What is an Inverted U Antenna?
Space-saving HF antennas are a godsend for radio amateurs living in restricted urban settings.
In this video from Jon (VU2JO), we see how a bent dipole antenna (aka Inverted U) can be used for a variety of Amateur Radio Bands.
The design and construction of this antenna are simple, with performance coming close to that of a "regular" flat top dipole antenna.
If you can't see the video, please insert this title URL into your browser search box: https://www.youtube.com/watch?v=A4hRdXPPc7I.
Thanks for joining us today.
Aloha es 73 de Russ (KH6JRM).
Here are some comments from Jon (VU2JO):
Today morning, during a discussion on 40 m, when I was mentioning about space constraints for HF antenna at my location, one ham told me about the Inverted U antenna, which has a horizontal portion and part of the antenna hanging down vertically from both sides. Inverted U antenna is also called a Bent Dipole Antenna as the half-wave dipole antenna is bent on both sides to fit in the available horizontal space. An online search told me that others have tried it for 40, 80 and even 160 m bands to suit their limited space. In one such application, an antenna tuner was used at the feedpoint and the hanging limbs were asymmetric as well. I thought that all these compromises would suit well to my location, except the antenna tuner at the feedpoint which would mean a remote antenna tuner. I do not have a remote antenna tuner and it is definitely more expensive and needs an additional wire or T-bias for providing power supply.
Of the articles which I came across online, one by K0NM on The Bent Dipole was most interesting and with computer modeling information. Though I do have EZNEC Pro+ antenna modeling software installed in my laptop, I have not learned to use it well because of my steep learning curve. The article by K0NM was originally published in QST Magazine, May 1997. There is a good comparison with a half-wave dipole. Both antennas have same length of half-wavelength of the frequency for which it is meant. Though the gain comes down as more of the dipole is bent, it is only about 0.6 dB down when the overall horizontal length is reduced to one-half of the original. Feedpoint impedance also comes down when more of the antenna is bent downwards and matches well with a 50 Ohms coaxial cable when it is bent about half.
Ground coupling losses will be more when the ends are near the ground, which seems to a great disadvantage in my case as I do not have an option for high vertical supports. Minimum of quarter wavelength height has been recommended to avoid ill effects of low antennas. You may be aware that low antennas are often called ‘cloud warmers’ as their radiation is directed upwards and not at low angles needed for long distance HF communication. The SWR bandwidth is also lower for the bent dipole antenna. K0NM has mentioned that he could make a 20 m dipole antenna in an hour and adjust the SWR by adjusting the length of the dangling wires, without the need for a tuner. That was indeed a great feat for an antenna with a span of just 17 feet. As I have not been that successful with the 20 m component of my 40/20 m fan dipole antenna, I am trying to figure out how to implement this Inverted U or Bent Dipole antenna for 20 m at my home.
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Aloha es 73 de Russ (KH6JRM).