VIDEO
In the rich and varied world of ham radio, antennas have played a pivotal role in the evolution of the hobby. Over the decades, numerous antenna designs have appeared, some enduring the test of time and others fading into obscurity. Among these, the DL7AB MultiBand antenna stands out as a fascinating piece of ham radio history, first introduced around 1960. This article delves into the origins, design, and practical applications of this historic antenna, highlighting its relevance even in modern amateur radio setups.
Introduction to the DL7AB MultiBand Antenna
The DL7AB antenna, attributed to the callsign DL7AB and dating back to the early 1960s, is an example of an end-fed (NFED) antenna designed for multiband operation. It was published in ham radio press at the time and has since become a noteworthy reference for operators interested in simple yet effective antenna solutions.
At its core, the DL7AB antenna is an end-fed half-wave wire antenna, originally designed for the 80-metre band but capable of covering multiple bands up to 10 metres. It uses balanced open-wire feeder line, a method that is still appreciated today for its efficiency and low loss characteristics. This antenna’s design cleverly incorporates a small coil near the feed point—a feature that was quite innovative for its time and remains relevant for improving antenna performance on higher frequency bands.
Understanding the NFED (End-Fed) Antenna Concept
End-fed antennas, or NFED antennas, have gained popularity in recent years because of their simplicity and effectiveness. As the name suggests, these antennas are fed at one end, which simplifies installation, especially in limited space or challenging environments.
The DL7AB antenna is based on the end-fed Zepp (Zeppelin) antenna concept, which uses a length of wire approximately half a wavelength long at the lowest operating frequency—in this case, around 40 metres long for 80 metres band operation. The antenna is fed with open-wire feeder line, also called balanced line, which offers much lower loss than coaxial cable. This was particularly significant in the 1960s when coaxial cable technology was less advanced and costly compared to today.
Why Open-Wire Feeder?
Open-wire feeder or balanced line is preferred in many antenna designs because it minimizes losses and is less susceptible to interference. In the DL7AB design, this type of feeder was essential because the antenna presents a high impedance at the feed point, especially on the fundamental 80-metre band and its harmonics.
Balanced feeders ensure that the radio frequency (RF) energy is efficiently transmitted from the transmitter to the antenna, reducing wasted power and improving overall performance. This efficiency is a key reason why balanced feeders were widely used in antenna systems before the widespread availability of advanced antenna tuning units (ATUs) and baluns.
The Significance of the Feed-Point Coil
One of the most interesting aspects of the DL7AB antenna is the inclusion of a small coil near the feed point. This coil acts as an inductive element to correct the antenna’s electrical length as it operates across multiple bands, from 80 metres through to 10 metres.
As the antenna is used on higher frequency bands—essentially harmonics of the fundamental 80-metre frequency—the resonant point tends to shift. This means the antenna may not naturally resonate at the desired frequencies without some form of compensation.
The coil serves this purpose by providing inductance that adjusts the antenna’s electrical characteristics, effectively “tuning” it for better performance on the higher bands. This concept is commonplace in modern NFED antennas, but it is fascinating to note that DL7AB had already incorporated it over 60 years ago.
Coil Dimensions and Placement
While exact coil dimensions and placement vary depending on the antenna’s construction and operating bands, the original design specifies a coil positioned near the feed point. Although specific lengths are not detailed here, the principle remains: the coil is small, placed close to the feed, and carefully sized to provide the necessary inductive reactance.
For modern experimenters, replicating this coil involves some trial and error, but the original documentation provides a valuable starting point for those interested in building or restoring historic antennas.
Operating Bands and Impedance Characteristics
The DL7AB antenna was designed for multiband operation, covering 80, 40, 20, 15, and 10 metres. On the fundamental 80-metre band, the antenna behaves as a half-wave radiator presenting a high impedance at the feed point, often in the thousands of ohms.
This high impedance characteristic continues across the harmonic bands, where the antenna also presents high impedance loads to the feed line. The balanced feeder line helps manage these impedances effectively, allowing the antenna to operate efficiently on multiple bands without requiring complex matching networks.
