For amateur radio operators, or “hams,” the quest for a simple, effective, and multiband wire antenna is a common one. Antennas that work well across multiple frequency bands without a complex tuner are highly desirable, especially for portable or space-constrained setups. This blog post explores one such design, a short aperiodic multiband antenna developed by Kurt Hübner, HB9KX, and Lothar S. Hübner, DK7ZB.
Multiband Wire Antenna – Understanding the Design
The aperiodic Multiband Wire antenna design featured in the accompanying image is a variant of a loaded dipole. Unlike a traditional dipole, which is cut to a specific length for a single frequency band, this antenna uses an offset feed point and a resistive load to achieve a broad SWR curve across multiple bands.
The key components are:
- The Antenna Wires: The multiband wire antenna consists of two wire sections of differing lengths, specifically 3.4m and 7.1m, creating an overall length of 10.5 meters. The asymmetrical design helps to achieve multiband operation.
- The 100-Ω Resistor: This is the heart of the aperiodic design. The 100-ohm resistor is connected in parallel to the feed point, bridging the two antenna wire sections. Its purpose is to intentionally introduce a resistive load that flattens the antenna’s impedance curve across a wide range of frequencies (10 to 40 meters). This makes the antenna less sensitive to frequency changes and allows it to be used on multiple bands without an external antenna tuner.
- The Common Mode Balun: Positioned just below the feed point, the balun is a crucial component. It suppresses “common mode currents,” which are unwanted RF currents that can flow on the outside of the coaxial cable shield. Without a balun, these currents could cause the feed line to radiate, distorting the antenna’s pattern and potentially causing RF interference in the shack. The balun ensures that the RF energy travels symmetrically into the antenna wires, leading to a cleaner signal and better performance.
- The 50-Ω Coaxial Cable: A standard 50-ohm coaxial cable is used to connect the balun at the antenna’s feed point to the transceiver in the shack.
The Trade-Off: Power vs. Convenience
The aperiodic design provides significant convenience by eliminating the need for an external antenna tuner on most bands. However, this ease of use comes at a cost. A portion of the transmit power is dissipated as heat in the 100-ohm resistor. This can result in a power loss of 2-3 dB, which translates to a noticeable reduction in signal strength on the receiving end.
For portable operations or situations where simplicity is a priority, this trade-off is often well worth it. The antenna allows for quick setup and operation across multiple bands with minimal fuss. For more permanent or high-power setups, a traditional tuned antenna with an external tuner might be a more efficient choice. This design is an excellent example of a compromise antenna that solves a practical problem for many radio amateurs.
When building the 100-Ω resistor for the aperiodic multiband wire antenna, the power rating is a critical factor due to the power dissipated as heat.
Multiband Wire Antenna – Power Rating and Construction
For a typical 100-watt transceiver, the load resistor should handle about one-third of the transmit power during CW (continuous wave) or SSB (single-sideband) operation. This means the resistor should have a power rating of at least 33 watts. The author used a nominal load of 36 watts for their design, which provides a small safety margin.
Specific method for building a suitable resistor:
- Resistor Type: Use 18 low-inductance metal oxide film resistors. Non-inductive resistors are essential to prevent unwanted reactive effects at radio frequencies. Wire-wound resistors, for example, tend to have significant inductance.
- Resistor Value: Each of the 18 resistors should have a value of 1.8 KΩ (1,800 ohms) and a power rating of 2 watts.
- Configuration: Connect all 18 resistors in parallel. The total resistance of resistors in parallel is calculated as 1 / (1/R1 + 1/R2 + … + 1/Rn). In this case, 1 / (18 / 1,800) = 100 Ω. The total power rating for a parallel configuration is the sum of the individual ratings, which is 18 resistors * 2 watts/resistor = 36 watts.
Mounting and Heat Dissipation
Because a significant amount of power is converted to heat, the resistor cannot be sealed in a small, weatherproof box without proper ventilation. The suggested solution is to mount the resistors on a non-conductive plate, such as a piece of PE (polyethylene) from a kitchen cutting board. This open construction allows for air circulation to cool the components while still protecting them from the elements.
Related Posts
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- RA9SUS Simple Loop Antenna covering 14–30 MHz
- Morgain Antenna: Dual-Band Dipole that Beats Traps
- How to Build a Reduced Size 40M Antenna – Complete DIY Guide
- Shortened Inverted V Antenna for 80 Meters (3.700 MHz)
For more information on the original design, please visit the DK7ZB website here.