# Aerial Feeders

A dipole in free space, fed at its centre by a balanced feeder, can radiate all the RF power fed to it with no radiation from the feeder. Such a system is illustrated in figure 1.

A dipole is a “resonant” aerial and its first “self resonance” is when its total length is approximately half a wavelength. The maximum current is then at its centre. The voltage between the two arms of the dipole at the centre feed point, divided by the current at this point defines the radiation resistance, which for this simple case is approximately 72 Ω resistive. If symmetry of the dipole relative to its surroundings and to earth is maintained, the dipole is said to be “balanced”. If a balanced feeder, (i.e. one having two closely spaces wires of equal diameter), is designed to have a “characteristic impedance” of this value, (72 Ω ) then the feeder is said to be matched to the dipole. In this case all the RF power supplied from the feeder is absorbed by the dipole and is radiated. I.e. no power is reflected back down the feeder. The voltages and currents in the two wires of the feeder will be equal in magnitude but in antiphase and the feeder is said to be in “differential mode”, i.e. there will be no “common mode” current, that is, no current flowing in phase down the two wires of the feeder.

A common method of feeding a dipole is with coaxial cable. This is very convenient because coax consists of a “live wire” in the centre and an earthed shield around it, so confining all the RF to the inside of the coax, at least until it reaches the aerial. The problem of feeding an aerial with coax is that coax really consists of three wires not two. Due to the “skin effect”, the inner and outer surfaces of the outer conductor, behave at RF as two separate surfaces which can support two separate currents. So what happens is that the RF currents sent up the coax on transmission consist of current on the outside of the inner conductor and an equal and opposite current on the inside of the outer conductor. On reaching the dipole all the current on the inner conductor is transferred to one limb of the dipole, but the current on the inside of the outer conductor splits between the other limb of the dipole and the outer surface of the coax. The currents are illustrated in figure 2, where the intended currents are shown as I1 and I2, but the unintended current on the outside of the coax is shown as I3.

``     John,   G0NVZ``