How to make an RF transformer-balun for antenna wire

In one of my articles, I described how to make a Fuchs antenna for a radio station operating in different frequency ranges. However, this antenna would need to be constantly tuned to different frequencies using a variable capacitor. How can I avoid having to re-tune the antenna while still transmitting at different frequencies? To do this, I need to make an RF transformer, which I'll call a balun.

You can find a balun in the connector for TV antennas. It converts the antenna's 300-ohm impedance (Pittockors dipole) to the 75-ohm input impedance of a TV or set-top box. This is called a 1:4 balun, meaning it divides the 300-ohm impedance by four.

The balun I made can be connected to a 40-meter-long wire, and this wire can radiate a signal well at any frequency from approximately 4 MHz to 20 MHz, with an SWR of around 1-2.

I connected my antenna wire to the nanovna device, and I can see that the SWR is 12 at 7.09 MHz, and this antenna has very high impedance at this frequency.This is very bad, since the transmitter power will not reach the antenna completely, but let's say 35 percent out of 100, the remaining power will be dissipated as heat or the power amplifier may fail.

To make a balun, you will need a PC40 ferrite ring with an outer diameter of 25 mm and an inner diameter of 15 mm.

Take three pieces of stranded wire, each 27 cm long, and wind them as shown in the photo.

The winding is 5 turns. Secure the wire with tape.

Use a multimeter in continuity mode to check the balun terminals. Terminal 1 should be connected to terminal 4.

Pin 2 is connected to pin 5, and pin 3 to pin 6.

Connect the balun terminals according to the diagram

Now it's time to check my antenna with a balun. As you can see, the SWR at 7.09 MHz is 1.23 and the impedance of my antenna is 58 ohms, which is good, because the output of my radio station is designed for 50 ohms.

At 20.4 MHz, the SWR is 1.65 and the antenna impedance is 47 ohms. My antenna can now operate perfectly from 20 to 4 MHz, without having to reconfigure anything in the antenna.

I connected the balun to the radio station and the antenna and counterpoise to the balun.