From: "K0FF" <K0FF@ARRL.NET>
To: "Amps" <amps@contesting.com>
Subject: [AMPS] Voltage at antenna end. de K0FF
Date sent: Mon, 11 Sep 2000 09:33:26 -0500
>
> W3BP was asking about the voltage at the end of an antenna. I've been
> thinking about that too, Looking at a center fed 1/2 wave dipole: At
> 1500Watts, the 50 Ohm feedpoint sees 273V at 5.5 Amps if my math is
> correct. The end of the antenna is a very high impedance, and I'm just
> guessing at 3300 Ohms. It could be more.
The end-impedance of a uniform diameter antenna can be
calculated by considering the antenna a single wire 1/4wl
transmission line.
Say you have a relatively thin antenna with a surge impedance of
600 ohms in the conductor. If the feedpoint is a balanced 50 ohm
feedpoint the standing wave ratio on the wire will be 600/25 = 24:1.
The end impedance will be 34,000 ohms.
This is modified by the fact you have "loading" on the open end
caused by a "loop" in the wire and the insulator, and by the fact the
wire radiates, but that would be the approximate impedance of a
single thin wire at the end in a dipole.
Making a loop of wire at the end of the antenna greatly reduces the
voltage for two reasons. It spreads the "voltage" out and reduces
the voltage gradient and it also loads the end of the antenna with a
"load" that reduces impedance.
Wire antennas at high power stations are often multiwire cages to
reduce surge impedance and lower voltage gradient.
On some of my wire antennas, the frayed wire ends have a "ball" of
molten metal on them after a short time of use. This is from corona.
The corona can melt or crack insulators, and melt support ropes.
73, Tom W8JI
w8ji@contesting.com
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