Electronics > RF, Microwave, Ham Radio

High power L matching

**e.sotillet**:

Hi guys, I usually work with small signals and today I need to use L matching network to connect a 50 ohm, 40dBm source to a 75ohm load.

But my question is: how I can select the proper capactors for this task? I will use air coil with relative thick wire but I dont know how to select the right capacitor. I think one major key parameter is the Q, rigth? I was thinking in High-Q RF capactors like those from ATC.

So, my questions are:

1.- What determine the selection of the capacitor?

2.- When can I use a 0603 capacitor and when I need to go with a bigger one?

Best regards

**TimFox**:

Capacitors have a maximum current as well as a maximum voltage.

To simplify, high Q gives a low ESR, with lower power dissipation for a given current.

You need to know either the manufacturer's current rating, or an estimate for power rating of the capacitor package, and calculate the RF current through the capacitor in your circuit.

In a resonant circuit used for power coupling, the current through the reactances may be larger than that delivered to the load ("circulating tank current").

+40 dBm = 10 W, so this is an important quantitative problem.

**M0HZH**:

You probably also need to design for output mismatches.

I use double the normal current as capacitor rating. For +40dBm you would have about 365mA of current into 75ohm, so a capacitor rated for 800mA RF current or more should be all right.

At these power levels, I see 0805 being the most common capacitor size.

**TimFox**:

Actually, the current through the capacitor will be higher than the current delivered to the load for a resonant Pi network, where the network Q value can be chosen to be 10 or so.

The L network does not have that degree of freedom.

For 50 to 75 ohms, with a shunt capacitor on the 75 ohm side, the reactance will be roughly -j 106 ohms, so the current through the capacitor is slightly less than the current delivered to the 75 ohm load (same voltage).

**Wallace Gasiewicz**:

I would just use an approximate Toroid Transformer.

4 turns primary and 5 turns secondary on the transformer

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