The insertion loss of an antenna cable is the amount by which it attenuates a Radio Frequency signal. It is expressed in decibels (dB).
To calculate the cable loss enter:
- Cable Type (e.g. LMR 200)
- Length of Cable
- Frequency of Operation
Background
What is Insertion Loss?
An important property of RF cables used in antennas is its insertion loss. The insertion loss of a cable is directly related to its frequency of operation. The higher the frequency, the greater the insertion loss.
The greater the insertion loss, the more the RF signal attenuation.
In fact, Insertion Loss = RF Attenuation
Insertion loss is typically specified in deciBels (dB).
Why is attenuation so critical?
For a Radio Receiver, the attenuation directly impacts its noise figure and therefore noise floor.
As the attenuation increases the receiver’s sensitivity decreases (the numerical dBm value that represents the smallest detectable signal increases).
As the receiver’s sensitivity is degraded, it is unable to detect weak signals. For this reason, it is important to keep the cable length to a minimum.
For a fixed length of cable, the loss increases with operating frequency. This means that a three meter length of antenna cable will cause greater reduction in the sensitivity of Wi-Fi (operating at 2400 MHz) relative to the sensitivity of an FM Radio (operating at 98 MHz)
For a Radio Transmitter, the attenuation reduces the power radiated out from the antenna. This in turn reduces the operating range of the communication equipment. The impact can be significant.
Insertion loss of 3 dB reduces the range by a factor of 2. In other words, if you could reach 10 miles, now with the attenuation you can only reach 5 miles.
Impact of cable material
The material with which a cable is built determines the amount by which it attenuates a signal. LMR cables [1] for instance come in different types – from LMR 195 to LMR 900. LMR 900 is the best quality while the quality of LMR 195 is relatively poorer.
If you want to use an antenna for Helium (operating below 1 GHz) LMR 400 is of adequate quality. For Wi-Fi, depending on cable length requirements, it might make sense to consider a higher quality cable such as LMR 600 or even LMR 900.
Use the cable loss in the Antenna Range Calculator. Remember to also include the loss due to connectors and adapters (which is not accounted for here) but may be significant as the frequency increases.
If you would like to see a specific cable type added to this list, please let us know.
How is Cable Loss Measured?
Cable loss is typically measured using one of the following methods:
- With a Signal generator and a Power Meter or Spectrum Analyzer. A signal generator is used to inject a Continuous wave (CW) signal at one end of the cable with a power of X dBm. The power at the other end is measured with the power meter or spectrum analyzer. Let’s say it’s measured to be Y dBm. Then the cable loss is X – Y. Units are dB, Here we can use the dBm dB calculator.
- With a Network Analyzer. The S parameter S21 (power measured at port 2 due to signal injected at port 1).
Example Calculation
WiMax networks [2] operate at frequencies from 2.3 GHz to 5 GHz.
- 100 feet LMR 400 cable has a loss of 6.5 dB at 2.3 GHz and 10 dB at 5 GHz.
- 10 feet of LMR 400 has a loss of 0.65 dB at 2.3 GHz and 1 dB at 5 GHz
For short cable runs, the insertion cable loss difference is negligible at 5 GHz relative to 2.3 GHz. However for long cable runs, the loss can be significant.
References
[1] LMR Cable by Times Microwave
[2] WiMax networks on Wikipedia