A while ago I wrote an article on making a trap dipole. Since then I have made lots more - and in doing so have gained some greater insights into how they work. The original article is here. My additional tips are:

1) When you add traps to the ends of the highest frequency section of the antenna they make the antenna appear shorter. This is a very odd effect but shows up when modeled using Eznec too. So resonate your 20m section a little low before adding the traps.

2) Traps show a high impedance at their resonant frequency, as you move away from that frequency, the impedance falls. This in turn reduces the isolating effect of the traps. The higher the Q of the traps, the quicker the impedance falls. Thus rather than resonate the traps a few percent below the highest frequency band, I now recommend resonanting them at the lower band edge. so for a 20-40m dipole, resonate your traps at 14.000MHz.

73 Richard G3CWI