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LF: Antenna Current

To: "RSGB LF Group" <[email protected]>
Subject: LF: Antenna Current
From: "Tom Boucher" <[email protected]>
Date: Tue, 8 May 2001 16:35:34 +0100
Reply-to: [email protected]
Sender: <[email protected]>
Hello all.

Dave G3YMC has reiterated one of the common misconceptions about loaded
antennas. There is no reason at all why the current out of the 'top' of
the loading coil should not be exactly the same as that into the
'bottom', providing it is a lumped inductance with no distributed
capacitance. It is a simple series resonant circuit and the same current
flows around the circuit.

Dave states that even if it were a lumped inductance with zero length,
the currents would be different. How can that possibly be so? Where does
the current go?

The tapering current shown in handbooks across the 160 metre whip coils
is due to their distributed capacitance and current flowing out of the
coil along its length instead of into the top section. Steve and Dick's
difference in current must be due to some distributed capacitance across
the coil.
I will take the same measurements when I am at home again next week end.
Wonder if I will get the same results? Probably not because my long 335
metre wire only needs about 250 microhenrys to resonate it on 136 KHz,
so the voltage gradient across the coil is a lot less than for a short

73, Tom G3OLB

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