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Why is copper better than steel for wire antenna?
Folks
So I'm reading the 2006 ARRL Handbook page 22.6, There is a single line stating "Steel wire is a poor conductor at RF; Avoid it." Any idea why? Or is this just one of those physical properties? So how much poorer than copper? Steel clothesline is easily obtained and not that expensive. Admittedly though I haven't done much research on copper or the other type of wires the Handbook mentions. Tony |
Why is copper better than steel for wire antenna?
On Fri, 06 Oct 2006 05:21:00 GMT, Tony VE6MVP
wrote: Folks So I'm reading the 2006 ARRL Handbook page 22.6, There is a single line stating "Steel wire is a poor conductor at RF; Avoid it." Any idea why? Or is this just one of those physical properties? So how much poorer than copper? Steel clothesline is easily obtained and not that expensive. Admittedly though I haven't done much research on copper or the other type of wires the Handbook mentions. Tony Hi Tony, Go ahead and use steel clothesline, it will work fine. 73's Richard Clark, KB7QHC |
Why is copper better than steel for wire antenna?
Steel has a resistivity many times that of copper. It varies a lot with
the alloy, so it's not possible to put a single number on it. But the real problem is that steel is ferromagnetic -- in other words, it has a high permeability. At radio frequencies, current flows in a thin layer near the surface of the conductor. (It actually continues clear to the center of the conductor, but the density decreases very rapidly with depth, so it's essentially zero anywhere except very near the surface.) This concentration of the current has the same result as passing the current through a wire of much smaller cross-sectional area: it greatly increases the resistance of the path carrying the current. The problem is that the thickness of this layer (more technically, the rate at which the current density decays with depth) is determined by, among other things, the permeability of the material -- the higher the permeability, the shallower the layer. So the higher the permeability, the higher the resistance. The permeability of steel is probably even more variable than resistivity, but I'd be surprised if you ever found any in common use with permeability under 100. Or if you found some with permeability of several thousand. Since the relationship between the depth of current flow and permeability is a square root, this means RF resistance of 10 to 100 or so times that of copper, as well as the higher resistance due to the higher DC material resistivity. If the antenna has a large enough surface area, even steel is fine. A common example is an FM mobile whip, which has insignificant loss, or a tower operated as a vertical. But because of the way the current depth and antenna size change with frequency, the loss with a given wire size gets greater and greater as you go lower in frequency, assuming the antenna stays the same size in terms of wavelength. So while moderate diameter steel wire might have insignificant loss on the higher frequency HF bands, that same wire might have substantial loss at the lower end of the HF range. Most hams can measure SWR, but almost none can quantitatively measure the strength of the signal their antennas radiate. And most run way more power than needed to communicate, so can easily lose quite a few dB without a major effect on communications. Consequently, the wider bandwidth gained due to loss in steel wire is considered an asset, while the few dB loss is probably not noticed. (Although hams spend a staggering amount of money trying to buy a few extra dB of gain. Go figure.) In fact, I recall an article some years ago -- in QST if I'm not mistaken -- featuring a wide-band 80 meter antenna whose secret was just that -- loss from using steel wire. If you try it, you might just like it! By the way, copper wire is easily obtained and not that expensive, either, should you choose to go for a stronger signal rather than wider bandwidth. Roy Lewallen, W7EL Tony VE6MVP wrote: Folks So I'm reading the 2006 ARRL Handbook page 22.6, There is a single line stating "Steel wire is a poor conductor at RF; Avoid it." Any idea why? Or is this just one of those physical properties? So how much poorer than copper? Steel clothesline is easily obtained and not that expensive. Admittedly though I haven't done much research on copper or the other type of wires the Handbook mentions. Tony |
Why is copper better than steel for wire antenna?
Use copper clad steal wire. Copper for the RF performance, and the
steal 'core' for strenth. Larger diam. usally yelds wider bandwidth. |
Why is copper better than steel for wire antenna?
Roy,
As my cobwebby brain remembers, for conductive materials such as aluminum through gold, the rough rule of thumb is that at 10 megacycles the skin depth is .01MM (01 is 10 backwards, only reason I remember) So 1mm is 0.0394" therefore a skin depth of 0.1mm is 0.00394", call it 4/1000 of an inch for round numbers... So, the other rough rule of thumb I have always used in my wasted career in industrial electronics is to have the conductive plating 5 times the skin depth... So, 0.020" would suffice for 10 megacycles... Now, that begs the question for steel, or zinc plated steel... Anyone interested can google up answers with a bit of personal effort... What has always intrigued me though, is the concept that a moving charge at RF frequences, spreads over the surface and penetrates only 0.020" the majority of the charge ( @10 mc ) , while still having lines of flux penetrating radially to the electrical center of the metal object... Yet, by the same token, if the metal shape is a hollow tube, no signal will be detected upon the inner skin of the tube... denny |
Why is copper better than steel for wire antenna?
