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## Re: LF: Measurement of antenna current

 To: "rsgb_lf_group" <[email protected]> Re: LF: Measurement of antenna current "Dave Sergeant" <[email protected]> Tue, 8 May 2001 08:27:15 +0100 <[email protected]> <[email protected]> [email protected] <[email protected]>
 ```From Dave G3YMC ``````There has been discussion on the difference in current below and above a vertical antenna's loading coil. I was not going to get involved in this discussion, but when I see some of the arguments put forward to justify this as being the same I had to. My response to Steve's results are that I would have expected an even greater difference. It is a fundamental property of any loaded antenna that the current distribution of such an antenna will reduce from a maximum at its base to zero at the top (or physical end of wire if there is a capacitive top). The classic diagrams of this distribution seem not to appear in the text books today, possibly because of less use of Top Band mobile. However it does appear in the RSGB Radio Communication Handbook 1968 edition p. 16.33 and the ARRL Antenna Book 1988 edition p. 16.4. Both these show maximum current at the base, a tapering current across the loading coil, and a rather smaller current above it tapering to zero at the top. There is no reason to believe the current at the two ends of the loading coil will be the same, and the fact that all of you will be aware you get a much higher voltage at the top than at the bottom would in anycase indicate otherwise. By the laws of nature the power at these two points will be the same less any difference due to radiation from it and electrical loss, so if the voltage is higher the current by definition must be less. There will also be a phase shift (90 degrees in the ideal case) between the voltages and currents. The ARRL Antenna Book perhaps gives the clearest explanation, where it likens the antenna to a transmission line with distributed inductance and capacitance (to ground). The loading coil represents an lumped inductance and a corresponding step change in current. Even if the inductance were of zero length the current would still be different. This is of course the reason why centre or top loading is better than base loading. The optimum radiation comes from those sections carrying the highest current, ie below the loading coil. Increase the length below the coil and you will improve the radiation. Thankyou Steve for confirming the operation of the classical base loaded vertical antenna! 73s Dave G3YMC [email protected] [email protected] http://www.dsergeant.btinternet.co.uk ```
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