Various answers include 1/8th of the wavelength all the way down to 1/10th, but would 1/15th or even 1/20th of the wavelength be sufficient for a 60 meter (5 MHz) horizontal NVIS antenna?
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1$\begingroup$ You might find the answers to ham.stackexchange.com/questions/746/… useful $\endgroup$– David Hoelzer ♦Commented Sep 26, 2021 at 13:57
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$\begingroup$ You didn't say ideal or maximum height for NVIS, which are both λ/4 high. K2CG's answer about loss vs. height is a good one. There is an NVIS myth propagating that says lower is always better. That's only good if the hams in your group are all close to each other and you want to limit how far away you can be heard. $\endgroup$– Mike WatersCommented Sep 26, 2021 at 14:41
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$\begingroup$ OP's related question with answers $\endgroup$– Mike WatersCommented Sep 26, 2021 at 14:44
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Minimum height to accomplish what? It'll work right down to the ground, it'll just get lossier and lossier the lower you hang it. The exact numbers depend on your ground, and the threshold of "good enough" depends on you, your power, and who you're trying to talk to, so there's no real hard limit. Some unsourced internet numbers suggest that at 0.05 wavelength above ground you'll have about 6dB of additional loss compared to 0.2 wavelength above ground, which seems manageable.
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1$\begingroup$ People have used buried wires as clandestine antennas, so the minimum height above ground is negative, from a technical point of view. Of course buried wires don't really make practical NVIS antennas... $\endgroup$ Commented Sep 27, 2021 at 15:40
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One of the comments for this question includes a link to a paper on blast-resistant ground level HF antenna's that the U.S. Airforce used or uses. So zero. Or negative if you count the buried cavity.
