Hi Rolf,
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I have not tuned above the 16 meter band, but the HF reception lower than this band seems fine. I have mostly tuned from 90 meters to 19 meters, and have not felt the sensitivity was lacking.
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My loop is only slightly larger than the recommended default, so I don't feel I'd have much to gain by increasing the size, since my main interest is generally <10 MHz on shortwave.
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Good luck with your own Azi-Loop!
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Best DX,
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Guy
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Guy Atkins