Handling the War Bands
While the antenna was primarily designed for the main amateur bands, the so-called “war bands” (intermediate frequencies between the main HF bands) are also handled reasonably well. The antenna presents a medium impedance on these bands, and the balanced feeder line can still feed the antenna with acceptable SWR (Standing Wave Ratio) values.
This flexibility means that the DL7AB antenna can be a versatile choice for operators wanting to cover a wide range of HF frequencies with a single wire antenna.
Feeding and Matching the DL7AB Antenna
One important note about the original DL7AB antenna is that it was designed in an era before the widespread availability of 49:1 ununs (unbalanced to unbalanced transformers) and modern antenna tuning units. Instead, operators typically used external balanced antenna matching units (ATUs) to match the high impedance antenna feed point to the transmitter’s output.
Balanced ATUs were common because they were easier to design to handle a wide range of impedances and reactances, and because balanced feeders were standard. These matching units could accommodate varying lengths of open-wire feeder without significant issues.
Modern Adaptations
Today, balanced ATUs are less common, but many modern antenna tuners, such as those from MFJ, include balanced line terminals or can be adapted to work with ladder line and balanced feeders.
If your antenna tuner does not have a balanced line input, a practical solution is to use a 4:1 balun between the balanced feeder and the unbalanced tuner input. This approach helps transform the impedance and allows efficient matching of the antenna to your transmitter.
While KW Electronics produced some excellent balanced ATUs in the past, these units are now rare and highly sought after. If you come across one, it is worth considering as a valuable addition to your station.
Why Consider the DL7AB Antenna Today?
With so many modern antenna designs available, why revisit the DL7AB antenna? The answer lies in its simplicity, efficiency, and historical significance.
- Simplicity: The antenna requires only a length of wire, some balanced feeder, and a suitable matching unit. This makes it straightforward to construct and install, even for newcomers to the hobby.
- Efficiency: Using balanced feeders and a carefully designed feed point coil, the antenna achieves good performance across multiple bands with low loss.
- Historical Insight: Understanding and experimenting with designs like the DL7AB antenna connects operators to the rich heritage of amateur radio and can inspire innovative modifications and improvements.
For those interested in exploring historic antennas, the DL7AB design offers an excellent project that combines theory, practical construction, and the opportunity to experience a piece of ham radio history firsthand.
Building Your Own DL7AB Antenna
Building a DL7AB antenna is a rewarding project that can be accomplished with basic materials and tools. Here are some general guidelines to get started:
- Wire Length: Cut a length of wire approximately 40 metres long for the main radiating element, suitable for 80-metre half-wave operation.
- Balanced Feeder: Use open-wire feeder or 450-ohm ladder line to feed the antenna. The exact length of feeder is flexible and can be chosen based on convenience.
- Feed Point Coil: Construct a small coil near the feed point. Start with about 10-15 turns of wire on a small diameter former (e.g., 2-3 cm), and experiment with coil length and turns to optimize performance.
- Matching Unit: Use a balanced antenna tuner if available. If not, incorporate a 4:1 balun between the feeder and an unbalanced tuner to facilitate matching.
- Installation: Erect the antenna as a horizontal wire at a reasonable height, ensuring the feeder is kept away from metal supports and grounded structures to minimize loss and interference.
With these steps, you can bring to life a classic antenna design that remains relevant and functional in today’s amateur radio environment.
Conclusion
The DL7AB MultiBand antenna is a remarkable example of early multiband antenna engineering. Its design, which dates back to around 1960, incorporates principles that remain valid and effective today—such as the use of balanced feeders, a feed point coil for inductive compensation, and a high-impedance end-fed half-wave element.
For amateur radio enthusiasts looking to explore historic antenna designs or seeking a simple, efficient multiband antenna, the DL7AB offers a compelling option. Its ease of construction, adaptability across multiple bands, and efficient use of balanced feeders make it a practical choice for many operators.
By revisiting and experimenting with antennas like the DL7AB, hams not only honour the legacy of pioneers in our hobby but also gain hands-on experience that can inform future antenna projects and innovations.
Whether you are a seasoned operator or a newcomer eager to learn, the DL7AB antenna is worth considering as part of your antenna arsenal. All you need is some wire, balanced line, and a willingness to experiment—and you might just be pleasantly surprised by the results. Enjoy your home radio, and happy experimenting!
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