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From: Alan G3NOQ
Date: 25 Nov 2002
Time: 03:40:37
Remote Name: 20.138.254.2
David - I agree with most of that but perhaps I could summarise some IEEE definitions connected with power flow, which cover all the points you raised abut losses in the feeder and ATU/antenna that are not commonly accounted as propagation losses, and if you could look them up some time your comments about them would gain even more authority :) :) . . . . .Directivity is "the ratio of the radiation intensity in a given direction to the radiation intensity averaged over all directions". Radiation efficiency is "the ratio of the total power radiated to the net power accepted by the antenna". Gain (strictly "absolute gain") is "the ratio of the radiation intensity in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically". A little thought shows that this is equal to the directivity reduced by the efficiency. Then there is "Realised gain" which is "the gain of an antenna reduced by the losses due to the mismatch of the antenna input impedance to a specified impedance". This can be shown to be the absolute gain minus the mismatch loss. Those definitions cover antenna losses pretty comprehensively and it is understood that the "antenna" includes any ATU that is required to match it to a standard system impedance like 50 or 75 ohms . . . . . I have no problem with thermal methods in general, they work well enough for measuring the efficiency of boilers, but my reservations are based on the difficulty of accurate measurements for HF antennas because of the small temperature rises and the inevitable heat-losses, and it is therefore easy to underestimate the rf losses seriously, which leads to grossly inflated results for efficiency and the temptation to reach conclusions about radiation resistance which have no basis in EM theory. . . . I have not answered all your points and this message is already long enough, but the guiding principle for antenna efficiency must be that it is a measure of the antenna's fitness for purpose, which is the ability to deliver a specified field strength at a specified distance, with a specified rf power input . . . . Best regards, Alan .