On Tue, Jun 9, 2020 at 09:07 AM, Mikek wrote:
Does -1.5db loss for 10 turns wound over 7 turns on a #43 binocular core sound
about correct, or would you expect lower?
Your loss is too high and you can get a fraction of a dB with the right ferrite mix selection, number of turns and winding arrangement.
From your design description it seems that you want to go as low as 500 kHz. Your primary was 7 turns with a system impedance of 50 ohms. The rule of thumb is that the reactance should be 4x50 = 200 ohms when nothing is connected to the secondary. Using the calculator at toroids.info we get 68 ohms at 500 kHz. using an FT50-43 toroid. You need 12 turns to get the desired 200 ohms on the primary which means 17 turns on the secondary for a 50:100 transformer. If you wind the primary on top of the secondary you will get a lot of capacitive coupling between the windings. There will also be a lot of capacitance between turns. You can try winding on opposite sides of the core and on top and compare your results.
It is generally best to have fewer turns and this means a core with higher permeability. I suggest using a FT50-75, a BN-43-202 or a BN-73-202. With the FT50-75 you only need 4 turns on the primary and 6 turns on the secondary. I like binocular cores because they give a high coupling factor between the primary and the secondary with only a few turns and less coupling capacitance. A BN-43-202 is a good choice and only requires 6 turns on the primary and 8 to 9 turns on the secondary. Also a BN73-202 with 3 turns on the primary and 4 turns on the secondary might work for you.
You can see my calculations in the graphics below. There is also a photo showing how to wind binocular cores for those not familiar with them.
Roger
Roger Need
