The high frequency properties of underdamped Josephson junction arrays consisting of parallel biased triangular cells having a Josephson contact in each branch are studied. At the applied magnetic field corresponding to half a flux quantum in every cell, the junctions transverse to the bias current ("horizontal" junctions) oscillate in phase. The radiation emitted by these junctions is measured using a double-side band superheterodyne receiver in the 80-120 GHz range. It is found that the radiation has a spectral linewidth of about 80 MHz, which is close to that expected from the RSCJ model for the DC biased "vertical" junctions. Our experimental data confirm that both the frequency and the linewidth of the horizontal junction oscillations are controlled by the rotating vertical junctions. In the double row array, when the two rows are simultaneously driven at the same voltage state, we observe a weak radiation peak, which can be explained in terms of a "mixed-mode" dynamics. Numerical simulations that predict this mechanism are in good accord with experiment.