Insights from a Ham Radio Operator
Author: Marius Lubbe ZS1ML
Research – tech: Lee Bezuidenhout ZS1GWL
Introduction
As an experienced ham radio operator, I’ve often found that the effectiveness of our transmissions greatly depends on the quality and design of the antenna system. Among numerous designs, one that has repeatedly caught my attention is the MFJ Octopus antenna. This novel multi-band antenna system ingeniously employs a dipole configuration and offers a multitude of advantages over traditional antenna designs.
In this exploration, I will share insights into the design of the MFJ Octopus antenna, elucidate its benefits, and guide you through the setup and tuning process for optimal performance. We will also delve into the role of Hamsticks in this antenna system, as well as their manufacturing process. Moreover, we will examine the myriad use cases for this versatile antenna system.
The Design of MFJ Octopus Antenna
The MFJ Octopus Antenna, often referred to as the MFJ-2104, -2104M, or -2106, depending on the model, stands out due to its compact, portable, and multi-band design. It follows a hub-and-spoke configuration, where the hub serves as a central support for four pairs of Hamsticks—portable monopole antennas that resonate at specific HF bands. Each pair of Hamsticks forms a dipole, and together, these four dipoles form a comprehensive multi-band antenna system.
This structure of the Octopus antenna resembles a four-armed spider or an “octopus,” hence the name. This unique setup enables operation on multiple bands, without the need for an antenna tuner.
Understanding Hamsticks and Their Manufacturing Process
Hamsticks are compact, portable monopole antennas, preferred for their flexibility in amateur (ham) radio operations. They are particularly suitable for mobile and portable scenarios due to their structural design.
Each Hamstick consists of a fiberglass rod that functions as an insulating core, around which a wire is tightly wound to form the antenna element. At the top, there’s a tuneable stinger—a retractable metal rod—that can be adjusted to alter the resonant frequency of the antenna.
The manufacturing process for Hamsticks starts with creating the insulating fiberglass rod. Then, a copper wire is wound tightly around the rod. A tuneable stinger is added at the top end of the antenna, and a standard 3/8-inch x 24 threaded connector is installed at the bottom. The antenna is then shielded with a protective layer to guard the windings from environmental damage. After manufacturing, each Hamstick undergoes mechanical inspections and electrical tests to ensure it resonates at the specified frequency.
The Advantage of Dipole Execution
The use of dipole antennas in the Octopus design offers several significant benefits. Firstly, it allows for multi-band operation thanks to the multiple pairs of Hamsticks. Secondly, dipoles are known for their efficiency, particularly on their resonant frequency, which leads to enhanced transmission and reception of signals. The Octopus antenna is also portable and easy to install, adding to its appeal. Furthermore, dipoles are balanced antennas, which can help reduce noise and improve signal quality.
Setting up the MFJ Octopus Antenna
Setting up the MFJ Octopus antenna is a relatively straightforward task. You begin by mounting the Octopus hub on an appropriate mast and attaching the Hamstick dipoles to the hub. Once they are securely fastened, the coaxial feedline is connected to the SO-239 connector on the hub. Finally, the antenna is raised to its operating position, ensuring to avoid power lines or other potential hazards.
Tuning the MFJ Octopus Antenna
Tuning the Octopus antenna involves adjusting the length of each Hamstick to resonate at the desired frequency within the band. This is achieved by moving the stinger in or out. An SWR meter can be used to check the SWR on the desired frequency. The aim is to achieve the lowest possible SWR, ideally below 1.5:1. If the SWR is too high, the length of the Hamsticks can be adjusted again. This process is repeated for each pair of Hamsticks on the Octopus antenna.
This is best done while the sections of your mast are still collapsed as it is a bit of a back-and-forth process to get the SWR spot on. But once the stinger on each Hamstick is tuned you can tighten them and keep them stored like that, ready for next time.
Overall, this process including setup took me about 15mins and I was done.
Use Cases for the MFJ Octopus Antenna
The unique design and numerous advantages of the MFJ Octopus antenna allow for a wide range of use cases.
- Portable Operations: The lightweight and compact design of the Octopus antenna makes it an excellent choice for portable operations such as field days, camping trips, and other outdoor radio activities.
- Emergency Communications: In emergency and disaster relief scenarios, the Octopus antenna is invaluable due to its rapid deployability, multi-band capabilities, and robust performance.
- Home Stations with Limited Space: For ham radio operators with limited space, the Octopus antenna offers an excellent solution due to its compact design, which allows installation in smaller spaces while still providing efficient multi-band operation.
- Educational Demonstrations: Its unique design makes the Octopus antenna ideal for demonstrating antenna principles and dipole operation.
- DX-peditions: For lightweight, easy-to-set-up antenna that can handle a range of frequencies, the Octopus antenna is an excellent choice.
- Mobile Operations: While larger than typical mobile antennas, the Octopus antenna is a suitable option for mobile operations where multi-band operation is desired.
- Contesting: For ham radio contests, the Octopus antenna’s ability to switch bands quickly without the need for an antenna tuner can be a great advantage.
Conclusion
The MFJ Octopus antenna is an efficient and versatile multi-band antenna system that combines the benefits of dipole antennas with the portability and convenience of Hamsticks. Its uses span portable operation, emergency communication, home stations with limited space, and contesting. Understanding its design and benefits, along with mastering the proper setup and tuning methods, can maximize the efficiency and flexibility of the Octopus antenna in various operating scenarios.