I think you dropped a decimal point, there, Denny. How did you get
from 0.01mm skin depth at 10MHz to talking about 0.1mm? If you stay at 0.01mm, it would be about 0.4 mils. The rule of thumb I remember is 2.6 mils at 1MHz, so it would be 0.8 mils at 10MHz. That's for _copper_ and as Roy wrote, steel will be much less. Another rule of thumb: the RF resistance of copper wire is about 1 milliohm/foot * sqrt(freq. in MHz) / diameter in inches. For 14AWG wire at 4MHz, that's about 31 milliohms per foot, which is pretty much inconsequential for a 75M half-wave dipole with about 75 ohms feedpoint radiation resistance. The resistivity of nonmagnetic stainless steel is roughly 50 times that of copper, so the loss would be about seven times as great at RF, assuming that the permeability really is low at RF; that wouldn't be bad. But high permeability would not be good, especially in a small diameter wire.. Moderately wide bandwidth, high strength, low loss dipole: a center support steel cable, surrounded by 4 or so small copper conductors in a "cage" spaced out from the center support to make a conductor perhaps 1/200 of a wavelength effective diameter. Cheers, Tom Denny wrote: Roy, As my cobwebby brain remembers, for conductive materials such as aluminum through gold, the rough rule of thumb is that at 10 megacycles the skin depth is .01MM (01 is 10 backwards, only reason I remember) So 1mm is 0.0394" therefore a skin depth of 0.1mm is 0.00394", call it 4/1000 of an inch for round numbers... So, the other rough rule of thumb I have always used in my wasted career in industrial electronics is to have the conductive plating 5 times the skin depth... So, 0.020" would suffice for 10 megacycles... Now, that begs the question for steel, or zinc plated steel... Anyone interested can google up answers with a bit of personal effort... What has always intrigued me though, is the concept that a moving charge at RF frequences, spreads over the surface and penetrates only 0.020" the majority of the charge ( @10 mc ) , while still having lines of flux penetrating radially to the electrical center of the metal object... Yet, by the same token, if the metal shape is a hollow tube, no signal will be detected upon the inner skin of the tube... denny |
Why is copper better than steel for wire antenna?
On 6 Oct 2006 01:15:58 -0700, "Will" wrote:
Use copper clad steal wire. Copper for the RF performance, and the steal 'core' for strenth. Yes, the ARRL handbook mentioned that. Tony |
Why is copper better than steel for wire antenna?
Roy, W7EL wrote:
"---the loss with a given wire size gets greater as you go lower in frequency,---. Effective resistance to r.f, is approximately proportional to the square root of the frequency due to "skin effect" as Roy mentioned in describing how current penetrates the conductor less completelty due to inductance deeper in the wire. So, loss is greater at higher frequency due to reduced effective cross-section in the wire. Conversely, the loss with a given wire size gets lower as you go down in frequency. Best regards, Richard Harrison, KB5WZI |
Why is copper better than steel for wire antenna?
On Fri, 06 Oct 2006 05:21:00 GMT, Tony VE6MVP
wrote: Folks So I'm reading the 2006 ARRL Handbook page 22.6, There is a single line stating "Steel wire is a poor conductor at RF; Avoid it." Any idea why? Or is this just one of those physical properties? So how much poorer than copper? Steel clothesline is easily obtained and not that expensive. Admittedly though I haven't done much research on copper or the other type of wires the Handbook mentions. I am guessing that the "steel clothesline" to which you refer is probably actually stranded (7x1?) heavy galvanised soft steel wire. The galvanising is zinc or zinc/aluminium alloy and its thickness has bearing on the answer for a specific frequency. The stranding also has adverse effect on the effective RF resistance, though not as predictable as the zinc coating. Though it works, there are a number of mechanisms that increase the loss, and the extent of some of them are quite difficult to predict or to measure (for the average amateur). The additional loss of steel wire is less important in an antenna design that is loaded with bulk resistance, eg T2FD. A reason why small guage stainless steel wire commonly used commercially on these antennas isn't necessarily unsound. But that application should not imply that small guage stainless steel is just as suited to a half wave folded dipole. Antenna wire would be one of the lowest cost elements of a complete system, which questions the cost effectiveness of savings. Owen -- |
Why is copper better than steel for wire antenna?
