At 12:38 AM +0000 8/8/03, Rob Atkinson, K5UJ wrote:
>One of my antennas is a dipole ~30 feet up that's ...88' long. I
>transmitted [in] the 80 cw band and walked around under the dipole
>with a cheap field strength meter..... [T]he field was strongest at
>the ends of the dipole and weakest near the feed point. I was not
>expecting this, mainly because with a 1/2 wave dipole I understand
>the current is max at the feedpoint....
You were within the "near field" region, where the antenna's
radiation field is dominated by its quasi-static field. Here, the
amplitude of the (oscillating) electric field is approximately that
of a static charge distribution equal to that on the antenna at the
moment of maximum charge (or voltage) and zero current; and the
amplitude of the (oscillating) magnetic field is approximately that
of the instantaneous current distribution in the antenna at the
moment of maximum current and zero charge (or voltage).
The max.-charge distribution is a sinusoidal function of distance
along the antenna, with zero at the center, whether the antenna is a
half-wavelength long, or shorter. The max.-current distribution is a
cosinusoidal function of distance along the antenna, with a peak at
the center, whether the antenna is a half-wavelength long, or shorter.
Your field-strength meter probably sensed the vertical component of
the electric field, by means of a short vertical whip. This
component is strongest under the ends of the dipole, because the
charge density on the wire is greatest there.
If you had been sensing magnetic field, e.g., with a small loop, you
would have found the strongest magnetic field under the center of the
dipole, directed horizontally, perpendicular to the wire.
In the near-field region, the electric field and the magnetic field
are not related as they are in the "far," or radiation-field. That
is, they are not necessarily perpendicular with the ratio |E|/|H|
equal to 377 ohms, and their cross-product is not in the direction of
radiation.
-Chuck, W1HIS
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