Antenna Fundamentals

21 07 2016

While there are an enormous variety of antennas, they
share basic characteristics and all are designed to radiate and
receive electromagnetic waves. In this chapter, we begin by
defining what an electromagnetic wave is and how it is described.
We then define the most important characteristics of
Antenna Fundamentals
Chapter 1
Figure 1.1 — Visualization of a magnetic field. The magnetic
lines of force that surround a conductor with an electric current
flowing in it are shown by iron filings and small compass
needles. The needles point in the direction of the magnetic
or H-field. The filings give a general view of the field
distribution in the plane perpendicular to the conductor.
Figure 1.2 — Visualization of an electric field, E=Vdc/d.
When the dc source is replaced with an ac source there
will be a displacement current (Id) flowing between the capacitor
plates.
an antenna — impedance, directivity and polarization — and
show how those characteristics are measured and displayed.
Finally, a section reviews how exposure to those waves affects
the human body and the measures necessary for all
amateurs to use antennas and electromagnetic waves safely.

1.1 Introduction to Electromagnetic Fields and Waves
1.1.1 E AND H FIELDS
In 1820 Hans Oerstad discovered that a current flowing
in a wire would deflect the needle of a nearby compass. We
attribute this effect to a magnetic or H-field, which at any
given location is denoted by the letter H. The magnetic field’s
amplitude is expressed in A/m (Amperes/meter) along with
a direction. (Direction can also be expressed as some value
of phase with respect to a reference.) Because a magnetic
field has both amplitude and direction, it is a vector. Symbols
representing a vector are printed in bold-face.
Figure 1.1 shows a typical experimental arrangement
that demonstrates the presence of a magnetic field. The shape
of the magnetic field is roughly shown by the distribution of
the iron filings. This field distribution is very similar to that
around a vertical antenna.
A compass needle (a small magnet itself) will try to
align itself parallel to H. As the compass is moved around
the conductor, the orientation of the needle changes accordingly.
The orientation of the needle gives the direction of H. If
you attempt to turn the needle away from alignment you will
discover a torque trying to restore the needle to its original position. The torque is proportional to the strength of the
magnetic field at that point. This strength is called the field
intensity or amplitude of H at that point. If a larger current
flows in the conductor the amplitude of H will increase in
proportion. Currents flowing in an antenna also generate an
H-field.
An antenna will also have an electric or E-field, which
can be visualized using a parallel-plate capacitor, as shown
in Figure 1.2. If we connect a battery with a dc potential
across the capacitor plates there will be an electric field E
established between the plates, as indicated by the lines and
directional arrows between the plates. (Like H, the electric
field also has an amplitude and direction and so is a vector as
well.) The magnitude of vector E is expressed in V/m (volts
per meter), so for a potential of V volts and a spacing of d
meters, E = V/d V/m. The amplitude of E will increase with
voltage and/or a smaller separation distance (d). In an antenna,
there will be ac potential differences between different
parts of the antenna and from the antenna to ground. These
ac potential differences establish the electric field associated
with the antenna.

 

Iklan




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