The Forgotten FB5 Ham Radio Antenna: A Compact Multi-Band Solution

The Forgotten FB5 Ham Radio Antenna: A Compact Multi-Band Solution

The Concept

In the world of ham radio, antennas have always been a topic of fascination and experimentation. Over the years, we've seen the rise and fall of various antenna designs, each with its own unique characteristics and capabilities. One such antenna that seems to have faded into obscurity is the FB5 Multiband Dipole, designed by the prolific antenna engineer, G4ZU, in the early 1960s.

About G4ZU

G4ZU, also known as Gordon Alfred Bird or "Dick" to his friends, was a prominent figure in the ham radio community. Born in 1918, he spent a significant portion of his career working in the telecommunications division of the General Post Office, based in London's St. Martins Le Grand. G4ZU was a prolific designer of antennas, each with a distinctive name, such as the famous G4ZU Mini-Beam and the "Jungle Job" antenna.

The G4ZU Antennas

G4ZU's approach to antenna design was marked by his innovative use of materials and his ability to create compact, multi-band solutions. The FB5 Multiband Dipole, which he designed around 1960 or 1962, was one such creation that showed great promise but has since faded from the collective memory of the ham radio community.

The Antenna Uses Ferrite Beads

The key to the FB5's multi-band capabilities lies in G4ZU's use of ferrite beads. These beads, when strategically placed along the antenna wire, can have a significant impact on the antenna's resonant frequency. By threading a series of ferrite beads onto the wire, G4ZU was able to create an antenna that could cover the 80, 40, 20, 15, and 10-meter bands without the need for a complex matching network.

How the Ferrite Bead Works

The ferrite beads work by adding inductance to the antenna wire at specific points. When placed at a point of high current, the beads have a maximum loading effect, effectively lowering the resonant frequency of the antenna. As the frequency changes, the point of maximum current along the wire also shifts, causing the beads to have a varying impact on the antenna's performance across the different bands.

The FB5 Diagram

The basic design of the FB5 Multiband Dipole, as shown in the diagram, consists of two 42-foot wire elements connected by a 10-foot section of 300-ohm ladder line. The 25 ferrite beads are placed on each side of the antenna, starting near the junction of the ladder line and the wire elements.

How the Antenna Works

According to G4ZU, the antenna is naturally resonant on the 20-meter band, with the ferrite beads having minimal loading effect at this frequency. This allows the antenna to resonate effectively on 20 meters without the need for a matching network. On the 21 and 28 MHz bands, the ferrite beads provide just enough loading to enable the antenna to resonate on these frequencies as well.

For the 80 and 40-meter bands, G4ZU claimed that the antenna would "more or less take care of itself," suggesting that the antenna's dimensions were well-suited for these lower frequencies. While a matching network may still be required, the VSWR (Voltage Standing Wave Ratio) would be low enough that most modern transceivers with internal matching capabilities could handle the antenna without issue.

The Matching Section

The 10-foot section of 300-ohm ladder line serves an important purpose in the FB5 design. G4ZU claimed that this matching section helped to produce a better radiation pattern, particularly on the 20-meter band, by preserving the antenna's lobes and preventing any breakup of the radiation pattern.

It's worth noting that while G4ZU specified 300-ohm ladder line, modern hams could potentially substitute 450-ohm line without any significant impact on the antenna's performance.

Stacking the Antenna

One of the most intriguing aspects of the FB5 Multiband Dipole is the possibility of stacking two antennas to achieve additional gain. G4ZU suggested that the lower antenna could be as low as 5 feet above the ground, with the upper antenna positioned 20 feet above the lower one, resulting in a total height of only 25 feet.

According to G4ZU, this stacked configuration would provide "useful gain" on the 10, 15, and 20-meter bands, potentially outperforming a full-size dipole on these higher frequencies. However, he did not provide specific gain figures, leaving the exact performance benefits up to further experimentation.

Summary

The FB5 Multiband Dipole, designed by the prolific G4ZU in the early 1960s, is a fascinating and largely forgotten antenna design. Its use of ferrite beads to create a compact, multi-band solution, as well as the possibility of stacking two antennas for added gain, make it an intriguing option for modern-day ham radio operators looking to experiment with unique antenna designs.

While the specifics of the ferrite beads and their modern-day equivalents may require some research and experimentation, the FB5 represents an interesting chapter in the history of ham radio antennas, and its rediscovery could lead to new insights and innovations in the field of antenna engineering.