  Bandgaps formed in a piezoelectric transducer with large coupling,
$k_\textrm{eff}^2$, qualitatively modify its electrical response. This regime
in which electrical loading strongly couples forward and backward waves occurs
in thin-film lithium niobate which has recently become available and amenable
to nanopatterning. In this work, we study how resonance and bandgaps modify the
design and performance of transducers and delay lines in thin-film lithium
niobate. These films are an attractive platform for GHz frequency applications
in low-power RF analog signal processing, optomechanics, and quantum devices
due to their high coupling, low loss, excellent optical properties, and
compatibility with superconducting quantum circuits. We demonstrate aluminum
IDTs in this platform for horizontal shear (SH) waves between $1.2$ and $3.3$
GHz and longitudinal waves between $2.1$ and $5.4$ GHz. For the SH waves, we
measure a piezoelectric coupling coefficient of $13\%$ and $6.0$ dB/mm
propagation losses in delay lines up to $1.2$ mm with a $300$ ns delay in air
at room temperature. Reflections from electrical loading when
$k_\textrm{eff}^2$ is large lead to a departure from the impulse response model
widely used to model surface acoustic wave devices. Finite element method
models and an experimental finger-pair sweep are used to characterize the role
of resonance in these transducers, illuminating the physics behind the
anomalously large motional admittances of these small-footprint IDTs.
