This project needs a balun between filter and antenna, to ensure maximum efficiency in transferring the RF from the transmitter(unbalanced) to the antenna(balanced). There are many types of balun, here are some details of the one I'll be using for this project.

I would normally not take a balun too seriously when I am using a co-ax feeder. Just put some turns of co-ax round a ferrite ring to stop any RF going back down the outside of the feeder and call it good. A simple balun, but one that does the job.

But in this case I don't have any feeder, I want to minimise loss. So resonant transmission line baluns are out too. I'm left with transformer baluns, and my choice is between an air cored one or a ferrite one. My gut feeling is that a ferrite one will have more loss than air cored, so I'll go for air core.

The classic air cored 1:1 balun is a trifiliar transformer as below:

Three wires are wound side by side on a former and connected as shown, with appropriate connectors. I may forgo the co-ax connector and wire straight into my filter.

I will use plastic pipe about 1" diameter, and wind about 15 or 20 trifiliar - three wires together - turns, that should be enough.

There you are, balun design, sorted!

(Edit) See the resulting balun in this project log!