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Introduction
In the world of amateur radio, the quest for efficient and compact antenna solutions is an ongoing pursuit. One such design that has captured the attention of enthusiasts for decades is the G4ZDU Mini Beam antenna. Conceived in the mid-1950s, this versatile antenna has become a staple in the toolbox of many radio operators, offering a compact and high-performing solution for the popular HF bands.
Overview
The G4ZDU Mini Beam is a tri-band antenna that covers the 20-meter, 15-meter, and 10-meter bands. While it is not a full-size antenna, it manages to deliver impressive performance, with a forward gain of around 3.5 dB on 20 meters and 5 dB on 15 and 10 meters. The front-to-back ratio is typically around 15 dB, making it a reliable performer in various operating conditions.
Design
The key to the G4ZDU Mini Beam's versatility lies in its innovative design. At the heart of the antenna is a driven element, which is effectively a dipole. This driven element is capable of accepting power over a wide frequency range, from 14 MHz to 30 MHz, covering not only the primary bands but also the 17-meter and 12-meter WARC bands.
To achieve this broad frequency coverage, the driven element is fed with 450-ohm ladder line, which provides a balanced feed and allows for greater flexibility in matching the antenna to the transceiver. The use of ladder line also means that the antenna can be rotated without significant issues related to nearby metal structures, such as the rotator.
Connection
Connecting the 450-ohm ladder line to the transceiver can be done in one of two ways. If the internal antenna tuning unit (ATU) of the transceiver has a wide enough range, it can be used to directly connect the ladder line. Alternatively, an external ATU with a balanced line connection can be employed, providing a convenient way to match the antenna to the transmitter.
Director
To add directivity and gain to the antenna, a director element is incorporated. This director is designed to be resonant just above the 30 MHz band, providing a boost in performance on the 10-meter band. However, the addition of a coil in the center of the director element introduces a new challenge – the potential loss of the 10-meter performance.
To overcome this, G4ZDU ingeniously added a quarter-wave stub across the coil, effectively shorting it out when the antenna is operating on the 10-meter band. This allows the director to maintain its resonance and contribute to the overall gain of the antenna on that band.
Reflector
The final element of the G4ZDU Mini Beam is the reflector. Similar to the director, the reflector is also loaded with a coil to bring it into resonance on the 20-meter band. Once again, a quarter-wave stub is used to maintain the reflector's effectiveness on the 10-meter band.
The combination of the driven element, director, and reflector results in a compact antenna that offers a multi-element configuration on the 10-meter and 15-meter bands, while still providing a two-element setup on the 20-meter band.
Conclusion
The G4ZDU Mini Beam is a testament to the ingenuity of amateur radio antenna design. By leveraging a combination of balanced feed, coils, and quarter-wave stubs, this compact antenna manages to deliver impressive performance across multiple HF bands, making it a popular choice among radio enthusiasts with limited space. Whether you're looking to build your own Mini Beam or simply appreciate the engineering behind it, this antenna offers a fascinating glimpse into the world of compact and versatile antenna solutions.
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