Dual-band blade antenna

Introduction

Monopole Antenna

A typical monopole antenna, pictured to the left, has an omni-directional pattern and is limited in its frequency range. Please keep in mind that an omni-directional (see omnidirectional antenna) radiation pattern applies to the azimuth patterns and does exhibit a null at zenith.

A monopole antenna can be thought of a dipole antenna where one end of the dipole antenna now becomes the ground plane for said monopole antenna. By this line of conceptual thinking, one can easily reach the conclusion that the radiation emanating from a monopole antenna exists in the half the space of similar dipole antenna. Therefore, the maximum gain is twice that or an addition 3dB of the maximum gain of typical dipole. Hence the nominal value of maximum for a monopole antenna is about 5.15dBi.

Stutzman puts is succinctly as follows:

... A monopole is a dipole that has been divided in half at its center feed point and fed against a ground plane...

This article cover one type of dual band blade monopoles. This is the slot inside a monopole. Computational Electromagnetic Modeling (CEM) will be used to give some graphics of operations for a more conceptual understanding.

Theory

Monopoles

Monopole equations can be arrived by inspection of dipole antenna derivations with the knowledge that all radiation occurs in half the volume when compared to said dipole antenna. This leads to the following equations:

Directivity

$D_{mono}={\frac {4\pi }{\Omega _{A,mono}}}=2D_{dipole}$

The leads directly to the ealier stated maximum gain relation to a dipole buy the definition of gain a $G=\epsilon D$ where $\epsilon$ is the antenna radiation efficiency.

Impedance

$Z_{A,mono}={\frac {1}{2}}Z_{A,Dipole}$

Radiation Resistance

$R_{r,mono}={\frac {1}{2}}R_{r,mono}$

As can be seen in section 1 of the linked radiation resistance article.

Blade Antennas

A bland antenna is an attempt to create a broader band monopole (when compared to a thin wire monopole). Most blade antennas are trapezoidal in shape. Variations have been on this shape for aerodynamic purposes and notches have been introduced in order to achieve a better broad band performance. These type of monopole antennas are generally used in aviation for VHF and UHF frequency ranges.

For more information see the Antenna Engineering Handbook.

Slot Antennas

A slot antenna can be viewed as a dipole with opposite polarization. This is due to the typical feed which sets the orientation of the E-field across the smallest linear dimension of the slot. The following equations can be used to 'translate' a vertical or horizontal slot antenna into its complement (dipole):

$E_{\theta s}=H_{\theta c}\qquad E_{\phi s}=H_{\phi c}\qquad H_{\theta s}=\,-{\frac {E_{\theta c}}{\eta _{o}^{2}}}\qquad H_{\phi s}=\,{\frac {E_{\phi }c}{\eta _{o}^{2}}}$

Where the subscript S denotes the opening on the screen and the subscript C denotes its compliment (a dipole). In addition, $\eta ={\sqrt {\tfrac {\mu }{\epsilon }}}$ where $\mu$ is the complex permeability and $\epsilon$ is the complex permittivity of the medium into which one is radiating. This assumes an unbounded medium. In addition, all of the slot equations assume a screen much less than a wavelength ($\ll {\tfrac {\lambda }{10}}$). If these were not held to be true, fringing and the existence of modes could not be ignored.

This is defined by Babinet's principle and Booker's Extension further expands this principle to include polarization. The simple equations from Babinet's principle are stated on the linked page for which the author has had input.

Dual Band Antennas

Dual band antennas are not a new idea. For years many manufactures have combined multiple elements to create antennas that operate in two separate bands (do not get this confused with so-called frequency independent antennas such as a log-periodic antenna).

One way to create a dual band blade antenna is to create a slot in a blade antenna that is less than or on the order of ${\tfrac {\lambda }{10}}$ so that the lower frequency does not 'see' the slot (it is a general rule of thumb that the perturbation created by a discontinuity less than ${\tfrac {\lambda }{10}}$ on a structure is negligible).

Computational Electromagnetic Modeling (CEM)

Computational Electromagnetic Modeling (CEM)uses various methods to numerically calculate an antenna pattern.

To the untrained eye, this may seem a trivial process. Although, with some research and thought, one will realize that all local structures affect the radiation pattern either by reflection, absorption, refraction, fringing, or being a part of the radiating structure. Some structure which are not local will also cause these items and more including blockage and 're-radiation'. With this in mind, the calculation become cumbersome.

Multiple algorithms exist in CEM. These include but are not limited to Method of Moments (MoM), Finite Element Method (FEM),

Two examples of software packages that use these methods in free-space are FEKO and WIPL-D.

References

--Wallace (talk) 22:13, 16 March 2008 (UTC)