On Fri, 06 Oct 2006 22:17:10 GMT, Owen Duffy wrote:
Antenna wire would be one of the lowest cost elements of a complete system, which questions the cost effectiveness of savings. Sure, but clothesline wire is easily available in this small town. Copper wire means I'd have to search it out in the nearest big city. Tony |
Why is copper better than steel for wire antenna?
Tony VE6MVP wrote:
On Fri, 06 Oct 2006 22:17:10 GMT, Owen Duffy wrote: Antenna wire would be one of the lowest cost elements of a complete system, which questions the cost effectiveness of savings. Sure, but clothesline wire is easily available in this small town. Copper wire means I'd have to search it out in the nearest big city. Tony All the wire antennas I've built for the last 20 years or so have been made out of electrical wire from the local home improvement store. They alway seem to outlive my interest in them. -- Jim Pennino Remove .spam.sux to reply. |
Why is copper better than steel for wire antenna?
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Why is copper better than steel for wire antenna?
On Thu, 05 Oct 2006 23:38:11 -0700, Roy Lewallen
wrote: I have to think about your response a bit and attempt to understand some things a bit better. My technical knowledge is limited but I'm learning lots. What I have in mind, before winter sets in, is to setup a horizontal loop just underneath the eaves of the house. Some eyes along the long wide of the house and stretched along the narrow width. And use a SGC Smartuner as per http://www.sgcworld.com/SmartunerProductPage.html. This will at least get me going on the HF side of things. Nothing fancy and with not a lot of range but enough to give me a taste of HF while I think about what I want to do with the trees in the lot and such. So one thing I would want to do is to ensure the wire is insulated from the eyes so there is no sparking or static. And clothesline is meant for the out of doors which is why I was thinking of it. Tony |
Why is copper better than steel for wire antenna?
Tony VE6MVP wrote:
On Sat, 07 Oct 2006 03:25:03 GMT, wrote: Antenna wire would be one of the lowest cost elements of a complete system, which questions the cost effectiveness of savings. Sure, but clothesline wire is easily available in this small town. Copper wire means I'd have to search it out in the nearest big city. Tony All the wire antennas I've built for the last 20 years or so have been made out of electrical wire from the local home improvement store. They alway seem to outlive my interest in them. Just standard household electrical wiring? So purchase some two wire (actually three wire if you include the ground wire) electrical cable and use the black and white wires? Will the insulation withstand the out doors? Or do you strip off the insulation and use them bare? Tony Standard, single strand, solid, electrical wire, normally with the insulation left on. I usually buy blue so it blends with the sky. Leaving the insulation on shortens the wire required ever so slightly. The insulation lasts for years on everything I've ever put up. -- Jim Pennino Remove .spam.sux to reply. |
Why is copper better than steel for wire antenna?
Tony VE6MVP wrote: On Sat, 07 Oct 2006 03:25:03 GMT, wrote: Antenna wire would be one of the lowest cost elements of a complete system, which questions the cost effectiveness of savings. Sure, but clothesline wire is easily available in this small town. Copper wire means I'd have to search it out in the nearest big city. Tony All the wire antennas I've built for the last 20 years or so have been made out of electrical wire from the local home improvement store. They alway seem to outlive my interest in them. Just standard household electrical wiring? So purchase some two wire (actually three wire if you include the ground wire) electrical cable and use the black and white wires? Will the insulation withstand the out doors? Not any of the multi-conductor household electrical wire ("Romex"), find a spool of insulated #14 single-conductor "household wire" at any decent neighborhood hardware store. Here in the southern provinces it's called "#14 THHN" which comes in both solid and stranded types and in a multitude of colors. I prefer stranded wire because it's less prone to bending fatigue failure than is solid wire. Theoretically If push comes to shove dial up a local electrician and ask where he gets the stuff. Personally I wouldn't string the wire thru bare screw eyes, I'd use the Radio Shack catalog number 15-853 screwin insulated "TV cable standoffs" to support it. Or do you strip off the insulation and use them bare? Leave the insulation alone, might get ugly after awhile but it lasts forever out in the elements and has no discernable effect on the performance of the wire as an HF loop antenna material. Tony Brian w3rv |
Why is copper better than steel for wire antenna?
