Antenna Materials & Efficiency
You can not make a very good antenna using insulators like PVC tubing, drinking straws, or nylon rope. I think we can all agree on that. Why? Materials like plastic are electrical insulators, and do not conduct electricity very well due to their low electrical conductivity and high electrical resistivity. With this in mind, consider that materials which conduct electricity also have a conductivity and resistivity number attached to them. This affects how well your antenna will perform.
Properties of Conductors
You can see in the table below that when it comes to the highest electrical conductivity and the lowest electrical resistivity, silver is king. However, copper is a close runner-up.
Lets look at conductivity in a graph form, since it’s visually more telling…
So it is clear that given the choice, you should construct your antenna from copper. But what if you don’t? How will your antenna perform if it’s constructed out of stainless steel, for example? Steel is an alloy of iron and carbon. If we look at the above graph, or table, we see that iron (Fe) is way down the list on electrical conductivity. Lower conductivity equals higher ohmic resistance, equals less efficiency.
Antenna Radiation Efficiency
How much of the power delivered to the antenna is being radiated in electromagnetic waves? Antenna radiation efficiency is defined by the IEEE as “the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.” This is the same definition of efficiency for any power conversion device, and any power which is accepted by the antenna but not radiated in the form of electromagnetic waves is dissipated in the form of heat.
Ohmic Resistance vs Radiation Resistance
There are two types of resistance which should be considered when it comes to an antenna. Ohmic resistance and radiation resistance.
Ohmic resistance is the most commonly understood property, being the resistance we are most familiar with. This can be measured with an ohm meter, multimeter, etc. Ohmic resistance stems from moving electrons due to the flow of electrical current. These electrons strike the atoms of the conductor material in which they are flowing, and they lose momentum. This lost momentum is converted into heat.
Radiation resistance, on the other hand, is created by the Lorenz-Abraham force, which is the resistance to moving electrons caused by the generation of electromagnetic waves. So, the higher the radiation resistance, the more efficient your antenna is at converting electrical current flow into radio waves. You will almost never see radiation resistance quoted in an antenna datasheet, since it is very difficult to measure accurately, and is still the subject of ongoing research.
While Direct Current (DC) flows through the entire conductor, Alternating Current (AC) flows only on the skin of a conductor. As the AC frequency increases, the skin conduction depth decreases. Here is a calculator which allows you to calculate the skin depth, depending on the material. In copper, at 3 MHz, the current is flowing on the surface of the conductor, to a depth of about 1.5 thousandths of an inch. At 30 MHz, that depth decreases to about 0.5 thousandths of an inch. At 1 GHz, it’s down to 0.08 thousandths of an inch. So, increasing the size of your antenna elements doesn’t do much to compensate for the ohmic resistance losses as much as you think, and it’s not a real solution for improving an antenna with poor efficiency due to the construction material.
However, due to the skin effect, if your antenna is built from a material with relatively poor conductivity, like steel for example, a decent plating of copper would go a long way to help the efficiency of the antenna.
Constructing antennas from materials with high permeability such as iron, steel, or nickel will result in a reduced antenna size, but also the depth of the skin effect will be reduced. Higher permeability contributes to higher inductance per meter of wire, and very small skin depth which contributes to higher resistance per meter of wire. Try looking at the skin depth for a given frequency in copper versus nickel in the skin depth calculator. See the highlighted column in the table below for permeability, relative to copper.
Using the right material
What is the right material with which to make an antenna? Easy, really. Something with high conductivity, low ohmic resistance, and low permeability. Ideally, silver, but that is ruled out due to cost. Copper? Yes. Aluminium? Yes. Copper plated steel wire? While not ideal, it’s a good compromise for skin effect conductivity and tensile strength, so yes.
Stainless steel whips used in mobile applications are used because that’s really the only material which is applicable to this application. It needs to be resilient, and that is more important than efficiency in this case.
Efficiency and VSWR
SWR is relied on far too much as an indicator of good or bad antenna performance. Your SWR meter is not telling you how well your antenna is performing, it is telling you how well matched its impedance is to your feedline impedance. You can have a very good impedance match (low SWR), but if your antenna is not efficient at converting the transmitted RF currents into electromagnetic waves, then it is not performing very well as an antenna, and is performing more like a dummy load, converting most of the RF currents into heat. A dummy load is (for our purposes) a 50 ohm non-inductive, ohmic resistor. It gives a very good VSWR over a wide frequency range, yet it is a very poor antenna.
“Horse Fence” Antennas
So called “horse fence antennas” are a curious type of antenna which is made from electric fence tape, like this. These antennas contain many strands of stainless steel wire, and boast very low SWR not just across one band of operation, but over many bands of operation over a wide frequency range. Would anyone like to hazard a guess as to why this may be? It’s a fine example of relatively high ohmic resistance coming into play in series with the antenna and helping to create the illusion of a great antenna via your SWR meter.
Some will say that the width of the antenna comes into play, as in the antenna aperture. While it is true that the antenna aperture does affect the VSWR bandwidth, like in the case of a cage dipole, this isn’t true of the horse fence antenna.
Additionally, I wonder how the cloth material in the horse fence antenna changes its dielectric properties when it’s wet vs dry, and how that affects the VSWR of the antenna? Most fans of this type of antenna will tell you that it doesn’t. Of course, this is a low Q antenna. I would like to see one of these antennas created using copper conductors.
Use copper, or aluminium, and get it up in the air, and don’t rely on SWR solely being an indicator of a good, efficient antenna.