How Do I Choose the Best HF Antenna?

A dielectric antenna is a round rod made of low-loss high-frequency dielectric material (generally polystyrene). One end of the antenna is fed by a coaxial cable or waveguide. The antenna shown in the overview diagram is a rod-shaped dielectric antenna fed with a coaxial line. In the overview, 1 is a dielectric rod; 2 is an extension of the inner conductor of the coaxial line to form a vibrator for exciting electromagnetic waves; 3 is a coaxial line; 4 is a metal sleeve. The role of the sleeve, in addition to clamping the dielectric rod, is more important to reflect electromagnetic waves, thereby ensuring that the electromagnetic waves are excited by the inner conductor of the coaxial line and propagated to the free end of the dielectric rod.

Dielectric antennas belong to the category of traveling wave antennas. The figure below shows one possible form of a dielectric antenna. It can be seen from the figure that the antenna is composed of a dielectric rod of a metal waveguide excited by an oscillator.

Because low-loss dielectric materials with specific dielectric constants can store electromagnetic wave energy, the stored energy is reflected back and forth in the medium to form electromagnetic oscillations, which can then be used to radiate and receive electromagnetic waves. Using this principle, a dielectric antenna can be designed. Dielectric antennas mainly include dielectric resonator antennas (DRA) and dielectric rod antennas. The dielectric antenna has a simple structure, low cost, light weight, and reliable performance, and is a new type of antenna.

The maximum radiation direction of a dielectric antenna, like all traveling wave antennas, is consistent with the antenna axis. If you want to make better use of the characteristics of the traveling wave antenna, it is best to concentrate the energy in the axial direction so that each unit of the dielectric rod also has the strongest radiation in this direction. If the equivalent current generated in the dielectric rod has a lateral component (ie, a component perpendicular to the Z axis), the above result may be obtained. In addition, this current component should have a certain main direction in the transverse plane. In order to satisfy these conditions, the wave excited in the dielectric rod must have a transverse electric field component, and this component should have a certain main direction. The most favorable wave pattern in this regard is

Its electromagnetic field distribution is shown in the figure below. It has a lateral component parallel to the X axis.

Assuming that the surface wave of the dielectric rod is a regular surface wave, the pattern of the dielectric antenna can be approximated by the following formula.

Here

Is the attenuation coefficient of the rod surface wave due to radiation and heat loss in the medium;

It is a factor that takes into account the unevenness of the field distribution in the cross section of the dielectric rod.

considering

Wave field distribution, factor

Can be approximated by the following function:

On the E-plane:

On the H-plane:

The angle is calculated from the axis of the dielectric rod.