how to choose the capacitor to operate in wideband

dear friends

i am using a capacitor to tune the electrical length of the microstrip line. currently its operating on 2.1 GHz, i calculated through software that if capacitor value is 15 pF it will make it operate at 700 MHz.
when capacitor is on the microstrip operates on 700 MHz
when capacitor is off it works on 2.1 GHz.
so how i choose the capacitor this 15 pF capacitor operating range from market
im very much confused

Simply chosse a small (0603 or below) standard SMD capacitor with NP0 dielectrica. Check the actual resonance frequency and correct the capacitance or microstrip length to tune the circuit, if necessary.

Try to select your lumped element size below the %5 of the wavelength that your circuit operates. %5 is not obligation but it is well accepted to transition from lumped to distributed analysis. Don't forget to consider dielectric medium effect when calculating your operating wavelenght.

Kind Regards
ALP