I'm looking into ways to match a 137MHz RX dipole antenna to an unbalanced coax + amplifier. I know there are several types of baluns which can be categorized under magnetic baluns (where there are separate primary and secondary windings) and current baluns which are essentially chokes.
I was hoping I could build a transformer type balun, since this would allow me to choose almost any impedance matching ratio I want. I have FT50-61 and FT50-63 toroid cores, which are advertised to work as wideband transformers from 10 MHz - 200 MHz.
I tried winding a few 1:1 transformers for measuring purposes and the results were pretty bad, I got at least -3.5 dB insertion loss. I tried using between 2 and 8 windings of different diameters of copper and silver for both primary and secondary.
This is a typical response (depending on how close the windings are and how many I put on it, I can move the peak left and right by a few ten MHz only):
Now I'm wondering if there is a systematic approach to getting good insertion loss, maybe by moving the peak of the curve to 137 MHz.
Any tips are appreciated.