Step 1: Understand the UHF frequency band.
UHF stands for Ultra High Frequency. The UHF band covers frequencies from $300\,\text{MHz}$ to $3\,\text{GHz}$, corresponding to wavelengths of $10\,\text{cm}$ to $1\,\text{m}$. These are the frequencies used by television broadcasting, mobile phones, and radar.
Step 2: Recall the different modes of electromagnetic wave propagation.
(i) Ground/Surface waves: follow the Earth's curvature; effective for low frequencies (below $2\,\text{MHz}$). (ii) Sky waves: reflected by the ionosphere; effective for medium and high frequencies ($2\,\text{MHz}$ to $30\,\text{MHz}$). (iii) Space waves (line-of-sight): travel in straight lines; used for very high and ultra high frequencies (above $30\,\text{MHz}$).
Step 3: Analyze why ground waves cannot carry UHF.
Ground waves are significantly attenuated at higher frequencies because the Earth absorbs more energy from high-frequency waves. They are practical only for frequencies below about $2\,\text{MHz}$. UHF is far too high for effective ground wave propagation.
Step 4: Analyze why sky waves cannot carry UHF.
The ionosphere reflects radio waves only up to about $30\,\text{MHz}$ (HF range). UHF waves at $300\,\text{MHz}$ to $3\,\text{GHz}$ simply pass through the ionosphere and cannot be reflected back to Earth.
Step 5: Confirm that space waves are used for UHF.
Space waves (also called line-of-sight waves) travel in straight lines from transmitter to receiver. They are used for frequencies above about $40\,\text{MHz}$. Since UHF falls in the range $300\,\text{MHz}$ to $3\,\text{GHz}$, UHF signals propagate as space waves. This is why UHF TV antennas and mobile towers must be in direct line-of-sight, and repeater stations are needed over long distances.
Step 6: State the final answer.
UHF frequencies propagate by means of: \[ \boxed{\text{Space waves (line-of-sight propagation)}} \]