Some radiosonde signals are successfully decoded at distances well in excess of the specified range for the radiosonde.
Experience reported here relates to Vaisala RS41-SGM radiosondes which are specified at 60mW max transmitter output and range up to 350km.
Free space model of the radio path
Whilst the path is over land, the radio path at high altitudes approaches a free space path, albeit with some ground reflection effects at the receiver.
The transmit antenna is probably close to a vertical dipole in performance, lets assume that it has a gain of 2.13dBi in the path direction.
Receive antennas will commonly be omni directional, and fairly low directivity, so lets just assume rx antenna system gain 0dBi.
If the path was free space, the rx field strength would be 13.8dBµV/m, tx power -115.5dBm. This model ignores ground reflection gain and losses, but it is in the ball park. Continue reading Radiosonde propagation – is aircraft enhancement significant?