Roy Lewallen wrote: Steel has a resistivity many times that of copper. It varies a lot with the alloy, so it's not possible to put a single number on it. But the real problem is that steel is ferromagnetic -- in other words, it has a high permeability. At radio frequencies, current flows in a thin layer near the surface of the conductor. (It actually continues clear to the center of the conductor, but the density decreases very rapidly with depth, so it's essentially zero anywhere except very near the surface.) This concentration of the current has the same result as passing the current through a wire of much smaller cross-sectional area: it greatly increases the resistance of the path carrying the current. The problem is that the thickness of this layer (more technically, the rate at which the current density decays with depth) is determined by, among other things, the permeability of the material -- the higher the permeability, the shallower the layer. So the higher the permeability, the higher the resistance. The permeability of steel is probably even more variable than resistivity, but I'd be surprised if you ever found any in common use with permeability under 100. Or if you found some with permeability of several thousand. Since the relationship between the depth of current flow and permeability is a square root, this means RF resistance of 10 to 100 or so times that of copper, as well as the higher resistance due to the higher DC material resistivity. If the antenna has a large enough surface area, even steel is fine. A common example is an FM mobile whip, which has insignificant loss, or a tower operated as a vertical. But because of the way the current depth and antenna size change with frequency, the loss with a given wire size gets greater and greater as you go lower in frequency, assuming the antenna stays the same size in terms of wavelength. So while moderate diameter steel wire might have insignificant loss on the higher frequency HF bands, that same wire might have substantial loss at the lower end of the HF range. Most hams can measure SWR, but almost none can quantitatively measure the strength of the signal their antennas radiate. And most run way more power than needed to communicate, so can easily lose quite a few dB without a major effect on communications. Consequently, the wider bandwidth gained due to loss in steel wire is considered an asset, while the few dB loss is probably not noticed. (Although hams spend a staggering amount of money trying to buy a few extra dB of gain. Go figure.) In fact, I recall an article some years ago -- in QST if I'm not mistaken -- featuring a wide-band 80 meter antenna whose secret was just that -- loss from using steel wire. If you try it, you might just like it! By the way, copper wire is easily obtained and not that expensive, either, should you choose to go for a stronger signal rather than wider bandwidth. Brings up a question in my mind which is related to the points you've made about conductivity vs. skin depth. As it then relates to bare vs. insulated copper wire for HF work. Bare copper wire out in the WX will oxidize which adds a layer of copper oxide on the O.D. of the wire over time. What is the effect of this layer on skin resistance losses at HF frequencies in practical terms? Somewhere along the line I picked up the notion that copper oxide is a pretty lousy conductor and the problem can be resolved by using insulated wire for wire antennas . . comments?? Roy Lewallen, W7EL Brian w3rv |
Why is copper better than steel for wire antenna?
On 6 Oct 2006 23:39:28 -0700, "Brian Kelly" wrote:
Somewhere along the line I picked up the notion that copper oxide is a pretty lousy conductor and the problem can be resolved by using insulated wire for wire antennas . . comments?? Hi Brian, Consider, insulation is pretty lousy conductor too. Current is going to conduct where it will, and ignore both insulation and oxide. Put a high resistance path in parallel with a very low resistance path (of identically the same length), which one will current choose? The problem of oxide is when it encounters a poor joint and creates a semiconductor. 73's Richard Clark, KB7QHC |
Why is copper better than steel for wire antenna?
The local price at Home Improvement stores is less than $25 for 500
feet of 14 gauge wire. I use electric fence insulators from the farm supply store. I have been disapointed in the mechanical strength of the Aluminum electric fence wire. On 6 Oct 2006 23:05:03 -0700, "Brian Kelly" wrote: John Ferrell W8CCW John Ferrell W8CCW |
Why is copper better than steel for wire antenna?
On Sat, 07 Oct 2006 14:20:38 -0400, John Ferrell
wrote: The local price at Home Improvement stores is less than $25 for 500 feet of 14 gauge wire. I use electric fence insulators from the farm supply store. Oh, ok. Electric fence wire. I hadn't thought of that. Tony |
Why is copper better than steel for wire antenna?
