VIDEO
In the world of amateur radio, some antenna designs stand the test of time due to their ingenuity and practicality. One such design is the ZL Special, a compact two-element beam antenna that dates back to the late 1940s and mid-1950s. This article explores the history, design, and performance of the ZL Special, drawing on personal experience and technical insights to determine if this classic HF mini beam is still a viable option for today’s ham radio enthusiasts.
A Trip Down Memory Lane: Discovering the ZL Special
My journey with the ZL Special began when I stumbled across an old copy of Shortwave Magazine from March 1959. The antenna design caught my eye immediately, and I vividly recall building my own version around 1960, shortly after I was licensed. The timing was perfect — the 10-metre band was wide open due to a Sunspot Maximum, providing excellent conditions for low-power DX contacts, especially with stations in the United States.
The ZL Special promised a lot: a two-element beam that was smaller than a full-sized Yagi, easy to build with simple materials like aluminum, wire, and wood, and capable of delivering impressive forward gain. I didn’t have the luxury of an antenna rotator back then, so I resorted to somewhat comical but effective means — jumping out of my bedroom window to rotate the antenna manually in the garden, turning up the receiver volume to listen for signals as I worked the beam into position.
The Origins and Development of the ZL Special
The concept of the ZL Special antenna was first conceived by ZL3MH (later ZL2Q) around 1949. The design involved two elements driven with one element out of phase with the other to achieve forward gain. The name “ZL Special” itself was coined by Fred Jud (G2BCX), a well-known and creative contributor to Shortwave Magazine and Practical Wireless during the 1950s. Fred was a notable figure in the amateur radio community, and I remember hearing him on the 160-metre band back in the day.
Two versions of the ZL Special emerged: one with elements of identical length and another where the reflector was slightly longer, as is typical in Yagi designs. Interestingly, the antenna’s directivity and front-to-back ratio were achieved by feeding the reflector out of phase at a specific angle, allowing the use of identical element lengths in some cases.
- Claims vs. Reality: Performance of the ZL Special
The ZL Special generated significant interest due to its claimed performance. It was said to offer forward gain comparable to a three-element Yagi—around 7 to 8 dB—which was exceptional for a two-element design. Additionally, it boasted a good front-to-back ratio and a compact footprint, making it ideal for limited space setups.
However, more recent tests have shown these claims to be somewhat optimistic. Real-world measurements indicate a forward gain closer to 4 to 4.5 dB, roughly equivalent to a standard two-element Yagi, and a front-to-back ratio around 15 dB, again typical for two-element beams. Despite this, the ZL Special’s compact size and ease of construction remain attractive features.
Understanding the ZL Special Design
The fundamental design of the ZL Special was documented by W4RNL and involves two lengths of 300-ohm ribbon cable acting as the antenna elements. The feed system is open circuit at both the feed and phasing points, with the phasing line length carefully calculated to be an eighth of a wavelength (approximately 4 feet 27 inches). This length creates a 135-degree phase shift, essential for achieving the antenna’s directional properties.
Feeding the antenna can be done in two ways:
- Using a 4:1 balun at the feed point and running coaxial cable to the transceiver.
- Running 300-ohm ribbon cable all the way back to the transceiver and using an antenna matching unit.
Contrary to conventional Yagi designs, there appears to be no significant advantage in making the reflector longer than the driven element; identical element lengths work well. Another benefit of the ZL Special is its tolerance for slight variations in dimensions and matching, resulting in consistent performance across the entire 10-metre band. This contrasts with typical Yagis, which often have a narrow “sweet spot” frequency range.
Why the 10 Metre Band?
The choice of the 10-metre band for the ZL Special was deliberate. At the time, this band was very active and open, particularly during sunspot maxima, providing excellent propagation conditions. Moreover, the compact size of the antenna suited the limited space and equipment available then — rotators were rare, and many operators had only a simple pole in the garden for mounting antennas.
Variations and Modern Adaptations
One notable variation of the ZL Special is the “compressed” version designed by G3KWH. This adaptation incorporated loading coils in both the driven and reflector elements and used coaxial matching lines, enabling coax feeding back to the transceiver. I built this version myself, though without a VSWR meter, so I couldn’t verify its performance rigorously. Nevertheless, it remains a practical design for those wanting to experiment with the ZL Special concept.
In recent years, the ZL Special has found new life in VHF antenna designs. While I haven’t personally used these, the basic principles of the ZL Special—compact size, ease of construction, and respectable gain—have been adapted for higher frequencies and are even used commercially in some portable antennas.
Radiation Patterns and Practical Considerations
Software modeling by W4RNL and others has provided insights into the ZL Special’s radiation patterns. The antenna exhibits respectable forward gain with a front-to-back ratio around 15 dB. However, the pattern to the rear can be somewhat bulbous, and the front-to-back ratio may vary with antenna orientation.
Different software tools sometimes produce varying results, highlighting the challenges of perfectly modeling antenna performance. Despite this, the ZL Special remains a popular and educational project for amateur radio operators interested in building and understanding directional antennas.
Conclusion: Is the ZL Special Still Worth Building?
The ZL Special may not live up to the most optimistic claims of its heyday, but it remains a compact, straightforward, and rewarding project for amateur radio enthusiasts.
Its relatively small size, ease of construction, and wide bandwidth make it a practical choice for those experimenting with directional antennas on the 10-metre band.
If you’re looking for a historic antenna design that offers valuable hands-on experience and decent performance without the complexity or expense of larger arrays, the ZL Special is well worth considering. Whether you want to relive a bit of radio history or simply build a functional mini beam, this antenna offers a lot of fun and learning along the way.
For those interested in building the ZL Special, detailed dimensions and design plans are readily available, and modern materials can make construction easier than ever. As always, enjoy your ham radio adventures and keep experimenting!
About the Author
Peter G3OJV, founder of Waters & Stanton, has been a licensed amateur radio operator for over six decades. Drawing on his extensive experience, Peter shares insights into classic and modern ham radio equipment to help others enjoy and succeed in this enduring hobby.
Additional Resources
- Waters & Stanton Ham Radio Store: https://www.hamradiostore.co.uk — a leading UK distributor of ham radio equipment with worldwide shipping and excellent customer support.
- Shortwave Magazine Archives: Explore historic issues for more on classic antenna designs and ham radio developments.
- Amateur Radio Forums and Communities: Engage with fellow hams to share your ZL Special build experiences and tips.
For more information please visit our online store or alternatively contact us and we'll be happy to assist you!