X603 Antennas - Quarter Wave Flexi Strand Radio Modem. 0 dB Gain.0 pages

THE INNER WORKINGS OF ANTENNA AP122 When considering antennas, the theoretical yardstick is the ideal isotropic antenna. If fedfrom a transmitter, this antenna radiates power with equal intensity in all directions.As with any antenna, there will be associated electric and magnetic field lines in space,due to the voltage applied to the antenna elements, and the current flowing in them. In the
immediate vicinity of the antenna, these fields are out of phase, and thus represent stored
circulating energy: this is called the near fieldђ region. This field drops of rapidly withdistance, it has almost disappeared at a distance of λ metres, i.e. one wavelength, from theantenna.But the antenna also generates electric and magnetic fields which are in phase, and theserepresent a flow of power away from the antenna - the far fieldђ. The magnetic field
strength corresponds to a magnetising field, measured in amps per metre. This, together
with the electric field strength in volts per metre defines the characteristic impedance offree space, which is 120 π or 377 Ω.Τ he ideal isotropic antenna looks purely resistive, and this resistance has two components.Resistance R >
r is the radiation resistanceђ - a notional non-dissipating resistancerepresenting the portђ via which power is radiated from the antenna. Loss resistance R >
1 istheohmic component of the antennas total resistance. Clearly the radiation efficiency Ҏ >
r is given by:- ׎ >
r = R >
r (1) R >
r + R >
1 In some cases, it is possible to make R >
1 negligible, but in practice an efficiency well shortof 100% must sometimes be tolerated.An ideal isotropic antenna located in free space, radiating a power P >
t will. result in apower density D , in watts per square metre, at a range d, in metres, is given by, P >
t (2) D = 4 ׏ d >
2 This assumes that d is much larger than the wavelength concerned, and is of courseindependent of the frequency. The term 4 d >
2 is the surface area of a sphere of radius d, centred on the antenna. The strength of the electric field , in volts per metre, in space atany point is given by, Վ = բȚ 377 D (3) Warwick Industrial Electronics Ltd Tel: +44 (0) 1455 233616 Fax: +44 (0) 1455 233179E-mail: sales@radiotelemetry.co.uk >