Feeding a Vertical Antenna from the Top: A Modified End-Fed Halfwave

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Introduction

If you have a small garden and want to operate on the 40m band, there's a modified end-fed halfwave antenna that you should consider. This antenna is half the length of a normal end-fed halfwave, yet it has the same bandwidth. Not only does it take up less space, but it can also be fed from the top instead of the bottom. In this blog, we will explore how to feed a vertical antenna from the top and the advantages it offers.

The Inverted L Antenna

The inverted L antenna is an L-shaped antenna that is inverted and typically fed at the base. The antenna consists of a vertical section and a horizontal section. The vertical section is fed with a coaxial cable, with the inner conductor connected to the vertical section and the shield connected to the ground. This configuration allows the antenna to be resonant, particularly on the lower frequency bands like 160m and 80m. However, the dimensions of the antenna are critical to achieve a low impedance at the base.

Feeding the Antenna from the Top

Although it may sound like a silly question, it is indeed possible to feed a vertical antenna from the top. In fact, this method has been explored in an article written by Peter Waters in May 1965. By feeding the antenna from the top, you eliminate the need for a long length of coaxial cable running from the base of the antenna to your shack. Instead, you can bring the end of the antenna close to your house and use a matching unit, such as a 9:1 unun, to connect it to your transceiver.

Advantages of Feeding from the Top

Feeding the antenna from the top offers several advantages. First, it allows for a shorter length of coaxial cable, which reduces losses and simplifies installation. Second, it helps reduce static buildup, as the antenna is directly connected to the ground, effectively dissipating any static buildup. This results in a quieter antenna with reduced noise problems. Additionally, feeding the antenna from the top makes it easier to achieve a good match with the internal matching unit on your transceiver.

Earth or Ground Plane

When using a vertical antenna, it's important to have an earth or ground plane. This helps improve the antenna's performance by providing a radial system for the antenna to work against. While it's ideal to have eight to ten quarter-wave radials, the length of the radials is not critical. Even shorter radials, as long as they can fit into your garden, can be effective in enhancing the antenna's performance.

Single Band vs. Multiband

The modified end-fed halfwave antenna can be either a single band or a multiband antenna, depending on its length. When the vertical section is a quarter-wave long and the total length of the wire is a quarter-wave on a specific band, the antenna is resonant and can be fed with a 49:1 unun. This configuration works well for a compact antenna on the 40m band. However, if you want to make the wire a bit longer to make it non-resonant, you can use a 9:1 unun for multiband operation.

Performance and Considerations

The inverted L antenna offers good performance, particularly for local contacts and some DX on the 40m band. It provides both vertical and horizontal polarization, resulting in a combination of low and high angle radiation. However, it's important to note that the antenna's performance is affected by phase cancellation, which becomes more significant at higher frequencies. Additionally, the presence of nearby structures, such as houses and concrete, can impact the antenna's efficiency. Despite these considerations, the inverted L antenna remains popular for LF bands like 160m and 80m.

Conclusion

Feeding a vertical antenna from the top offers a practical solution for those with limited space or an upstairs shack. The modified end-fed halfwave antenna provides good performance, low angle radiation, and the ability to work both local and DX stations. Whether you choose a single band or a multiband configuration, the inverted L antenna is a reliable option for amateur radio operators. So, if you have a small garden and want to operate on the 40m band, give the modified end-fed halfwave antenna a try. Enjoy your ham radio experience and happy operating!