Wire dipole on 160m

Discussion with a correspondent recently ranged onto the effect of some conductor materials in antennas. This posting reports the feed point impedance and internal efficiency (meaning power not turned into heat in the structure divided by power input at the feed point) of a 160m half wave dipole (80m long, centre fed, 20m above average ground and using 2.6mm diameter single core wire).

The table values are calculated using NEC-4 and apply only to the scenario described. (NEC-2 does not model skin effect properly for ferro-magnetic conductors.)

Wire typeFeed point impedance (Ω)Internal efficiency (%)Loss (dB)
316 stainless steel59.1+j7.8878.171.07

Note the wide range in internal efficiencies.

Importantly, the 316 stainless entry applies strictly to 316 stainless steel alloy which is non-magnetic, and would not apply to any stainless alloys that are ferro magnetic, even weakly so.

Stranded wire constructions will perform worse than a circular conductor.

For clad wires with one type of core (often steel) and a higher conductivity coating (eg copper, aluminium, silver), the RF performance depends greatly on the thickness of the coating. If the coating is more than three skin depths in thickness, performance will be almost as good as a homogenous conductor of the same material whereas if the coating is a small fraction of a skin depth, performance will be close to that of the base material.

For example, 2.7mm Galagher XL fence wire has an aluminium cladding of 223µm which is more than three times the skin depth in Aluminium at 1.8MHz (63µm) so performance will be close to Aluminium in the table (perhaps a little better due to its slightly larger diameter).

On the other hand, blue fence wire (polyurethane coated, Zal plated HT steel wire) has so little Zal coating that it might as well be steel and will have performance close to steel in the table.