How do you know when the dipole is set at the right length for the band in use? Measure it of course…. (make the dipole out of 30m Aluminum tape measures).
5m + 5m = Half Wave on 20m.
10m + 10m = Half Wave on 40m.
20m + 20m = Half Wave on 80m
Metric really is that easy! (you do need to measure a little longer than this measurement due to end effect) . Makes a superb roll-up doublet if fed with 450 Ohm feeder. Just make it as big as you can fit in your location.