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Does the particular metal used in antenna elements make any difference in element length or spacing, i.e. 1/2" aluminum tube vs 1/2" copper tube?

I know that length and diameter have an effect on resonance and impedance, but does the particular metal?

My question is specifically about element length vs type of metal element.

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  • $\begingroup$ Edited to clarify the difference between the two questions. $\endgroup$ – David Thorpe Oct 27 '17 at 12:44
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    $\begingroup$ The biggest difference between copper and aluminum and probably the major factor to consider is: cost and weight. Copper is more in both categories. $\endgroup$ – K7PEH Nov 5 '17 at 2:23
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    $\begingroup$ I agree. Cost and weight are dominant factors. The formulas for wave speeds in copper vs aluminum are pretty much the same. I suppose some physicist somewhere could explain that there is a slight difference. But in my experience most hams and even professionals do not make any distinction in the speed part of the formula. $\endgroup$ – SDsolar Apr 11 '18 at 3:00
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Changing the metal in an antenna is equivalent to changing the diameter of the elements, but most metals used for antennas are so close in conductivity that the equivalent diameter change is negligible for most antenna models.

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    $\begingroup$ "Most" metals? This is true for a few metals including copper and aluminum. Most other metals have conductivity significantly less than those two. And some are even magnetic, a big no-no for most antennas unless they are properly plated (such as Copperweld®). Just sayin'. $\endgroup$ – Mike Waters Nov 4 '17 at 19:53
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    $\begingroup$ Ditto, Mike. Since the OP focuses on those two netals this answer may be a tad broad. But I believe your comment it brings it back into the fold, $\endgroup$ – SDsolar Apr 11 '18 at 3:01
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    $\begingroup$ I said most metals used in antennas...which would exclude magnetic metals and hopefully exclude poor conductors. $\endgroup$ – user10489 Apr 14 at 12:48
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Besides mechanical differences, the primary difference between aluminum and copper in antenna construction is RF resistance. Copper will have slightly less RF resistance for the same surface area. Increasing the surface area slightly allows aluminum to exhibit the same RF resistance as copper.

RF resistance is unique due to the tendency of the RF current to crowd around the surface of the conductor. As the frequency is increased, the RF current crowds even more toward the surface, occupying an increasingly thinner layer of the metal. Because of this effect the surface area of the conductor becomes a dominant factor in determining the RF resistance and conductors can be hollow tubes since no appreciable current would flow in the core.

But specifically to your question, RF resistance has no appreciable effect on resonance or element spacing. It will potentially impact antenna efficiency and gain. This is critical in certain classes of antennas such as small loop antennas where the radiation resistance is quite low.

With typical antennas such as a yagi, the difference in efficiency and gain due to RF resistance differences between aluminum and copper will most likely be neglible. If you are concerned about your specific case, provide more details regarding the type of antenna and the intended frequency range of operation.

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    $\begingroup$ My immediate application is a 2-meter, three-dipole horizontal, triangular array - an adaptation of the "Big Wheel" design, presented in QST of March 2008. For my construction plan, 1/2" hard-drawn copper pipe is more readily available than the 1/2" aluminum tube used in the prototype in the article. $\endgroup$ – David Thorpe Oct 27 '17 at 12:37
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    $\begingroup$ That will work great. Have fun with the project. $\endgroup$ – Glenn W9IQ Oct 27 '17 at 13:03
  • $\begingroup$ In cases where the impedance is a bigger factor than you like, but not huge, the diameter of the element can be increased. Also wire and tubing of various sizes and strengths can be clad with copper or other metal (aluminum w/ copper cladding, steel w/ copper cladding, etc.). At lower frequencies a thicker cladding helps because the signal is deeper in the material. $\endgroup$ – Keith Martineau Oct 27 '17 at 23:35
  • $\begingroup$ I think I'm missing a link here. This answer makes me think RF resistance equals or is caused by skin effect. Or vice versa? $\endgroup$ – William - Rem Oct 31 '17 at 14:27
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    $\begingroup$ @william-rem The skin effect alters the resistance seen by current at RF frequencies. If you consider the cross sectional area occupied by the RF current, the RF resistance is nearly equal to the DC resistance of that same cross sectional area. The skin effect simply makes the cross sectional area smaller as the frequency increases. $\endgroup$ – Glenn W9IQ Oct 31 '17 at 14:45
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Leif SM5BSZ has made investigations on the matter by measuring Q of antenna elements in an enclosed box. Look at the fastening methods to the boom that change Q drastically, especially with iron involved.

