Choice of material for the antenna

What metal can be used to make an antenna, and what is undesirable? What criteria should be followed in the choice of material? These questions will surely arise before you if you seriously undertook to make a good antenna yourself. Let's try to find answers.

In search of an answer, the most important thing is to understand the fact that at high frequencies the current does not flow in the entire cross-section of the conductor, but only in a thin surface layer. This phenomenon is called the “skin effect”. The thickness of the skin layer is usually taken as the distance from the metal surface, at which the current value is 0.37 of the value on the surface. In this case, the resistance of the skin layer of such thickness at a given frequency is equal to its resistance at DC. The skin layer thickness decreases with increasing frequency. At the same time, already in the UHF range, it is several microns. This implies the conclusion that it is critical for us what the surface layer of the wire is made of, and the composition of its center is not important.

 We immediately exclude unsuitable metals for the antenna – these are ferromagnets - iron, nickel, and alloys based on them, for example, steel. However, bimetallic wires with a steel core and copper coating are quite suitable. It is due to the presence of the skin effect. So, all metals and alloys that can be used as the surface layer of antenna elements can be divided into two groups:

1. silver, copper, aluminum.
2. brass, zinc, duralumin.

Within the groups, metals differ little in terms of losses and are practically equivalent. Antennas made of metals and alloys of the 2nd group will be low in gain to antennas made of metals of the 1st group from 0.04 to 0.16 dB, which is almost not noticeable in practice. Obviously, not only copper-plated bimetal is suitable, but also solid copper wire or wire from any of the six metals listed above.
About all kinds of alloys should be said separately. Examples:

• The manganin alloy has 85% copper, and its resistance is 30 times higher than that of pure copper.
• nickel-chromium is an alloy of nickel 70% + chromium 15% + iron 15% has a resistance 10 times higher than that of any of its components.

Therefore, you should not at all expect that the presence of the word “copper” or “aluminum” in the name or in the composition of the alloy will provide it with low resistance.

 Due to the skin effect, the roughness of the wire surface increases its resistance, since the path length for the RF current increases. Under the causing of atmospheric influences, the wire usually oxidizes, its surface becomes rougher and the properties of the antenna degenerate. In this case, several cases can be separated:

• Copper-plated bimetallic wires typically oxidize heavily over time, acquiring a very poor surface, so they need periodic polishing. The surface can be sanded with fine sandpaper along the wire.
• Galvanized steel, on the contrary, improves its properties over time and is well suited for long-time structures.
• Aluminum and tubes from its alloys (duralumin) are the best for passive and active elements that do not require soldering. The surface oxide film is quite strong and reliably protects the metal from further oxidation. However, duralumin alloys need additional insulation from assertive atmosphere by varnishing.
• Brass, in terms of strength, surface condition and stability, soldering reliability, can be used for any antenna elements. The disadvantage is increased breakability at the frost.
• Often, copper or aluminum wires for electrical wiring are used as the material for the antenna making. This is quite acceptable, but it is preferable to choose a single-core wire. The electricians measure wires by cross-section, but we need to know the diameter of the wire in the calculations. Here is the matching table:

Although the paintwork is able to protect the wire from oxidation, it still breaks down over time and needs to be periodically updated. Of course! If you care about the condition of your car, then the antenna also needs care.

In terms of mechanical properties, the ability to withstand loads, for example, the weight of birds, in order of decreasing strength, metals are arranged in the following sequence: steel bimetallic wire, hard brass, duralumin, copper, aluminum. Separately, it is worth mentioning that replacing solid metal with PCB at high frequencies without recalculating the antenna is unacceptable.

When connecting the feeder to the antenna, electrochemical corrosion is possible at the contact point of different metals. The following table shows the compatibility of different metals in a contact joint outdoors:

“+” — compatible, “-” — not compatible, “0” — neutral.

When choosing a place to install the antenna on the roof, it is necessary to avoid the proximity of chimneys and ventilation pipes, since the outgoing gases have a destructive effect on the metal of the antenna. Over time, under the influence of an aggressive environment, the antenna is gradually destroyed, but the good news is that IT technologies are spreading and developing at a much faster pace.