Antenna Materials and Resistivity
Sometime back , I mentioned about talking to a mobile on 40 meters and that he had mentioned the base section of his antenna would get hot when running an amplifier. The reason for this problem is because the chromium plating, being a mixture of several different metals has a lot of resistance compared to copper. This increase in resistance caused the large amount of current flowing in the antenna at 600+ watts to heat up the mast section. This is due to the skin effect where the RF current flows on the surface of a conductor. This has been covered in some depth in a past article in antenneX.
For those who don’t remember, but would like a refresher, the RF current does not flow through the entire conductor like DC or low frequency AC does. Instead, RF only flows on the surface of a conductor. If the surface of a conductor has a corroded surface, the resistance will increase. This, in turn, will cause losses due to the famous I^2*R formula which states that the Current Squared times the Resistance will equal the power in watts being used. So, if the surface resistance of a conductor is large and the current is also high, then the power lost in the surface will be large and detectable as heat.
This is what is happening with the base section of the above antenna. The chrome plating was literally burning up the RF flowing in the antenna base section. So why is the base section chrome plated? It looks good, that is why—and many hams aren’t aware the chrome is wasting some of their power. You are probably thinking it can’t be that much power. But if you think about it, getting a ½ inch (1.27 cm) diameter, 3 ft. (.912 m.) long tubing section too hot to touch, takes a lot of power—power that could be more effectively radiated into the air.
Other base sections I have owned and used were the ones that came with the Hustler and Master Mobile antennas. These were made of one of the aluminum derivatives considerably more stronger than pure aluminum. I also have built mobile mast sections of copper tubing that work well as a mobile mast section. But, the tubing had to be polished to a brilliant shine and painted. Copper oxide has a very high resistance compared to clean copper and so there will be a loss in the copper as oxides form on the surface when exposed to the elements. Therefore it needs to be protected from the elements using paint.
So why are mobile masts made of aluminum instead of copper? Cost mainly, and aluminum is easier to work with in this particular application. In a past article in antenneX, construction of a high-powered mobile antenna was described and this is a good way to minimize mobile losses on the lower frequencies. Building your own unit is not hard, it just takes a little time and you will get a much better antenna than you can buy. As far as other materials, copper is the material of choice. Aluminum is too hard for most of us to work with as it cannot be soldered or welded with methods available to the majority of us. Silver and gold are rather expensive so we are left with copper as the preferred choice.
That takes care of mobile mast sections, but how about the rest of the antennas we use? For the dipoles, it is not hard to get the copper multi-strand wire needed. Aluminum tubing can be found strong enough for beams and booms for those beams. Verticals have been put up using TV mast sections as well as aluminum irrigation pipe. One time I used galvanized downspout tubing and it worked well. But the best choice of materials available should be limited to selections that make good conductors.
There three primary materials we should use and they are annealed copper, hard-drawn copper and aluminum. Copper-clad wire, which is copper-plated steel wire, was used frequently for antennas in times past and I understand that it is now back on the market. Phosphor wire is also good antenna wire since it is used mostly in high impedance antennas. Where the resistivity of the material used for your antenna becomes really important is in low band mobile whips and the small transmitting loops. In these type of antennas, impedance is so low that any excess resistivity will increase losses to such a point where the antenna’s efficiency is severely impaired.
Unfortunately, many may not be able to get enough copper wire to make an antenna and have to make do with other conductors. I have in the past used barbed wire, electric fence wire, and some scrap wire that had to be soldered together to get enough length to make the particular antenna I needed.
Below is a short chart that will help you figure out what materials best used for an antenna. It is also color-coded to give some easy indication as to resistivity of each metal listed. This list is also found in the ARRL Handbook, along with an explanation of resistance and how it is determined. You will note, all materials resistivity is compared to copper (annealed) as the standard for measurement index at 1.00. The green color indicates an excellent conductor for antenna usage. Yellow is the next best and the red color is to be avoided as the figures show. Thus, at .94, silver is the lowest and lead is the highest at 12.80:
Chart of Resistivity
Type
Copper (Annealed)
Silver
Copper (Hard Drawn)
Gold
Aluminum (Pure)
Chromium
Phosphor Bronze
Zinc
Brass
Cadmium
Nickel
Iron (Pure)
Tin
Steel
Lead
Index
1.00
0.94
1.03
1.40
1.60
1.80
2.80 – 5.40
3.40
3.70 – 4.90
4.40
5.10
5.68
6.70
7.60 – 12.70
12.80
Unfortunately, many may not be able to get enough copper wire to make an antenna and have to make do with other conductors. I have in the past used barbed wire, electric fence wire, and some scrap wire that had to be soldered together to get enough length to make the particular antenna I needed.
When using aluminum tubing in sections, be sure to apply some sort of protective coating on the joints. There are many coatings specifically useful for aluminum connections of the types used in the electric industry for aluminum wire. These coatings will do well and you should further wrap the joints with some sort of weatherproof tape to keep the coating from washing away in the rain. By doing this, the connections will not corrode and cause your signal to gradually deteriorate. Additionally, I solder all my connections to my antennas that I can—not twisted and wrapped in tape, but soldered and covered in silastic or some other weather proofing material.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 5th March 2024