Re: Measurement Receiver Inductors


Hi Mike

Toroid turns calculators and tables give approximate L values with tolerances that may vary with toroid AL variations, wire spacing and to some extent frequency in a functional sense ( we usually put them in circuits with stray L and C )  Wire gauge may effect turns spacing and also perhaps how the wires orient and stay put on the toroid core. Even the target L can influence these as more windings might mean thinner gauge wire and wire turns can bunch up and not stay in place compared to a small uH coil wound with thicker wire etc.

With nH coils @ VHF, the lead length may affect precision. I don’t use turns calculators and just measure them with a LC meter and scrunch or spread the windings to get the desire L.  When I did use turns calculators, I always wound 1 less turn than calculated and then applied the scrunch/spread method to get the desired L.

In most L-networks, a tuning cap proves critical. Most of mine apply 2 caps [ L-C-C] network because 2 caps gives you 3 tweeks. The other tweek is to again scrunch or spread the inductor while peaking the trim cap(s) for the best impedance match. SPICE might not account for stray L and C and the reactive components at 1 or both ports.

If you experiment with a return loss bridge and a simple L or L-C-C network to match a network into 50 ohms --- while measuring RL on the 50 ohm port, you will learn just how “touchy” the tuning can get.

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