Copper vs. Aluminum
What are the properties of copper and aluminum, besides cost, that favor selecting one over another for antennas, ground cables, etc? What difference does it make electrically, efficiency wise, if any?
If one was building an wire antenna, would it make a difference to use aluminum wire vs the same size copper?
What about yagis? Those are built from aluminum tubing, mostly. Assuming cost was no object, would copper result in a better performance?
These questions are not entirely theoretical. I am planning a salt water counterpoise for my BigIR vertical which will sit on the dock behind the house, in a channel off San Diego Bay. The idea is to encase a sheet of metal in plexiglass, sealed, mounted to be in the water and connected to the vertical as a ground/counterpoise/radial substitute. Copper foil is about $10/s.f. Aluminum isn't. Will it matter?
73 de W6OGC
Copper has about 60% better conductivity (less resistance per foot) than aluminum. Copper has greater tensile strength (pull apart) than aluminum. Copper is more expensive and heavier than aluminum.
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If one studies why capacitor plates made of aluminum
reign over the use of copper one finds out why antennas made of aluminum
reign supreme in efficiency.
Aluminum machines better in some ways. It also has tremendous strength for its weight.
Titanium is even better for the S/W ratio but it's not as good a conductor. Aluminum is a great choice for antennas made from tubing. It's a lousy choice for anything that needs to be buried beneath ground or exposed to salt water.
Beams aren't (usually) made from copper because it's heavy, expensive and doesn't make as much sense as aluminum for this. Everything's been tried, there's nothing new to invent here unless someone discovers a new element that's stronger and lighter than aluminum and also highly electrically conductive.
Copper is a better electrical conductor, but current in antennas generally isn't high enough to matter much.
I think aircraft designers use aluminum is for the strength to weight ratio. For its weight, it's very strong and the proper alloys can take tons of vibration over billions of cycles without degradation.
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Seems like a lot of extra work and expense. A plate submerged in salt water will not reduce ground losses under the antenna. It will work about the same as a wire tossed into the water (which seems like an easier alternative). It can make the SWR happy, but it will not reduce ground losses. For that you need elevated radials (above the salt water). A vertical over salt water will work much better than one over earth even if it has a poor ground, so you will be very happy with the performance even with a poor ground. A good radial system will be better. The performance should be about the same, or maybe a little less than, a no-radial vertical in the same location.
The reason is that RF penetration into salt water is only about 7 inches at 2 MHz and about 2 inches at 30 MHz. A submerged wire will not pick up fields from the antenna and return them to the base of the antenna. Those fields from the antenna hitting the salt water directly under the antenna will be dissipated in the water. Current driven into the radials (which will be exactly the same as that going into the vertical) will be dissipated in the water. It has no way to escape or even see the fields from the vertical. It's analogous to using a well casing as a radial system.
That seems to be an issue of controversy. According to SteppIR,
Originally Posted by K4SAV
Due to the fact that RF does not penetrate more than 2 inches into the water, direct coupling (a wire in the water) is difficult. Objects like metal floats or boats, providing they are large
enough, can make good grounds in salt water. If you are using a metal boat or large metal object,
corrosion is no longer a problem because the large surface capacitively couples to the water.
When using a small metal float (3 ft x 3 ft is just enough to “connect” to salt water), you
want to be certain that the metal does not corrode over time.
That means I'll be S9+20 all bands all over the world!
Proximity to the ocean improves the far field loss of a vertical and allows very low angle radiation
Rudy Severn, with whom I have been corresponding, said, "Because the skin depth in seawater is small at HF you only need to get down 18" or so even for 80m operation. A sheet of copper 18" wide by 4-5' formed into a cylinder would work very well." and went on to point out the corrosion possibilities.
There are at least 4 ways I have thought of, or have been suggested.
1. Copper pipe fixed along and just below the edges of the dock, all the way around it, a length of about 95 feet or so. Advantage is no direct immersion in salt water.
2. A cylinder of copper 18" deep, about 4 inches in diameter, wrapped around a PVC pipe. This would require a zinc to prevent or minimize corrosion/electrolysis, or perhaps could be wrapped with some sort of flexible plastic sheet to avoid direct contact with water. Now I have realized that I misread it. He suggested the copper sheet by 4-5' wide formed into a cylinder.
