I’ve just watched three excellent videos on baluns.
First some theory and testing a couple of baluns
Building and testing a 1:1 balun
Building and testing a 4:1 balun (making use of the 1:1 as well)
A lot of the work in here is based on the work done (in German) by DG0SA at http://www.dg0sa.de/inhaltbalun.htm
Given what I saw in the first video I suspect that what I’m using at the top of my 40/20m dipole is, to not put too fine a point on it, a piece of crap.
The wire that was used in video two (silver plated PTFE AWG18) is horrendously expensive in Australia (almost AUS$90 for a 25m roll), and you’d realistically want two rolls of different colours. That or be very, very careful with your labelling.
Having a look around DG0SA’s web site I found a 1:1 50Ω balun for 200 watts that uses AWG22, which is is a little under AUS$50 per roll of 25m.
The toroid suggested is a FT240-34, which is a type 34 ferrite toroid of about 60mm diameter. Element14 has them around AUS$8.
I’m being very tempted to to make one of these.