  We describe a new technique for constraining the radio loud population of
active galactic nuclei at high redshift by measuring the imprint of 21 cm
spectral absorption features (the 21 cm forest) on the 21 cm power spectrum.
Using semi-numerical simulations of the intergalactic medium and a
semi-empirical source population we show that the 21 cm forest dominates a
distinctive region of $k$-space, $k \gtrsim 0.5 \text{Mpc}^{-1}$. By simulating
foregrounds and noise for current and potential radio arrays, we find that a
next generation instrument with a collecting area on the order of $\sim
0.1\text{km}^2$ (such as the Hydrogen Epoch of Reionization Array) may
separately constrain the X-ray heating history at large spatial scales and
radio loud active galactic nuclei of the model we study at small ones. We
extrapolate our detectability predictions for a single radio loud active
galactic nuclei population to arbitrary source scenarios by analytically
relating the 21 cm forest power spectrum to the optical depth power spectrum
and an integral over the radio luminosity function.
