Inverted L Antenna
The Topband, as the 160-meter band is also known, poses a challenge when it comes to designing a compact antenna. That is why many amateur radio operators skip this band, which, however, is a mistake during the solar minimum when openings on the 10-meter band cannot be expected. For the CQWW Contest 2008, I set out to build a portable antenna that could also be used under portable conditions. I was inspired by DK5WL’s article “http://www.160m.de/Invisible%20Antenna080406.pdf” titled “Invisible” DX Antenna for the Low Bands.
The principle is essentially very simple—the inverted L is a resonant quarter-wave antenna with a predominantly vertical component. This is particularly advantageous for DX operations.
After discovering that Jirko OK5IM was selling 12.5-meter-tall fiberglass telescopic masts, the decision was made. I tested the antenna in the IARU HF Championship, and the results were impressive; this was later confirmed in the CQWW Contest, where I likely set a new Welsh record with this antenna.
Dimensions of the 160-meter inverted L antenna
The antenna’s dimensions are clear from the diagram. With these dimensions, the antenna resonated at 1,850 kHz. When using a 12.5-meter telescopic mast, the radiator impedance will be approximately 12 ohms; however, the total impedance is also influenced by the impedance of the ground system. If only 2–3 radials laid on the ground are used, the antenna can be fed directly with a 50-ohm coaxial cable.
Antenna Base
Although the efficiency of such an antenna will be only 24% compared to an ideal, lossless vertical, nobody has such a vertical anyway, HI. However, the “poor quality” of the ground has a positive effect on the antenna’s bandwidth. The bandwidth for a PSV < 2 is approximately 90 kHz.
Heinz-Josef described a method for increasing efficiency very well. It is suitable for home implementation and involves reducing losses by using a larger number of radials and a transformer circuit that transforms the lower impedance to 50 ohms. A second option is to extend the vertical section of the antenna. A laminate telescopic mast can be extended by inserting a duralumin tube into the base of the antenna, or you can purchase a telescopic mast with greater height.
Even one person can easily set up an inverted L. The joints on the telescopic sections are secured with electrical tape to prevent loosening. The wire is also secured to the telescopic section with electrical tape. The termination is housed in a box containing a PL connector and terminals for connecting the dipole and radials. At the top of the opposite telescopic section is an end insulator—in my case, Teflon. Both telescopic sections are secured in this portable configuration with four thin climbing ropes.
Tuning is also possible using a SWR meter. First, we tune the antenna to the desired frequency by adjusting the length of the radiator, where the SWR should be < 2. Then, by shaping the antenna—specifically, by adding or shortening the radials—we fine-tune the antenna so that the SWR is as low as possible at this frequency.
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