Impedance transformer
The purpose of the transformer is to present a ~650-Ohm input impedance to the antenna when connected to a 50-Ohm preamplifier, and minimize direct coupling between the antenna wires and preamplifier input. I tried several different cores and found that good parameters could be achieved with Amidon BN-73-202. The transformer shown below has 7 and 2 turns in the primary and secondary windings respectively, its impedance transformation ratio is 12.25. I tried two designs, with the leads leaving the core at the opposite sides and at the same side, but could not see any difference in performance.
Here is a frequency response of two such transformers, connected back to back
and measured using HamVNA
[502]
.
The losses are about 0.5 dB per transformer on all HF bands. Two turns in the primary winding is the bare minimum for this core. I have also tried 1 : 4 turns, but the losses increased to 1.9 dB. More turns result in higher inter-winding capacitance, which for the 2:7 design was measured to be ~6 Pf.
The input impedance of the transformer with a 50-Ohm resistor across its secondary winding was about 550 Ohms, fairly flat across the HF bands:
