor) for 'current electricity' but 'electricity is not electrical energy that can move in pure vaccum and the return path is the entire universe.

 

Hello Forum,

I always thought, naively, that a circuit powered by a DC or RF generator with a transmission line connecting it to a load, needed two conductors to close the circuit. One conductor being the signal conductor while the other the return conductor.

Is the return conductor always necessary in a circuit? If so, why?
The voltage is a concept that needs to be applied at two points (conductors).

Does current in a circuit always need to form a closed circuit? Why?
With two conductors we can then have two currents, equal and opposite. But what do we do with a transmission line made of only one conductor? Both equal and opposite currents exist on conductor: zero net current...maybe?

If we connect a AC battery terminal to a single wire, that wire becomes an antenna.

Has anyone heard about single-wre transmission lines (Goubau line)?
How does a single-conductor transmission line like the Goubau line work?

Is the longitudinal current inside it equal to zero? If it is zero, why so?
How about the voltage on this line?
All I know the field propagates as a surface wave wrapped outside the single metal wire. I tried to find simple conceptual explanation but not luck so far.

(Microwave waveguide are composed of only one conductor too. But the field (modes) are trapped inside the guide and propagate by reflecting off the guide walls. There is then propagation without any conductors, free space propagation (free space can be viewed as an infinite spherical waveguide).

Thanks
antennaboy

@antennaboy You need to get a much MUCH better grasp on what voltage, current, and impedance or resistance (depending) is and their relationship to one another. It is because of your lack of actual understanding that you are confused by some things are are relatively simple. I suggest this as a starting point:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

You don't understand Ohm's Law yet- when you do, all your questions will be answered. KVL/KCL and Thevenin will help. The simple answer to your question is an unequivocal 'YES'. You must always have atleast 1 return _PATH_. Not just a wire, but a path. And the only actual difference between AC and DC is time- AC has a time domain, DC does not.

 

If referenced properly, AC systems can of course use the Earth itself for a return path simply because the distance the electrons actually have to move decreases with frequency and so a "virtual circuit" is often easily established. Whereas DC systems can't employ such a method so easily excepting very short distances. It just takes to much power to be practical in most situations.

 

If referenced properly, AC systems can of course use the Earth itself for a return path simply because the distance the electrons actually have to move decreases with frequency and so a "virtual circuit" is often easily established. Whereas DC systems can't employ such a method so easily excepting very short distances. It just takes to much power to be practical in most situations.

There are losses but a HVDC earth return path is a feature in transmission lines like the: https://en.wikipedia.org/wiki/Pacific_DC_Intertie

The Pacific Intertie consists of:[4]

• The Celilo Converter Station which converts three phase 60 Hz AC at 230 to 500 kV to ±500 kV DC (1000 kV pole-to-pole) at
  
  45°35′39″N 121°6′51″WCoordinates:
  
  45°35′39″N 121°6′51″W.
  • The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two-foot (60 cm) trench of petroleum coke, which behaves as an electrode, arranged in a ring of 2.0 miles (3,255 m) circumference at Rice Flats (near Rice, Oregon), which is 6.6 miles (10.6 km) SSE of Celilo. It is connected to the converter station by two aerial 644 mm2 (0.998 in2) ACSR (aluminum conductor, steel reinforced) conductors, which end at a "dead-end" tower situated at
    
    45°29′51″N 121°03′53″W.
• A 846-mile (1,361 km) overhead transmission line consisting of two steel-cored ACSR conductors, each 1.6 inches (4.1 cm) in diameter with a conducting cross-sectional area of 1,171 mm2 (1.815 in2), carrying 500 kV.
  • The two lines when combined have a capacity of 3.1 gigawatts (in bipolar mode).
• The Sylmar Converter Station (
  
  34°18′42″N 118°28′53″W) which converts DC to 230 kV AC (a process also called inverting) and is phase-synchronized with the L.A. power grid.
  • The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach[5] suspended in concrete enclosures about 2 to 3 feet (0.5 to 1 m) above the ocean floor. The grounding array, which is 30 miles (48 km) from the converter station and is connected by a pair of 644 mm2 (0.998 in2) ACSR conductors, which are in the sections north of Kenter Canyon Terminal Tower at
    
    34°04′04.99″N 118°29′18.5″W installed instead of the ground conductors on the pylons. It runs from Kenter Canyon Terminal Tower, via DWP Receiving Station U (Tarzana; a former switching station), Receiving Station J (Northridge) and Rinaldi Receiving Station (also a former switching station) to Sylmar Converter Station. On the section between Receiving Stations J and Rinaldi, one of the two shielding conductors on each of two parallel-running 230 kV transmission lines is used as electrode line conductor.

https://curtinmaritime.com/projects/sylmar-ground-return-system-ladwp/

 

There are losses but a HVDC earth return path is a feature in transmission lines like the: https://en.wikipedia.org/wiki/Pacific_DC_Intertie


https://curtinmaritime.com/projects/sylmar-ground-return-system-ladwp/

Nice! But those are still properly two-wire systems (plus a ground wire of course). I think the TS was referring to true single-wire transmission. Which isn't easy to do with DC using a grounded rod connected to Earth serving as the "phantom" pole. AFAIK only AC is capable of doing that with much success. Correct me if I'm wrong of course.

 

Nice! But those are still properly two-wire systems (plus a ground wire of course). I think the TS was referring to true single-wire transmission. Which isn't easy to do with DC using a grounded rod connected to Earth serving as the "phantom" pole. AFAIK only AC is capable of doing that with much success. Correct me if I'm wrong of course.

The original earth-return DC telegraph systems that were installed all across the USA were very successful until the introduction of electric trolly-cars. The interference they caused in the earth return path made the telegraph systems useless.

 

The original earth-return DC telegraph systems that were installed all across the USA were very successful until the introduction of electric trolly-cars. The interference they caused in the earth return path made the telegraph systems useless.

Ah ok, so very sensitive to electrical "noise" but otherwise very reliable and efficient. Well thanks for pointing that out then. I had no idea!

 

All circuits are two conductor no matter if the second conductor is a wire or the earth or a theoretical conductive plane.
That green "ground" wire has nothing to do with circuits, it is a safety feature, not a circuit element.

 

Nice! But those are still properly two-wire systems (plus a ground wire of course). I think the TS was referring to true single-wire transmission. Which isn't easy to do with DC using a grounded rod connected to Earth serving as the "phantom" pole. AFAIK only AC is capable of doing that with much success. Correct me if I'm wrong of course.

The two transmission lines on the pole can be DC bipolar +- neutral electrode or DC monopolar using one or both wires with the earth electrode as the second 'return' wire.

https://www.epri.com/research/products/1020116
https://www.osti.gov/servlets/purl/580585

 

In single wire telegraph transmissions, the return path is called the "earth return".
https://en.wikipedia.org/wiki/Earth-return_telegraph

Thank you.