  We evaluate the spin-wave spectrum and dynamic susceptibility in a layered
superconductors with helical interlayer magnetic structure. We especially focus
on the structure in which the moments rotate 90$^{\circ}$ from layer to layer
realized in the iron pnictide RbEuFe$_{4}$As$_{4}$. The spin-wave spectrum in
superconductors is strongly renormalized due to the long-range electromagnetic
interactions between the oscillating magnetic moments. This leads to strong
enhancement of the frequency of the mode coupled with uniform field and this
enhancement exists only within a narrow range of the c-axis wave vectors of the
order of the inverse London penetration depth. The key feature of materials
like RbEuFe$_{4}$As$_{4}$ is that this uniform mode corresponds to the maximum
frequency of the spin-wave spectrum with respect to c-axis wave vector. As a
consequence, the high-frequency surface resistance acquires a very distinct
asymmetric feature spreading between the bare and renormalized frequencies. We
also consider excitation of spin waves with Josephson effect in a tunneling
contact between helical-magnetic and conventional superconductors and study the
interplay between the spin-wave features and geometrical cavity resonances in
the current-voltage characteristics.
