In amateur radio satellite communication, the use of directional antennas is a fundamental necessity. Operating on both the VHF and UHF bands with a single antenna can present a considerable challenge. In this article, we introduce a dual-band antenna design that covers both VHF and UHF bands, accomplished by combining two distinct antenna types: the Moxon antenna for VHF frequencies and the Yagi antenna for UHF frequencies. One of the notable advantages of this design is that it utilizes a single feeding point.
The Moxon antenna, also known as the Moxon rectangle, stands as a simple yet mechanically robust antenna configuration composed of two elements: a driven element and a parasitic element. It derives its name from the renowned radio amateur, Les Moxon, with the call sign G6XN. In essence, the Moxon antenna resembles a Yagi-Uda antenna with two folded dipole elements, one serving as the driven element and the other as the reflector element. A tunable gap exists between these two folded dipoles, allowing for adjustments to minimize the VSWR (Voltage Standing Wave Ratio) at VHF frequencies. Consequently, it is a mechanically tunable antenna that eliminates the need for an impedance matching network. In our dual-band design, as illustrated below, the Moxon segment of the antenna serves as the excitation point.
The accompanying figure also depicts the Yagi-Uda portion of the dual antenna. This section adheres to the conventional Yagi array configuration, comprising a reflector element, a driven element, and three directors. However, the driven element in this context does not receive direct excitation; instead, it is powered indirectly through electromagnetic induction from the driven element of the Moxon array. The gap separating the Moxon and Yagi sections of the combined antenna can be mechanically adjusted to minimize the VSWR at UHF frequencies.
In the analyzed frequency ranges, this VHF/UHF dual-band antenna exhibits self-resonance with an input impedance close to 50 Ohms, obviating the requirement for a matching network. To optimize performance in each band, fine-tune the gaps as indicated in the figure below.
Additionally, the figure provides VSWR curves as a function of frequency. In the upper section, it is evident that the antenna resonates at 147 MHz, while in the lower section, it resonates at 442 MHz. The figure also presents radiation patterns for both frequency bands, with gains of 6.3 dBi at VHF and 12 dBi at UHF.
Detailed AN-SOF models, along with antenna dimensions and calculations, are available for download through the buttons located below the figure.