[Amps] Toroid for RF power meter

[Will Matney]

All,

I found this while researching toroids. If your building a RF power meter (wattmeter or SWR meter), Collins design, and want to use a transformer in it here's the following.

For use from 1.5 to 30 MHz, the transformer can be an Amidon Associates T68-2 toroid. That's an iron powder type 2 (red) material. It uses 36 turns of 26 gage magnet wire for the secondary. The toroid size has an OD of 0.690 and an ID of 0.370. The primary is one pass of the incoming RF which can be any wire just going through the center of the toroid carrying the power to the load. One turn means to pass through it, not one wrap around it. For lower frequencies, they specify a ferrite with a permeability of 125 which would be 61 mix. The two chokes, after the signal from the transformer is rectified, should have an XL of 100,000 ohms. The two rectifiers are matched hot-carrier diodes #HP2800. The two 10 ohm resistors off the diodes anode should be carbon and closely matched. This also uses two 5 pF piston trimmers off each side to null it out. If it's hard to null, a Faraday shield was recommended.

Best,

Will