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12 meter element:
I made mine 221 inches.   On my MFJ-259b, the SWR is very sharp 1.6 throughout the 12m band

17 meter element:
I made mine 312".   SWR is 3.8 and very broad and almost undetectable in the MFJ259b.

It also tunes up great on 20 meters.    I use an antennqa tuner to briing everything in to 1.0 SWR