Well, 20 may be a problem and so may 10, as on those bands it's an even multiple of 1/2 wave length. Generally speaking a full wave antenna (or 2 full waves) is not a good choice, There is one exception that I know of but generally full waves don't work well
(you get a good swr but at at more than 10 wavelengths from the antenna the ERP is zero)
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