Feeders and Antennas

Revision part 2

Learning outcomes of this part

Know that the gain of an antenna is measured in dB, and understand how to calculate the e.r.p. for a known RF power and antenna gain (in multiples of 3 dB).

Know that a three-element yagi has a half-wave driven element, a reflector that is slightly longer than the driven element and a director that is slightly shorter than the driven element.

Know that yagi antennas may have more than one director.

Know that electromagnetic radiation comprises both an electrical field and a magnetic field.

Know that the two fields are at right-angles to each other and that the direction of propagation is at right-angles to both fields.

Know that it is the electrical field that defines polarisation of the wave.

Know that VHF and UHF signals will normally be received best when the transmitter and the receiver have the same antenna polarisation.

Know that polarisation is less important at HF due the effects of ionospheric reflection.

Know the concept of an antenna polar diagram.

Know how to recognise the directions of maximum and minimum radiation.

Know how to recognise polar diagrams for the half-wave dipole and yagi antennas.

Know the use of a dummy load and its construction.