A total wire length of near 1/4 wavelength on the lowest frequency of operation can work if you have a good ground. Otherwise nearer 1/2 wavelength is better. However exactly 1/2 wavelength means extremely high impedances. That can present high feedpoint voltages and difficulties with some antenna couplers. So you might want to go a few metres longer or shorter than 1/2 wavelength to avoid these extremes.
A bit over 20 metres is a good general purpose length for casual operating on bands like 40, 20 or 30 metres. Although I'd suggest (say) 35 or 45 metres for better results on 80m.
* W4RNL's all band inverted L
* The Inverted L antenna and NVIS
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