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Best wire for long wire antenna
Owen Duffy wrote:
"JERD" wrote in : I am getting back to the HF bands after some 20 years 'rest'. What is the best type of wire to use for long wire antennas? The wire will be under some tension to keep it above some local trees! You might find the article 'Sufficient sag for wire antenna spans for wind survival' at http://www.vk1od.net/rigging/sag.htm of interest. You might wonder why annealed copper in almost any form (enamelled, or PVC insulated, especially PVC insulated) is so popular. I endorse Roy's support for Copperweld, but you will not find it readily available in Oz. There are a few suppliers of HDC (ATN and Discount Cables come to mind). Stainless steel (2mm dia) is used in some of the commercial antennas (eg Bushcomm), but you need to consider whether the additional loss is an issue for the specific antenna. BTW, MIG wire (copper plated steel wire) is not Copperweld, Copperweld has a substantial copper thickness (eg 30%, depending on the grade) which is chosen to be at least several skin depths in thickess so that the RF resistance is similar to copper, but the strength comes largely from the copper core. I think you mean the strength comes from the steel core? FWIW, when they say 30%, that means that the wire has 30% of the conductivity of the same diameter copper wire, not that it is 30% copper. You have to look in the mfrs tables to see what the cladding thickness is. Odd that there aren't suppliers in Australia.. Maybe because there isn't an existing telegraph/telephone cable infrastructure in rural areas? In the U.S., there were and still are millions of miles of the stuff installed before carrier telephony, coax, optical fibers, etc became common. And, it's still used for MV power distribution, much of which is still above ground. |
#2
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Best wire for long wire antenna
Jim Lux wrote in
: Owen Duffy wrote: .... thickess so that the RF resistance is similar to copper, but the strength comes largely from the copper core. I think you mean the strength comes from the steel core? Yes, thanks Jim. FWIW, when they say 30%, that means that the wire has 30% of the conductivity of the same diameter copper wire, not that it is 30% copper. You have to look in the mfrs tables to see what the cladding thickness is. Yes, IIRC it is rated on its 60Hz resistance as a competitor to HDC aerial power applications. Odd that there aren't suppliers in Australia.. Maybe because there isn't an existing telegraph/telephone cable infrastructure in rural areas? In the U.S., there were and still are millions of miles of the stuff installed before carrier telephony, coax, optical fibers, etc became common. And, it's still used for MV power distribution, much of which is still above ground. Yes, pity that. Remember that we had a monopolistic telecomms market until ~1995. I did scavenge a littel copper clad steel (3.2mm) on one occasion, but most aerial phone cables were HDC. My article mentions an aluminium clad HT steel fencewire which looks interesting for antennas, but it does bring the issue of corrosion resistance connection to aluminium. Unfortunately, this wire is only available in 1km rolls. It turns out that HDC is probably the best available wire in Oz (JERD is a VK5), but gal fencewire or stainless steel are eminently suited to lossy antennas like TTFD etc. I do remember Roy discussing the resistivity of rust, but I don't recall figures... does anyone have any figures for the resistivity of the rust coating on a rusted steel wire? I guess its permeability is also relevant to RF resistance. Owen |
#3
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Best wire for long wire antenna
Owen Duffy wrote:
. . . I do remember Roy discussing the resistivity of rust, but I don't recall figures... does anyone have any figures for the resistivity of the rust coating on a rusted steel wire? I guess its permeability is also relevant to RF resistance. No, I don't believe I've mentioned rust, although I've commented on copper and silver salts a couple of times. I wouldn't try to guess what the resistivity of rust might be, since it would surely vary a great deal with the amount of hydration. A highly resistive coating won't add appreciable loss, nor will a highly conductive one. There's an in-between range which will. But keep in mind that the skin depth is inversely proportional to the square root of the conductivity, so a thick coating with twice the DC resistivity will increase the RF resistivity only by a factor of about 1.4. The real problem with thinly plated steel wire is that if and when the coating corrodes off -- or if it's too thin to begin with -- the current ends up flowing in the steel itself. Steel is terribly lossy stuff at RF chiefly because of its permeability, not that its conductivity is all that hot to begin with. Skin depth is inversely proportional to the square root of permeability, so a steel with permeability of 100 has 1/10 the skin depth it otherwise would, resulting in 10 times the RF resistance. If you consider half wavelength antennas at various frequencies all made from the same size wire, you find that the loss gets worse as frequency decreases. So it's often more of a problem on the lower frequency bands. Some stainless steels are magnetic and some aren't. Magnetic ones are much lossier at RF for the same reason as ordinary steel. Roy Lewallen, W7EL |
#4
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Best wire for long wire antenna
The 'best' wire for long antennas is whatever you happen to have
the most of that will support it's own weight. After that, it's more a matter of what color insulation you think is nice. And after that, it's whatever you can convince yourself that you can afford. Have several pieces of different wire size/type? If they will support their weight, splice the @#$ things! Paint it all pink. Pink works very well for antennas. - 'Doc |
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