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Re: LF: Measurement of antenna current

To: "rsgb_lf_group" <[email protected]>
Subject: Re: LF: Measurement of antenna current
From: "Dave Sergeant" <[email protected]>
Date: Tue, 8 May 2001 08:27:15 +0100
References: <[email protected]> <[email protected]>
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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
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