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Very high. Harmonics work the other way around — antennas can be useful on higher bands than the design band, but almost never lower bands. A dipole cut for 40m might be good on 15m. An end-fed or a loop for 40m might be good on 20m and 10m. But pretty much any design of 40m antenna is going to be a half-size antenna on 80m, and very unlikely to be tunable ...
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