Can I use 14/2 UF-B wire as transmission line?

Question

I'm wanting to build an antenna system utilizing twin lead feedline running to an 80 meter doublet. I'd like to avoid the typical hassles that accompany 450 or 600 ohm ladder line so I'm considering using simple 2 conductor wire as the feedline. I found an online impedance calculator that suggests that regular 2-conductor 14 gauge lamp wire would have a close-to-acceptable impedance, but unfortunately lamp wire typically isn't rated to 600 volts (I'm planning on running several hundred watts of power to the antenna), nor would I expect it to survive long-term outdoors. So I'm thinking a good 14/2 UF-B cable might be a workable solution, except for the presence of a ground wire between the 2 conductors in the cable.

My question is this: what effect if any would the presence of the ground wire have on the HF transmission characteristics of the cable, as compared to an identical transmission line that didn't have the ground wire?

It seems to me (at least in theory) that the simple presence of the ground wire itself should have no effect at all, because of the way that a twin lead feedline behaves in conjunction with a doublet at any given frequency: at any particular point along the length of the feedline, the voltage and/or current in either of the conductors exactly matches but is 180 degrees out of phase with the energy in the other conductor. So the net effect that the near-field electromagnetic wave emanating from each conductor has on any nearby metal object (including a ground wire inside the cable) is ZERO. In fact, consider why a properly setup twin lead feedline presents no discernible RF issues in the shack; the RF energy radiating from either conductor is effectively and exactly cancelled by the opposite-phase energy radiating from the other conductor.

I'm not concerned at this time with any considerations of cost, etc., so please refrain from such comments as "Jeez, that's expensive cable, you can build a ladder line much cheaper!". My main concern is whether the presence of a third wire in the jacket would present any problems with the feedline function of the cable.

Please advise, and kind thanks in advance.

Answer 1

The ground wire doesn't cause any major trouble, because it's in the centre, at zero potential, as you say. It does reduce the impedance of the line though, because it effectively brings the two live wires closer together.

The 14/2 UF-B wire looks like this:

Here are a few simulations I made in good old Quickfield. I've just taken one quarter of the line, because of symmetry, to make best use of the limited mesh in the student edition. You can see the outline of the plastic, and there is no calculation inside the metal. The simulation boundary is a lot larger, for safety, but this is the interesting part.

The fields in the wire look like this:

The characteristic impedance is 124 ohms.

With the ground wire removed, the fields look like this:

and the impedance is 150 ohms.

This change in impedance is not a big deal, but you aren't getting the typical 300-450 ohm line impedance of an open wire line, if that matters.

The next problem is that the plastic used on mains cable is optimised for toughness, UV stability and fire safety, not for its RF properties. It is probably PVC of some sort, and likely quite lossy.

You can test the loss in the line with a VNA, or try transmitting full power through a short piece, into a load. Unroll it first!

You can also try out the plastic alone by placing some in the microwave oven. If the plastic warms up in the microwave, it is lossy at RF. (No metal, just plastic. Start with a glass of water at the other side of the microwave, but if it's not heating up, try without water to see if the glass heats up before the plastic).

This cable also tends to concentrate all the fields inside the plastic, while in 300-ohm cable the fields are mostly in the air. You even see 450-ohm line with holes cut in the centre dielectric, to further reduce the loss (especially when wet).

You will probably find this cable has too much loss, and might even melt with a few hundred watts in it, especially if it's after the tuner, at high VSWR.

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Oh, if you are happy with 150 ohms, then a pair of RG-6 cables will work much better than this wire, and be cheaper. Connect the inners to the balanced side of your balun, and ground both outer conductors. It will also not be affected by rain, and OK to mount directly to metal.