On 6 Oct 2006 23:05:03 -0700, "Brian Kelly" wrote:
Not any of the multi-conductor household electrical wire ("Romex"), find a spool of insulated #14 single-conductor "household wire" at any decent neighborhood hardware store. My cursory glance as I walked by the small town stores didn't see any such but a few other stores, such as farm supply store, should have such. Personally I wouldn't string the wire thru bare screw eyes, I'd use the Radio Shack catalog number 15-853 screwin insulated "TV cable standoffs" to support it. We don't have a Radio Shack store within a hundred miles. But I get the idea. I'll go looking for some such. Tony |
Why is copper better than steel for wire antenna?
On Sat, 07 Oct 2006 14:20:38 -0400, John Ferrell
wrote: I have been disapointed in the mechanical strength of the Aluminum electric fence wire. Aluminium is not very good material for fence wire and not usually a substitute for steel in general fencing as it lacks the strength of steel. There are fence wires made from a steel core (typically high tensile) and an aluminium (or aluminium / ~5% zinc) coating, sometimes with a polymer coating over the top. These products are appearing as the new "longlife galvanised" fence wires. Commonly the aluminium thickness is around 30 microns, way less than skin depth at low HF, so they can be expected to perform about as well as the high tensile steel core. There are other products with a 200 micron cladding of 60% conductivity aluminium over a high tensile core, and they look a good prospect for antenna wire, 80% RF conductivity and 10000% strength compared to the same diameter HDC. For example Gallagher XL 2.7mm diameter wire (200 micron aluminium cladding) should have the same loss as 2.3mm dia HDC, but over 10 times the Gross Breaking Strength. To determine their likely loss as antenna wires, you need to know the coating thickness. Owen -- |
Why is copper better than steel for wire antenna?
Either you didn't read the remainder of what I wrote, or I failed to
explain it clearly. I was speaking of antennas of a constant length in terms of wavelength as frequency is changed, for example half wavelength dipoles. If you cut the frequency in half, the skin depth increases by a factor of the square root of two, so (assuming a conductor at least several skin depths in radius) the resistivity decreases by the square root of two. But to maintain a constant antenna length in terms of wavelength, the wire length doubles. So the total wire resistance at the lower frequency is greater by a factor of the square root of two. In other words, if you make two half wavelength dipoles out of the same diameter and kind of wire, and cut one for 1 MHz and the other for 2 MHz, the 1 MHz dipole will have about 1.4 times the resistance of the 2 MHz one. That's why you're more likely to see the loss of steel wire in lower frequency antennas. Roy Lewallen, W7EL Richard Harrison wrote: Roy, W7EL wrote: "---the loss with a given wire size gets greater as you go lower in frequency,---. Effective resistance to r.f, is approximately proportional to the square root of the frequency due to "skin effect" as Roy mentioned in describing how current penetrates the conductor less completelty due to inductance deeper in the wire. So, loss is greater at higher frequency due to reduced effective cross-section in the wire. Conversely, the loss with a given wire size gets lower as you go down in frequency. Best regards, Richard Harrison, KB5WZI |
Why is copper better than steel for wire antenna?
Roy, W7EL wrote:
"Either you didn`t read the remainder of what I wrote, or I failed to explain it clearly." The fault was mine, not Roy`s. It is true that if you scale an antenna for half the frequency by doubling its length without increasing cross-section of the wire, its resistance increases. Resistance is rho(l/a) where rho is the resistivity, l is the length of the wire, and a is the area of the wire`s cross-section. Roy noted that lowering frequency by half means a wire twice as long which tends to double the wire`s resistance but skin effect increases penetration of the wire at the lower frequency. This reduces resistance by 1/sq.Rt.2. The same antenna using twice the length of the same wire but at half the frequency thus will have 1.414 times the effective resistance of the double frequency antenna. Best regards, Richard Harrison, KB5WZI |
Why is copper better than steel for wire antenna?
Electric fence wire is aluminum, comes on 1/4 mile, 1/2 mile and mile rolls,
works great for ground radials but is to soft for antennas, IMHO. Best wire I have found for antennas is what the local phone co-op calls "field wire". They lay it down during the winter when they can not trench in a new copper line. In the spring they roll it up and toss it away after they trench in the new line. This stuff is 7 strands of steel, covered with a UV protective outer plastic shell. Stuff will last for years, stretch and return to shape, and if you bundle enough together you can pull a car out of the ditch in a pinch. Have had it up for years as a 40 and 80 meter dipole. Hard as the dickens to work with, almost impossible to solder, but, it makes a dipole even northeast Montana winters can not break, something to be said for that. Just my two cents worth. Sam |
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