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Clear Springs Press |
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Clear Springs Press |
When comparing antenna performance specifications and reports, understanding the meaning of decibels and the nature of the S meter is helpful.
Power and antenna gain measurements are expressed in dB. The decibel (dB) is a logarithmic unit that indicates the ratio of a power measurement relative to a reference level. A ratio in decibels is ten times the base 10 logarithm of the ratio of two power quantities.
dB=10xlog(P1/P2)
A change in power ratio by a factor of 10 is a 10 dB change. A change in power ratio by a factor of two is a 3 dB change.
Comparison of antenna gain is generally referenced either to a standard resonant dipole (dBd) to an isotropic antenna (dBi). An isotropic antenna is a theoretical antenna in free space which uniformly distributes energy in all directions. A dipole in free space has a figure 8 radiation pattern. The dipole's gain in the direction of its pattern is 2.15 dBi compared to 0 dBi for the isotropic antenna.
0 dBd = 2.15 dBi
Real world dipoles don't live in open space, they live above a ground. The ground affects the antenna radiation pattern and relative gain. So taking a ground into account, the gain of a real world dipole can be as high as 8.5 dBi.
This matters because it is sometimes a source of confusion when comparing antenna specifications. Mixing up dB and dBi reference points and free space versus over ground can cause some confusion.
Most transmitters and transceivers have an "S" meter built in. Its purpose is to provide a relative indication of the strength of a received signal. The signal report ranges from S1 which is faint and barely perceptible to S9 which is very strong. The meter is supposed to be calibrated to read S9 when a 50uv signal is received. In practice, this standard is not always followed. Each S unit down from S9 is supposed to mean a reduction of 6 dB of signal strength.
The S meter will read in S units and dB above S9. Signal strength reports are given in S units or the number of dB's over S9, like "10 over 9" or "10 by 9" meaning 10 dB over S9. S9 is generally in the middle of the scale.
Because there is apparently a lot of variation in the way S meters are implemented and calibrated, one cannot accurately compare the S meter readings on different transceivers. Accurate measurement and comparison of received signal strength requires more test equipment than many amateurs have available. The S meter is still useful for rough comparison of signal strength between stations and antennas using the same receiver.
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