Homemade Transformer Voltage

A real transformer for a low frequency has a laminated iron core and hundreds or thousands of turns of wire.
Your transformer has an air core and only a few turns then it might work for low current at 100MHz.

I would wind the primary first. Let's say that you are feeding the primary with 6VAC. Keep count of the turns, and wind until very little current flows with 6VAC applied. We want enough reactance in the primary, where very little current flows. An appropriate core material can reduce the number of turns needed. Using the turn count from the primary, now wind the appropriate number of secondary turns for the voltage you want.

With the secondary open, there should be very little current in the primary. When you allow current to flow in the secondary, the reactance of the secondary, will cancel the reactance in the primary, allowing more primary current. It's automatic and called transformer action. It automatically throttles the current. That's the basic idea. Of course there is wire size, insulation, heat and hum to consider.....for the voltages and currents used.