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We propose a novel design of internal impedance matching networks for axial-mode helical antennas. This network comprises a single wire attached to the helix. One of the main challenges when designing an internal matching network is its strong electromagnetic coupling with the antenna. The matching network must hence be designed in the presence of the antenna, which slows down the design process. To overcome this problem, we formulate an equivalent thin-wire model of the complete helix, including the matching wire (matching network) and the dielectric support. This computationally low-demanding model can be analyzed extremely rapidly, yielding accurate results, which are in excellent agreement with alternative numerical solutions and measurements.

Axial-mode helical antennas have been known for a long time [

The input impedance of axial-mode helical antennas is close to 150 Ω [

An alternative solution, an impedance transformer similar to a quarter-wave transformer, comprising the helix reflector and a metallic plate vertically attached to the beginning of the helical conductor, was presented in [

Following these ideas, originated from [

We consider a helical antenna made of a copper wire with a radius

(a) A helical antenna with a matching wire. (b) A sketch of the matching wire, whose geometry is governed by (

Next, we propose a matching network made of a single wire connected to the beginning of the helical conductor, as shown in Figure

The matching wire is made of the same copper wire as that in the helix (this will further simplify the formulation of the equivalent model). The geometry of the matching wire is parameterized by the following equations in Cartesian coordinates, as in [

The overall geometrical profile of the matching wire is determined by the parameter

The parametrization (

As it will be described in Section

When a helix is internally matched by a vertically profiled metallic plate [

The complete models of the matched helix are typically tailored for simulations to be carried out by the most commonly used rigorous full-wave numerical techniques, that is, the finite element method (FEM) or the method of moments (MoM). In comparison with these models, the equivalent wire model is much more computationally efficient, especially when wires are simulated by the MoM [

Consider a short wire segment wound around a PVC tube. Since the curvature of this structure is relatively small, the wire can be locally considered straight, laying down on a flat PVC substrate. We expect that neglecting the curvature of this structure will not significantly affect the validity of equivalence and that we can introduce uniform transmission lines necessary for the derivation of the equivalence [

Contemplating structures suitable for the equivalence, we consider two-wire transmission lines (a) with a PVC substrate and (b) without a PVC substrate, as described in [

Cross sections of (a) the original and (b) the equivalent two-wire transmission lines.

The cross section of the equivalent two-wire transmission line is shown in Figure

By employing the quasi-static analysis [

Adding the PUL inductance to a wire is a standard feature in modern full-wave EM simulators [

The equivalent (purely metallic, MoM) model of the helix with the matching wire is assembled in the software WIPL-D Pro [

The complete full-wave FEM model (with the explicit presence of the PVC tube) of the matched helix is assembled in the software ANSYS HFSS [

The reflection coefficient of the helical antenna with the matching wire: comparison of three simulation models and measurements.

Also presented in Figure

A photograph of the internally matched helix prototype. The stencil for the matching wire, preprinted on a piece of paper, is also shown.

Finally, we will examine the influence of the PVC tube on the helix reflection coefficient. By starting from the complete WIPL-D Pro model, we simply omit the PVC tube. Thus obtained “incomplete” wire results are also presented in Figure

We have proposed a simple and inexpensive wire-based internal matching network for axial-mode helical antennas. We have also formulated an equivalent thin-wire-based EM model of internally matched helical antennas. The proposed model yields results which are in an excellent agreement with the complete full-wave FEM simulations. The accuracy of the proposed model has also been validated by measurements on a fabricated antenna. In the presented example, the proposed model reduces the simulation run time by more than 100 times and 400 times, compared to FEM and MoM full-wave analysis of complete models, respectively, while maintaining excellent accuracy. We remark that when employing the equivalence from [

The authors declare that there is no conflict of interest regarding the publication of this article.

This work was supported in part by the Ministry of Education, Science and Technological Development of Serbia under Grant TR32005.