I'd say these things aren't an exact science, depending on locaton, height and other factors, just start with a too-long wire and bend it back on itself before finaly cutting it and even then leave some spare in case you need to easily lengthen it again. If it's wet, it'll need shortening for resonance, when dry, need lengthening... unless of course you get it within a happy broadband medium coverage range as required. Even depends how flat top the dipole is, usually they sag from feedline, or may be bent etc. Nothing like having fun up and down and up and down adjusting and readjusting for resonance. Then try adding additional wires for other bands that will interact and prepare for hours of up and down :)

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