HALF-DIPOLE ANTENNAS BASIC INFORMATION ELECTRONICS TUTORIALS

WHAT ARE HALF-DIPOLE ANTENNAS?

Developing a theory of antenna radiation requires many simplifications. Designing radiating antennas is not the aim of this book. Our interests relate to the field strengths of existing transmitters (interference), the fields that are inadvertently radiated by circuits and the impact of radiation that enters a circuit.


The analysis of a dipole antenna starts out by dividing the antenna into individual current elements. These elements of current cannot exist any more than the element of current that was used to explain magnetics.

This dipole element has zero resistance and zero diameter, a very neat trick indeed. To simplify the problem, the current in the dipole element is assumed to be sinusoidal at the frequency of interest. transmission line connected to the root or grounded end of the antenna.

Each element of antenna current contributes to the radiation of the total antenna. The field around the antenna is current flowing in the capacitances of space. This goes back to the statement that a changing E field is a current in space.

This changing current further implies an H field. Note that the field energy flowing out of the transmission line flows onto the antenna. In the space around the antenna, both the E and H fields intensity are changing as a function of height and radial distance.

At the tip of the antenna, the current flow is zero. In the analysis the current pattern follows a half sine wave from the root of the antenna to the tip.

The energy in this antenna field is leaving and returning to the antenna at the same time. There is a net transfer of energy into space that does not return. Theoretically, Poynting’s vector can be used to determine the net energy that flows past any surface area that surrounds the antenna.

If there is a good impedance match at the end of the transmission line, no energy will reflect back on the transmission line. Matching the transmission line to the antenna is a major consideration in antenna design.

The current flow in the antenna causes an E and H field that diminishes in intensity with distance from the antenna. The ratio of the E-field intensity to the H-field intensity far from the antenna is a constant. This constant is known as the impedance of free space. The ratio of E to H is 377 OHMS when the waves have little curvature.

Near the antenna, the ratio of E to H is dominated by the E field. This type of field is said to be a high impedance electric field. A distance of ¸ALPHA/ 2PI  from the antenna is called the near-field/far-field interface.

At 1 MHz the wavelength ¸ is 300 m. This is the distance a wave travels in free space at the speed of light for 1 MICROSECONDS (the speed of light is approximately 300,000,000 m/s). Beyond 50 m the ratio of E to H is 377 OHMS .

Beyond this distance both fields fall off linearly with distance and the radiation takes on a character known as a plane wave even though there is always some curvature to the wave.

Assume that the interface distance is 100 m and that the E field at this point is 10 V=m; then the field strength is 5 V=m at 200 m. At this distance the H field is 5=377 A=m. Beyond the interface distance the wave is always considered a plane wave. Inside the interface distance the ratio of E field to H field increases linearly.

Near the dipole the field is said to be a high impedance E field. Very close to the antenna the E and H field intensities are complex in nature and the ratio of E to H does not follow a simple set of rules.

The E field inside the near-field/far-field interface can be calculated as follows. Assume that the interface distance is 100 m and the E field is 10 V=m at this distance. At 50 m the wave impedance is doubled. The power crossing the two spherical surfaces must be equal.

This requires the E field must increase by a factor of $2 and the H field must decrease by a factor SQRT OF 2. At this distance the ratio of E to H is 377 X 2 OHMS.