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From: David Jefferies
Date: 22 Nov 2002
Time: 13:25:12
Remote Name: 195.92.168.171
Marc, Alan, List, <><><> The question you are really asking is "where does the energy go?". Let's take it bit by bit. The transmitter generates energy and launches it as an outgoing wave on the feeder. The feeder is a bit lossy and can absorb some. The antenna may be mis-matched and will reflect some. This reflected energy may be reabsorbed in the transmitter internal impedance (resistance) or in the feed....multiple reflections may happen between transmitter and antenna terminals up and down the feed. What is certain is that not all the power launched by the transmitter is necessarily delivered to the antenna. The IEEE definitions and standards usually ignore this source of loss in defining "antenna efficiency". <><><> Of the power delivered to the antenna, some (as Marc H suggests) is dumped into Joule heating of the antenna wires or rods, causing the temperature to rise. But there are other sources of dissipation; the antenna induces currents in the ground or earth which heats the soil in the near field region. This heating is over and above the propagation loss of the ground, which is taken care of in standard "radiation field over earth" simulations. The antenna fields also induce currents in nearby structures; wires, guys, trees, walls, support structures, all of which may be within the radian sphere or near-near field region.<><><> Then we have energy which escapes from the radian sphere. Some of the transmitter energy actually makes it out there :-). But this energy is absorbed by the lossy ground in the far field region, by obstacles and trees and leaves and houses and rain and sea water etc etc so if as Alan suggests you then measure the field strength of what is left, you will get quite a different number from that suggested by Marc. Mike Underhill takes another approach, and tries to separate the various loss mechanisms by measuring their frequency dependences. This seems sensible and reasonable to me, but it can only be done with an antenna that is tunable over a wide range. We have opened a can of wriggling worms here; what is quite clear is that the standard broadcast engineering definition of "efficiency" is limited, though practically useful, if we want to examine where the power from the transmitter actually lands up.<><><> Like Alan, I hope this helps some more. <><> Regards, David.