capacitor selection for antenna tuning

Hello Guys,

In antenna tuning for EM Prox (LF) can we use X7R capacitors? They recommend NP0 capacitor only for antenna tuning... If I use X7R what will happen?

X7R is a class II dielectric while NP0 is a class I dielectric. Class I will tend to have higher stability in terms of voltage/temperature, and generally lower losses. Also class I dielectrics tend to have less piezoelectric effects. But these classifications are quite broad and if you really want to be sure then you should look at specific devices and their datasheets.

Intended coupler bandwidth and respective frequency stability requirements will answer the question. I won't use X7R for any kind of resonant circuit.

Yes, for high frequencies (>30MHz) X7R capacitors are not recommended for resonators, but for lower frequencies they should be fine.

http://www.avx.com/docs/catalogs/cx7r.pdf

What frequency?

The term "proximity card" refers to the older 125 kHz devices as distinct to the newer 13.56 MHz contactless smartcards.

You can also use diodes used as a varicap for fine tuning which increase in pF size with power rating but wide tolerance for any diode.

Definitely! X7R normally used for digital and power area not RF/antenna field.