Also known as: attenuation, loss
Attenuation is the reduction of signal strength as energy passes through a medium, cable, connector, or obstacle.1 It is expressed in decibels and subtracts directly from a power budget — a loss of 6 dB means only a quarter of the power survives.
How it works
Attenuation comes from two mechanisms: spreading, where a fixed amount of power is diluted over a larger area or volume, and absorption, where the medium converts RF energy into heat. Both are conventionally logged in decibels because losses then simply add to the gains and other losses in a chain, and a total is a running sum.
The main contributors in a receive path are:
- Cable loss. Coax loss rises with frequency and length. A cheap RG-58 run might lose several dB per 10 m at 400 MHz; low-loss cable and short runs cut this. Because a lossy cable ahead of the amplifier also degrades noise figure, the loss before the first amplifier is doubly harmful.
- Connector and component insertion loss. Every connector, filter, splitter, and switch adds a fraction of a dB to a few dB. These accumulate.
- Obstacles. Walls, foliage, and terrain absorb and scatter energy; loss through material generally worsens at higher frequencies, which is why UHF penetrates buildings less well than VHF.
- Atmospheric absorption. Above ~10 GHz, oxygen and water vapour absorb RF, and rain adds “rain fade”; below UHF this is negligible.2
The frequency dependence is the recurring theme: for most of these mechanisms, higher frequencies attenuate more. That single fact shapes band choice, cable choice, and site planning.
In practice
Attenuation is not always the enemy. A deliberate attenuator is inserted to protect a receiver from strong signals that would otherwise overload the front end and drive the ADC past full scale, spraying intermodulation across the band. Trading a few dB of wanted signal for headroom against a nearby transmitter is often a net win. Filters attenuate out-of-band energy for the same protective reason.
In a link budget, every loss is entered as a negative dB term: transmit power, plus antenna gains, minus path loss, minus cable and connector attenuation, yields the received power compared against receiver sensitivity. Knowing where the dB are going tells you where to spend effort — usually shortening the cable run and adding a preamp at the antenna.
Relevance to SDR
Free-space spreading is a specific kind of attenuation called path loss (or free-space path loss in the idealised case). For the operator, controllable attenuation lives in the feedline: keeping coax runs short and connectors clean minimises loss between antenna and receiver, preserving SNR. GopherTrunk sees only what survives the path and the cable — attenuation upstream of the SDR is loss it cannot recover.
Sources
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Attenuation — Wikipedia, the general definition, spreading-versus-absorption mechanisms, and causes of signal loss. ↩
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Atmospheric attenuation — Wikipedia, absorption by oxygen, water vapour, and rain at higher frequencies. ↩