Impedance matching and tuning of RF coil

[uzi kiko]

Hi everybody.
I am trying to understand the concept of impedance matching and tuning of RF coil.

As far as I understand there are 2 stages:
At the first stage I should match the coil to the source (Let's say with L network method) and the second stage is to tune coil to the desirable frequency (tuning = tuning the coil so the desirable frequency will be the resonant frequency) using a tuning capacitor.

Now at the first stage I am matching the coil circuit including the tuning capacitor but at the second stage I am changing the capacitor value. But will not this change affect on the impedance matching?

Thanks a lot

 

[Jeff Rosenbury]

Hi.

For a simple tank circuit the center frequency will be: f = 1 ÷ 2π√LC.

This is the frequency where the inductor, AKA coil, (L) and the capacitor (C) have the same impedance.
In the first stage we select a coil that exactly matches our desired frequency. However manufacturing variances mean we will likely be off by a little. So we need to adjust a component a tiny fraction. It doesn't really matter which we adjust. You seem to have an adjustable capacitor, so adjust that. (Adjustable inductors are usually slightly non-linear, so frowned upon by high end audiophiles).

 

[sophiecentaur]

we select a coil that exactly matches our desired frequency.

If the match is for a transmitting antenna then the design will need a good fractional bandwidth so the centre frequency of the matching network may not need to be particularly accurate - it will be a Low Q system, radiating as much Power as possible.