For the characteristics of a radio link involving a transmitter, a receiver, their antennas, the associated circuits and the propagation medium, the following terms, definitions and notations should be employed:
Free-space basic transmission loss
The ratio, usually expressed in decibels, for a radio link between the power radiated by the transmitting antenna and the power that would be available at a conjugately matched receiver antenna input if the actual antennas were replaced by loss-free isotropic antennas located in a perfectly dielectric, homogeneous, isotropic and unlimited environment, the distance between the antennas being retained.
Here free space is defined as a perfect vacuum which may be considered of infinite extent in all directions, so that free space propagation is the propagation of a radio wave radiating in free space.
Basic transmission loss (of a radio link)
The ratio, usually expressed in decibels, for a radio link between the power radiated by the transmitting antenna and the power that would be available at a conjugately matched receiver antenna input if the antennas were replaced by isotropic antennas with the same polarization as the real antennas, including the attenuation effects on the propagation path, but with the effects of obstacles close to the antennas being disregarded.
Lb =Lbf + Lm dB
where Lm is the loss relative to free space
NOTE – The loss relative to free space, Lm, may be divided into losses of different types, such as:
- absorption loss (ionospheric or atmospheric gases, precipitation, clouds, etc.);
- diffraction loss as for ground waves;
- effective reflection or scattering loss as in the ionospheric case including the results of any focusing or defocusing due to curvature of a reflecting layer;
- polarization coupling loss; this can arise from any polarization mismatch between the antennas for the particular ray path considered;
- aperture-to-medium coupling loss or antenna gain degradation, which may be due to the presence of substantial scatter phenomena on the path;
- beam spreading loss;
- effect of wave interference between the direct ray and rays reflected from the ground, other obstacles or atmospheric layers;
- clutter loss;
- building entry loss
Transmission loss (of a radio link)
The ratio, usually expressed in decibels, for a radio link between the power radiated by the transmitting antenna and the power that would be available at a conjugately matched receiver antenna input if actual antenna radiation patterns are substituted with no losses in the radio-frequency circuits.
NOTE 1 – The transmission loss may be expressed by:
L = Lb − Gt – Gr dB
where Gt and Gr are the directivity gains of the transmitting and receiving antennas, respectively, in the direction of propagation.
NOTE 2 – The effect of the local ground close to the antenna is included in computing the antenna gain, but not in the basic transmission loss.
The ratio, usually expressed in decibels, for a radio link of the radio-frequency power input to the terminals of the transmitting antenna and the resultant radio-frequency signal power available at the terminals of the receiving antenna.
𝐿𝑠 = 𝐿 + 𝐿𝑡𝑐 + 𝐿𝑟 dB
where Ltc and Lrc are the losses, expressed in decibels, in the transmitting and receiving antennas circuits respectively, excluding the dissipation associated with the antennas radiation, i.e. the definitions of Ltc and Lrc are 10 log (r’/r), where r’ is the resistive component of the antenna circuit and r is the radiation resistance.
Total loss (of a radio link)
The ratio, usually expressed in decibels, between the power supplied by the transmitter of a radio link and the power supplied to the corresponding receiver in real installation, propagation and operational conditions. This is determined at the input or at the output of the transmitting and receiving antenna feed lines. The feed lines may include radio-frequency filters or multiplexers.