  High harmonic generation (HHG) has become a multipurpose source of coherent
XUV radiation used in various applications. One of the notable aspects of HHG
is its wide spectrum consisting of many harmonic orders. This might represent a
bottleneck in HHG utility for applications requiring a single wavelength. We
propose a method to generate radiation consisting of a single high-order
harmonic frequency employing Bessel-Gauss driving beam and a periodically
modulated gaseous medium. We validate it by analytical calculations and
numerical simulations. Our method provides a way to generate monochromatic
harmonic radiation directly from the source without the need for additional
monochromatizing optics. Thus, it represents a substantial enhancement of the
flux and simplification of the setup for numerous applications requiring
monochromatic short-wavelength radiation.
