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Copper vs. Aluminum

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W6OGC, Dec 21, 2011.

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  1. W6OGC

    W6OGC Ham Member QRZ Page

    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?
     
  2. KD5SPX

    KD5SPX Ham Member QRZ Page

    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.

    Search Google for Copper vs Aluminum
     
  3. KB9MZ

    KB9MZ Ham Member QRZ Page

    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.
    Art
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    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.
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    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.

    Jerry, K4SAV
     
  6. W6OGC

    W6OGC Ham Member QRZ Page

    That seems to be an issue of controversy. According to SteppIR,

    and

    That means I'll be S9+20 all bands all over the world!

    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.
     
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    You have asked good questions and maybe surprised by the answers.

    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.
     
  8. NA0AA

    NA0AA Ham Member QRZ Page

    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.
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    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.

    "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.

    Jerry, K4SAV
     
  10. KB9MZ

    KB9MZ Ham Member QRZ Page

    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!
    Art
     
  11. WB5WPA

    WB5WPA Ham Member QRZ Page

    http://forums.qrz.com/showthread.php?325762-Copper-vs.-Aluminum&p=2416287#post2416287

    "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."

    I'm going to have look at that slide presentation, but I imagine for a 15 Amp circuit they use a larger gauge Aluminum wire as wire sizing in 'bundles' (as opposed to something operating in 'free air') takes into consideration the heat generated by the I^2*R = P loss due to the size of the wire selected?

    Jim de WB5WPA
     
  12. WB5WPA

    WB5WPA Ham Member QRZ Page

    "Conductivity alone is not the measure of efficiency in radiation terms of classical physics but just an old wives tale!"

    Art, I have not seen much written on that in either 'classical' or contemporary works on the subject.

    Jim de WB5WPA
     
  13. KB9MZ

    KB9MZ Ham Member QRZ Page

    Isn't it a matter of voltage breakdown of generated oxides together with losses introduced by the presence of heat which a lossy addition to relative efficiency? Metamaterials consist of smooth aluminum surface where
    oxides generation is prevented by self healing insulation which is why their consideration is being considered for energy change with high efficiency via surface traveling energy. Jo
    ims comments are correct with respect to the court hearings on consumer house fires when aluminum is used
    in a current carrying form. General Electric was released from these hearings when they showed that their connection methods impinged beyond the metal surface oxides into virgin material which thus preserved the connection free of oxide generation for dependable electrical connections which thus did not provide the generation of heat that eventually created house fires and where others used pressure connections alone.
    Art
     
  14. W8JI

    W8JI Ham Member QRZ Page


    I have no doubt what I am about to tell you is true, because I have looked at this carefully in the past and have seen the problems people have found with antennas near saltwater. Let's go down a list:

    1.) People claiming aluminum has some conductivity advantage are absolutely wrong. Copper is the better conductor. Any table listing resistivity will show that. Copper, or the correct grade stainless, is a better choice because of reduced corrosion.

    2.) Conductivity in the metal does not matter much, because resistivity is so low for any metal, the loss advantage of copper, or one metal over another, is meaningless. Variable capacitors are sometimes manufactured with aluminum plates only because they do not rust, are cheap, and are soft and easy to polish. It has nothing to do with a conductivity advantage over copper.

    3.) Anyone telling you you can stick a pipe in saltwater, or use a small plate, is wrong. They have either not tried it themselves, or they have not run the numbers for loss. Current density in the water will be so high around the contact area there will be dozens of ohms loss, plus the tuning will change with the tides (if you have tides).

    Typically for 80 meters, a reasonably low resistance ground would require about a fifteen foot radial area, or larger, for coupling to the water. The problem you will have is some tuning change if the water level changes in relationship to the vertical. The ground does not need to be in the water, and actually should not be in the water. It will actually work better above the water, so it couples capacitively to the water.

    The problem you are going to have is skin depth, as well as resistivity of the water where currents concentrate. Skin depth is less than a foot on 160 meters. This means if you stuck a 100 foot deep rod down into the water, at less than 1 foot down only 37 % of the current remains on 160 meters. In the second several inches from that point, only 37% of that 37% remains, or about 14% of the initial current.

    As frequency is increased, the current decays even faster with depth.

    This is why you should NEVER submerge a ground plate for RF in saltwater.

    When broadcast towers are installed over saltwater, the ground systems are very large area copper mesh screens suspended well above high tide. Some contact is made with water, but only for lightning and low frequency grounding. Radials, or RF carrying ground conductors, are never submerged.

    Here is the N6LF paper, that I found after typing most of the above comments:

    http://rudys.typepad.com/files/seawater-ground-article.pdf

    I found Rudy's paper while searching for BC ground systems over saltwater, to show you a picture. I was looking for pictures of an AM BC station I did some work at out in the SF bay area, but I cannot recall the call sign.

    Here's something that looks very similar. The squares at the tower bases are the ground screen systems that suspend above water level.

    wsun2.jpg

    73 Tom
     
    Last edited: Dec 22, 2011
  15. KB9MZ

    KB9MZ Ham Member QRZ Page

    rough surfaces of conductive materials prevent the orderly formation of displacement current, the same as indented imperfections. This fact is used as a measure of non destructive testing of materials in industry , especially the oil pipes in Alaska and similar climes. Surface defects are where harmonic fatigue failures begin.
    Art
     
  16. W6RZ

    W6RZ Ham Member QRZ Page

    I'm going to guess it was KGO-AM. It's a three tower array that's located on what looks like a small man made peninsula in the bay (although in shallow water).

    http://maps.google.com/maps?f=q&hl=en&q=37.52639,+-122.10056+(KGO-AM)&om=1

     
  17. K9KJM

    K9KJM Ham Member QRZ Page

    I agree with W8JI's comments.

    The bottom line to me is that copper is the superior conductor, Period. Aluminum can be used to function almost as well as copper in many cases, Above ground and away from saltwater, For a much lower cost.

    When placed in or on the soil, In most all cases, Go with copper conductors.
     
  18. NH7RO

    NH7RO Ham Member QRZ Page

    Been years since I lived in the Bay Area but my first thought was KDIA and/or KSFO right next to the Bay Bridge Toll Plaza, not that it really matters that much. Don't really need a photo to illustrate things as your explanation seems pretty clear to me; makes complete sense.

    Corrosion is a major culprit in more ways than one (i.e., think of the corroding galvanized mast that couples with RF and rectifies it into RFI...).

    Great explanation as usual, Tom; you should write books (if you don't already).

    73, Jeff
     
  19. W6RZ

    W6RZ Ham Member QRZ Page

  20. WA9SVD

    WA9SVD Ham Member QRZ Page

    All antenna materials are a compromise. Copper is preferable to Aluminum (Aluminium for our British brethren) because it DOES have higher conductivity, and thus slightly lower ohmic loss and slightly higher radiation efficiency. Aluminum is usually chosen because of it's strength/weight ratio, and because it IS relatively conductive. (Most wire antennas are constructed with Cu wire rather than Al, although the absolute performance difference probably wouldn't be noticeable.) Practically speaking, I can't imagine something like a 20 Meter Yagi being constructed from Cu tubing; it wouldn't be able to support it's own weight, or be impossibly heave.
    For that matter, if price and mechanical strength were NOT an issue, we would better choose Silver as the element of choice for all our work; it has higher conductivity than ANY element known. But it's so "mechanically soft" (and expensive) it's impractical to use.
     
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