Why is coax unbalanced ?

[W5DXP]

I have some big coax that has a hollow center conductor. Now do we have four paths? Is it now balanced?

Simple answer: The laws of physics prevents RF current from flowing on the inside of the hollow center conductor because any and all RF fields will cancel.

[K8JD]

So you are saying the coax is somehow isolated and the coax connector on the radio is not riveted or screwed to the chassis ??
Very unusual. Most modern rigs have the threaded, or outer shield part of the antenna connector grounded to he chassis ! Even without an actual "ground wire" connected to an external ground rod there is some coupling to earth or house wiring potential. (A ground) Even a hand held is designed with a HAND holding the radio and cpacitive coupling serves as an RF ground, a counterpoise to the little stubby antenna on the radio.

I don't agree.

I don't have ANY connection to ground on my HF rig or the antenna which is fed with coax. It's still unbalanced. Same for the little RG-174 that connects an HT to an external antenna. Coax is also used in space and it's still unbalanced.

unbalanced.

[N3OX]

I have some big coax that has a hollow center conductor. Now do we have four paths?

No, not really. The inside of the center conductor is a waveguide below cutoff at frequencies where you are likely to use the coax. Electromagnetic energy inside the inner conductor will decay exponentially away from the cut end and by the time you get in a short distance the currents on the inside of the inner conductor will be nearly nothing.

[WA6TKD]

In a balanced line neither conductor is connected to ground (chassis). A 3 conductor microphone cable (XLR termination) has a balanced pair of audio wires not connected to ground (chassis) and a grounded conductor usually a shield of some kind.
Coax is unbalanced because the shield is connected to ground (chassis) on at least one end and maybe both ends. I don't believe that skin effect or whether the current is on the inside surface or outside surface has anything to do with balanced or unbalanced.

73
Bob

+1 I feel you have answered the question that was asked. I think many here are answering a question that wasn't asked.

[G0EVA]

All said before is true - particularly considering the 3 conductor model.

To feed a ground plane antenna to common mode currents on the outer surface, of the outer conductor may be an advantage, BUT if such currents are present when feeding a BALANCED antenna e.g. dipole, doublet, yagi, quad lopp, etc; then the currents on the outer RADIATE, and alter the radiation pattern.

I said may, because "at resonance" all the power delivered to the antenna, ought to radiate, rather than partially leak back down the outer surface of the co-axial cable.

When using 50 ohm co-ax to feed an appriximately 70 ohm antenna (dipole), whose characteristic impedance can vary between near zero and 130 ohms depending on its height, and surroundings - all balanced antennas need to be fed with a balanced to unbalaced (BALUN) matching element.

My preferred means of feeding of diploes etc. is to use twin feeder - BALUN being at the feed end as a) twin is nuch less lossy than c0-ax, ; and b) tends not to radiate if antenna is also balanced.

[G0EVA]

Alternate answer

is it called unbalanced because you are if you feed a dipole with it?

BTW why use 50 ohms when dipole theoretically is 72 ohms impedance = another unbalanced reason?

[G3TXQ]

I said may, because "at resonance" all the power delivered to the antenna, ought to radiate, rather than partially leak back down the outer surface of the co-axial cable.

Why? The antenna could present a perfect 50 Ohm resistive balanced differential load to the coax, but if the CM path back along the braid is low there will be significant CM current flowing that route.

73,
Steve G3TXQ

[K8ERV]

No, not really. The inside of the center conductor is a waveguide below cutoff at frequencies where you are likely to use the coax. Electromagnetic energy inside the inner conductor will decay exponentially away from the cut end and by the time you get in a short distance the currents on the inside of the inner conductor will be nearly nothing.

Right, that is why the viewing screen in a microwave oven doesn't leak (much).

TOM K8ERV Montrose Colo

[W5DXP]

The antenna could present a perfect 50 Ohm resistive balanced differential load to the coax, but if the CM path back along the braid is low there will be significant CM current flowing that route.

Yes, assume a worst case 1/2WL path down the outside of the coax braid. The source end of that 1/2WL of coax braid is RF ground. The current coming from the inside of the coax at the feedpoint has a choice between the antenna element's 25 ohm impedance and the outside coax braid's e.g. 5 ohm impedance to ground. Which path will most of the RF current take?

[WB2UAQ]

Ah, no, Walt (who is a friend of mine) did NOT change unbalanced line into balanced line by his ingenious use of ferrite chokes. What he did manage to do was to abate the shield of the feedline from radiating and acting as an antenna, as does the PROPER length of coax wound in a coil, with or without a toroid. None of this changes unbalanced coax into balanced feedline.

The source is the transmitter. Matching the impedance of the load and the source is called conjugate matching. Ferrites do not change the impedance of anything. They simply act upon unwanted radiation and keep it where we want it, in the antenna.

With all due respect, I made no mention of changing impedance or matching. If a 50 ohm unbalanced to balanced conversion is NOT being accomplished by the use of the ferrite beads and in some cases a ferrite cylinder than a lot of existing radios should be dead right now This is very common practice in PA stages. An RF source that was once referenced to RF ground plane is now able to accomplish phase inversion or is now able to drive a push-pull stage, etc.