I'm in need of a transformer to match a five foot diameter, three turn loop to a sound card's input jack, but a rather high turns ratio is required. If I were to wind three separate transformers, each with only one third of the overall ratio, then connect the primaries in parallel and the secondaries in series, would that produce the desired effect?
73, J.B., VE3EAR
What`s the audio card microphone input impedance. Mr. google says around 1-20Kohm. So very different as with a basic receiver with 50 or 75ohm.
If you know the two values, you can also plot it on paper/screen using a smithchart and use the corresponding cap/inductance to match the two.
73 Frank
I think your plan sounds unusual and may not work. Perhaps you can use a strategy similar to that of small transmitting loop antennas (or magnetic loop antennas). In this kind of antenna, a single-turn loop (of maybe 1 meter diameter) is resonated with a variable capacitor. To match this antenna to the 50 ohms of a transmitter (or receiver), a small coupling loop of wire, about 1/5th of the diameter of the main loop, is placed inside the main loop, typically opposite of the variable capacitor and very near the surface of the main loop. By adjusting the size, position, and orientation of this small coupling loop, a perfect match to 50 ohms can be obtained.
I recently did an experiment where I grabbed a random length of wire, which happened to be a 1-meter-long USB cable. I coiled this up into a small loop of three turns, about 10 cm in diameter, and held it in shape with some tape. Then I made a smaller coupling loop about 2 cm in diameter. I connected the coupling loop to the 50-ohm input of my NanoVNA network analyzer. I found that the main loop was resonant around 50 MHz. By carefully adjusting the size, shape, and position of the smaller loop, I could quickly obtain a perfect 50 ohm resistive match with 1:1 SWR.
You might be able to use a similar technique with your loop antenna. For example, here's a shortwave loop antenna that uses a smaller coupling loop with variable taps (on the coupling loop) to achieve an impedance match. As the article says, the same technique can be extended to lower or higher frequencies. https://swling.com/blog/2021/10/bobs-updated-passive-resonant-transformer-coupled-loop-antenna-for-shortwave/#more-50550
You would probably need a VNA or similar instrument to confirm that your coupling loop is actually transforming the antenna's impedance to the value that you desire.
Two other methods can also be used: an inductive tap on the loop antenna (an autotransformer approach), or a capacitive matching network. You can read about those approaches here: https://vaedrah.angelfire.com/antenna.htm . Here's an image from that page.
Since your loop is rather large -- five feet (!) in diameter -- perhaps the most physically practical approach would be the capacitive matching network, which allows the matching network and the feedpoint to be located at the same location. The inductive matching methods (inductive tap, or coupling loop) require that the matching network and feedpoint be physically located opposite of the point where the variable capacitor is located, which can make things more complicated to construct physically.
Again, a VNA is a pretty handy, if not indispensable, tool to have if you want to really confirm the behaviour of your impedance matching network.
I'm not an engineer. I assume inductance is inductance for any circuit. I use this calculator to try and get to the target and test it. https://www.translatorscafe.com/unit-converter/en-US/calculator/parallel-inductance/