Range is a system property, not a transmitter specification
The usable range of an RF listening device is determined by the complete radio link. Transmitter power matters, but so do antenna efficiency, operating frequency, receiver noise performance, modulation, bandwidth, local interference and the physical environment. Two devices with the same nominal RF output can therefore behave very differently in practice.
The link-budget idea
A link budget accounts for the gains and losses between transmitter and receiver. At a high level, engineers compare the power expected at the receiver input with the receiver sensitivity required for reliable demodulation. The difference is the available margin. More margin usually means better resilience to fading, obstruction and interference.
Free-space path loss
In unobstructed conditions, radio signals spread as they travel and the received power falls with distance. Free-space path loss is a useful reference calculation, but indoor and urban environments rarely behave like free space. Walls, reinforced concrete, metal furniture, vehicles and people introduce additional attenuation and multipath.
Why receiver sensitivity matters
Receiver sensitivity is the minimum signal level at which the receiver can still meet its required audio or data performance. A narrowband, well-designed receiver can often work with signals far below levels that would be obvious on a general-purpose RF detector. Digital systems may also show a threshold effect: performance can remain good until the signal-to-noise ratio falls below the demodulator requirement.
Antenna efficiency can dominate miniature systems
Small enclosures often force the antenna to be electrically short or placed close to batteries, wiring and other conductive structures. That can reduce radiation efficiency significantly. In miniature equipment, improving the antenna installation can produce a larger practical improvement than simply increasing transmitter output power.
Fade margin and real environments
A professional design should include margin for shadowing, orientation changes, multipath fading and local RF noise. A link that works only under ideal line-of-sight conditions is not a robust operational link. Range claims are therefore most meaningful when the test conditions and antenna arrangements are stated.
What to compare when assessing range
Rather than looking only at a headline distance, compare operating frequency, RF output, antenna type, receiver sensitivity, occupied bandwidth, modulation, installation environment and whether the quoted range is line-of-sight or representative of real buildings. This gives a much more realistic view of performance.