I was chatting on the weekend with a new ham who was inspired by an article in Amateur Radio to build an OCF dipole using galvanised fence wire.
I have not seen the article, but he recalled that it recommended single core 1.25mm galvanised wire as quite suited to the task of an 80m OCF dipole. The wire consists of a mild steel core with a thin layer of zinc.
AC currents tend to flow in the surface of conductors rather than deep in the core, this is known as skin effect and a quantity known as the Skin Depth can be calculated. Skin Depth is the depth in a sufficiently large conductor at which the current density has fallen to 37% of that at the surface. Almost all of the current is contained in a layer of three Skin Depths in thickness, so if a conductor is at least three Skin Depths in outer radial thickness, it has RF resistance similar to that of a solid conductor of the same material.
AS/NZS 4680:199 requires that the zinc coating thickness of new 1.25mm wire is 35-45µm. A reasonable allowance for corrosion of the zinc is up to 6µm per year, so in as little as 5 years, the zinc could be of negligible thickness though it may last longer in some environments.
Above 90MHz, the zinc coating will be sufficient to deliver zinc like performance of the wire. At frequencies below that RF resistance will be higher, approaching that of the steel core as frequency is decreased.
80m OCF of 1.25mm galvanised wire
The Skin Depth of zinc at 3.5MHz is 65µm, three Skin Depths is 195µm and clearly the coating thickness of new wire at around 40µm doesn’t come close to three Skin Depths so we cannot expect zinc like performance.
Due to weathering effect, a conservative approach is to consider that the wire is no better than mild steel wire. Mild steel wire is a poor RF conductor because of its high resistivity and high permeability.
An NEC-4 model of an 40.5m long dipole at 10m above average ground fed 33% from one end and using mild steel wire converts 17% of the RF power delivered to the feed point into heat and 83% is radiated. When feed line losses are taken into account, less than 70% of the transmitter power is typically radiated.
Now copper is not lossless, the loss in a dipole using 2mm copper is about 3%… a whole lot better than towards 84%
- Galvanised steel wire may be quite lossy for wire antennas on HF.
- Readers ought critically assess the content of Ham Radio magazines and journals.
- Duffy, O. 2009. Loss in antenna conductor materials . VK1OD.net (offline).
- ———. 2009b. Skin depth calculator. VK1OD.net (offline).
- Robinson, J Ed. nd. INGAL specifier’s manual.
- Terman 1955. Electronic and Radio Engineering: McGraw-Hill New York.