3. A floating "sandwich", plexiglass bottom, copper sheet, topped with enough Styrofoam to float, any convenient size. I've read that 3'x3' would be adequate. It could be made larger, or a different shape, too.
4. A sandwich of copper foil between two sheets of Plexiglas, sealed so water would not penetrate, 2' of plastic with 18" of copper foil, mounted vertically so the foil is in the water all the way, fixed to the dock, with plastic 2' by 6' or so covering a copper foil 18" by 66".
Rudy warned me about attaching radials to the dock which, when made of treated wood, might prove to be a fire hazard as it has on at least one occasion.
The purpose of the present inquiry is to gather enough info to see if the metal has to be copper or perhaps aluminum will do as well.
73 de W6OGC
You have asked good questions and maybe surprised by the answers.
Originally Posted by W6OGC
Electrically, Aluminum got a bad rap in the early days between 1941 to the mid 70's. Multiple outages and many fires were attributed to the use of aluminum. Much research went into the primary causes and problems. What was found is 3 contributing factors. Poor workmanship, thermal expansion, and creep.
Workman was a result of cutting corners and ignorance. Aluminum will for a hard layer of oxidation when exposed to air. Electricians and installers failed to recognize this fact and did not properly treat connection points with a oxidation inhibitor. This is easily solved by proper training, but still can be a problem today.
Thermal expansion between dissimilar metals was a huge problem when the industry switched from using brass screws to steel screws in branch circuit feeders associated with 10 and 12 AWG circuits. This caused connection points to loosen with different rates of expansion and contraction over time.. This issue was solved in th elate 70's by changing the alloys used. But note you must use UL tested and listed products rated for aluminum/Copper termination.
Creep is the non elastic stretch of conductors caused by thermal expansion and tensile forces. This cause terminations to loosen over time requiring re torquing termination connections. Again this was solved by adjusting the alloys used to make the wire.
True that aluminum has higher resistance than copper but is a non issue. Today's aluminum building power wire uses a compact design so what works with say 12 AWG coppers works with 12 AWG aluminum. As for antenna construction it is of no concern what soever, and in fact aluminum is superior to copper.
Now for the down side of aluminum. It is completely useless for ground electrode applications. You cannot use it if it comes into contact with earth. All electrical and building codes forbid its use.
If you want more details look at this SLIDE PRESENTATION.
Aluminum is the preferred material due to conductivity, and it's strength in common alloys for our applications. It's not the strongest ultimate strength, but it's more than adequate and it's extremely cost effective. The alloy strength in flexing means that the yagis don't fail due to metal fatigue, which is of course a real bonus!
If you HAVE the copper, trade it in and you can buy 3 times as much aluminum or more with the money!
I've made some verticals with scrap copper, but the cost of new copper is so expensive as to be prohibitive for MY wallet.
There are a couple of good sources for the nesting aluminum tubing, Cycle 24 looks pretty competitive, of course DX Engineering. You might find it locally but I cannot.
BTW, I've never compared beam prices vs. aluminum tube stock costs, but given the amount of materials used in the designs in the ARRL antenna books, the commercial antennas would have to be VERY expensive indeed, or you want to build a design you cannot buy.
Another nice thing about aluminum is that it takes paint so nicely.
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It's a common misconception. The object of a radial system is not to capacitive couple to earth (or water). It is to shield the antenna from the earth (at least partially). If capacitive coupling was the goal, we would all be burying metal plates in the ground instead of putting down radial wires. That could have much more capacitance. Also an elevated radial system wouldn't work well at all because that has low capacitance to earth. In fact we know it works better.
Originally Posted by W6OGC
"Connecting" to salt water is not the goal of a radial system unless the goal is to make the SWR low, which it will do. So a big metal plate may make your radio happy but it will do nothing for improving ground loss directly under the antenna.
Far field loss is different from near field field loss, which is the reason I said a vertical over salt water will work better than one over earth even if it has a poor ground.
Aluminum is superior to copper as it allows for max efficiency for surface rotary
travels of displacement current and the travel of electrons between adjacent plates.
This makes it superior to copper and noble materials such as gold but it is less efficient when compared to graphite of a particular layered form. Conductivity alone is not the measure of efficiency in radiation terms of classical physics but just an old wives tale!