The influence of resistivity to an actual antenna depends on grade of directivity optimization, the theoretical Q = bandwith / center frequency of the antenna.

For normal low Q antennas, like a dipole or ground plane antenna, the influence have negligble effect of performance.

See it from the source here:


Losses in yagi antenna elements at 413 MHz (sm5bsz.com)

Excerpt:

There are several mechanisms by which element losses may increase above the values computed by the modelling software.

  • Reduced surface conductivity due to corrosion.
  • Ohmic losses due to Eddy currents in the boom tube or other conducting materials near the element center.
  • Magnetic losses in washers, screws and other magnetic materials near the element center.
  • Dielectric losses in surface coatings used to prevent corrosion.
  • Dielectric losses in plastic plugs at the element tips.

Losses in yagi antenna elements at 144 MHz (sm5bsz.com)

Excerpt:

At VHF frequencies element losses are much less important than they are at UHF frequencies because system noise temperatures are not much lower than the environmental temperature and therefore element losses only affect antenna gain, not the system noise temperature for the receiver even when the antenna points into cold sky.

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The simple answer is: There is no difference between copper and aluminium. Now, the simple answer is misleading because copper has to be coated in order to not form the badly conducting oxide while aluminium does not have to be coated because aluminium oxide is a good dielectric. The coating needed on copper can affect the electrical length slightly. Better would be gold plated copper. A VERY thin layer of gold would prevent oxidation and allow most of the current to flow in the copper. Even better, silver plated "whatever" with a very thin layer of gold on top. Silver forms a sulphide which is lossy while gold is inert.The silver layer can be fairly thin and what is beneath does not matter.

Much better than optimizing conductivity is to make a design with larger element diameters:-)

In case you want an invisible antenna, a 144 MHz yagi with 1 mm piano wires as elements with appropriate silver/gold coating on a very thin boom tube could give very good performance while not frustrating neighbours.

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When an electromagnetic (EM) signal is traveling along a conductor at the speed of light, it is not traveling IN the conductor but is instead traveling in the medium that surrounds the conductor. If the conductors are bare copper and bare aluminum, the medium through which the EM signals travel is air so the velocity factor (VF) would be the same for both conductors. EM signals are not electrons traveling in a conductor but rather are photons traveling in the air (or insulation) surrounding the exterior surface of the conductor. Photons, not electrons, are what travel at the speed of light in the medium. EM signals require an ever changing electric field and that cannot happen inside the conductor.

The thing that slows down an EM signal is not the type of metal in the conductor but instead the type of medium surrounding the conductor, e.g. air or the type of insulation through which the photons must travel.

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  • $\begingroup$ This is interesting, Cecil. How does this correlate with the skin effect, where electrons flow in (on?) the conductor at a very shallow depth? $\endgroup$ – Mike Waters Mar 21 at 19:50
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    $\begingroup$ In a nutshell: The inability of RF EM photons to travel deep inside a conductor is the cause of skin effect. Free electrons exist throughout a conductor but only the ones closer to the surface of the conductor can absorb and emit RF EM photons in a bucket-brigade operation that passes the photons along the conductor at the speed of light. The electrons simply vibrate in place at the signal frequency. $\endgroup$ – Cecil - W5DXP Mar 22 at 0:28

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