Who has any insights on whether braided conductors make better ground conductors than solid ones of the same DC resistance, and why?
I’ve found several references that say braids are better because they have higher surface area. Some cite the “skin effect”. But it does not seem to me that surface area compressed into a small region would be helpful. I think large flat conductors, or “ground planes”, provide superior HF grounding because of their shape and the multitude of different path lengths implicit therein (or said another way their ability to intercept magnetic field lines in many locations simultaneously). I think turning a solid wire into a braid of the same gage would do no more good than using a metal pipe filled with fine metal powder having high surface area. Turning a solid wire into a larger diameter, hollow conductor may well be an improvement (it increases the capacitance for example). I’m looking for the surface area contribution per se that would be independant of overall shape.
But then there is Litz (Litzenstraudt? sp?) wire, the odd looking stuff they wrap ferrite rods with to make AM radio receiver antennae. This wire is multiple identical fine copper strands that are individually insulated and then braided in a pattern that keeps each strand near the surface for the same percentage of its length. The wire is terminated by removing all the insulation and soldering the ends all together. While there could be old wive’s tales about grounding braids, I know radio manufacturers are not going to go to the expense and trouble of using Litz wire to make 10,000,000 radios if they don’t measure a worthwhile improvement.
If braids are better ground conductors, then why aren’t they used for the radiating elements of antennae?
I’ve always thought that braided cables were preferred simply because they are flexible. Anything that is not mechanically grounded (via bolts or whatever) doesn’t need a special cable to ground it electrically (since it’s grounded via the chassis), and anything that isn’t mechanically grounded will be likely to want to move around and therefore will need to be grounded with a cable that offers some flexibility. Braided cable is easier to use in all respects.
A good examply is the ground cable on a car battery. If that was a solid piece of AWG10, you’d never be able to replace your battery without a crow bar.
As to why flexible conductors aren’t used as antenna elements, antenna need to be cut to exact physical dimensions. A little too long (or short) and you’ll have standing (or reflected) waves. Change the physical characteristics of an antenna and you’ll also screw up the electrical characteristics.
Transmitting antenna are high impedance voltage driven devices. An antenna is a transducer, changing high voltage from the transmitting amplifier into the desired electromagnetic & electrostatic waves. No need for large current carrying wires up there.
Braided cable of the same outside diameter as another piece of solid wire has about the same DC resistance, provided that the braided wire is not hopelessly corroded.
Flexible wire is not used for antennas for the simple reason that it IS flexible. An antenna made of braided wire would flop over like an extremely aged penis in dire need of a dose of Viagra. Since the antenna’s resonance frequency is dependent on the relative positions of the antenna rod and the ground plane, you would get crappy reception with a limp rod.
Antennas today normally have an impedance of fifty ohms (for two way radios) or seventy five ohms (normal car radios.) A car radio antenna cable doesn’t have to carry either high voltages or high currents. The antenna cable for a two way radio doesn’t have to handle high voltages, but it does have to deal with high currents.
In truly high performance radio systems - microwave relay stations, FM broadcast transmitters, and the like - they use a solid tube with a solid wire run through the middle. This gives them a very low DC resistance, and a perfectly controlled impedance.
Braided wire is used because it is flexible, not because of any better electrical characteristics.
Wow, I haven’t heard of Litz wire in 20 years. I used to work for a company that made CAT scanners back in the late 70s and early 80s. Hospitals are pretty picky about ground leakage and I think the standard was 20 milliamps. Anyhow, somebody in engineering put in a requirement for Litz wire for grounding the gantry and it worked better than a normal stranded ground wire. I’m not sure why it worked but at the time I was told that it had to do with the frequency. Might I ask why you’re interested in something as obscure as Litz wire ?
High frequency signals tend to ride along the skin of a conductor, while lower frequency and DC signals will stray towards the core. Thus, braided conductors will maximize surface area for a given amount of material.
I can see how this would keep the current uniform throughout the wire. For a solid wire, the skin effect causes current to flow predominantly near the wire surface. For the current to do this on Litz wire, it would have to jump the insulation between conductors. A less resistive path is to just follow the strand into the wire middle, so the current does this. Since the whole wire is being used, a Litz wire of the same metallic cross-sectional area (i.e. not including the insulation) would be a better high frequency conductor.
Whether they use it because of this, or because it’s flexible, or some other reason, I don’t know.
If all the individual strands were parallel, with some always in the middle, you wouldn’t get this effect, and the center strands would have lower current at high frequencies.
Several points in reply to the above:
Flexibility isn’t what I was referring to in regards to antennae. If you think of a whip antenna on your car, or a Yagi beam for your television set or Ham set, then, sure, it has to be rigid to accomplish the mechanical design of the antenna. But plenty of antennae use flexible wires, for example as a trap dipole strung between high trees or for example as a longwire. There’s an antenna somewhere in New England that communicates with submarines around the world; it’s flexible cable lying on the ground in a loop several miles in diameter.
Antennae do sometimes have to carry large currents as well as very high voltages, if they are transmitting (receiving antennae don’t). Transmitting antennae generally are made to resonate and preferrably have many times more current circulating around in the radiating element than they have entering from the transmitter amp. It’s expensive to create this resonating current and having a less resistive element for it to circulate on is very worth doing. If I remember right the ground radials for AM radio stations (which depending on how you look at it count as radiating elements more than as connections to the EArth) are usually about an inch in diameter, stranded Bronze cable (but stranding would be needed for flexibility whether or not it helped conductance).
>>Anyhow, somebody in engineering put in a requirement for Litz wire for grounding the gantry and it worked better than a normal stranded ground wire. I’m not sure why it worked but at the time I was told that it had to do with the frequency.<< -Yeah, see, there’s something about this point that could absolutely dictate the answer to my question. This is what I am getting at. But I don’t know exactly what gives. >>Might I ask why you’re interested in something as obscure as Litz wire ?<< Only because it seems to me as though the justification for using Litz wire (and there must be some justification) would also explain why braid is better for HF grounding, or else explain why many people mistakenly believe it is.
>>If all the individual strands were parallel, with some always in the middle, you wouldn’t get this effect, and the center strands would have lower current at high frequencies.<<
>>High frequency signals tend to ride along the skin of a conductor, while lower frequency and DC signals will stray towards the core. Thus, braided conductors will maximize surface area for a given amount of material.<<
********* Aha! Now, these two posted comments seem to me to contradict one another. Does charge try to get to the surface of a conductor, or does charge try to get away from other like charge? Because those two things are equivalent only in a solid conductor. If you have a stranded conductor, the charge may not travel on the surface of each strand, but rather on the surface of the bundle, demonstrating that it was the overall shape that mattered, that charge was just displaying its dislike of self. So which is it? Posters, do I take your answers correctly to be mutually exclusive in the context of multistranded conductors? BTW ZenBeam I think you get the point of Litz wire.
They aren’t really contradictory: both the overall shape of the “bundle” and each individual conductor matter. The issue is somewhat complicated to explain easily because of capacitive effects between close proximity conductors, but that site should give you a look into the “equivalent circuit” model of transmission lines for high frequency signals. It isn’t that tough to understand with some electronic background, but otherwise the case is totally lost.
This site goes into some less complicated details.