Measuring Signal Strength - RSSI and SNR

RSSI is the received signal strength and it is measured as a grade value ranging from 0 to 100. Each grade value has an equivlient dBM. RSSI are negative and represent the level of signal loss which can be experienced between the transmitter and receiver with the receiver still being able to receive the signal correctly. Because RSSI is relative and based on grades the RSSI figures cannot always be compared between manufacturers equipment, one may use grades from 0 to 50 and another may use from 0 to 100.

Signal to noise ratio (SNR) is the amount of signal compared to the amount of surrounding noise. A higher value is better for SNR because there is more signal compared to noise.

Measuring Signal Strength:
Signal strength can be measured using two types of values; relative and absolute. The absolute value is a static measurement which is taken at a point in time, which compared to the relative measurement is based on a change from one value to another. Relative measurements are easier so you will most often see these used when describing the signal power.

The measures for absolute measurements include:
Watt - which is the energy spent / emitted / consumed per second. 1 Watt is 1 Joule of energy per second. 1 Watt is also 1 volt with 1 ampere of power. This is confusing and I don't fully understand it! However I'm hoping it will become more apparent through my wireless journey
Milliwatt (mW) - 1W = 1000 mW

The values used for relative measurements are:
Decibel (dB) - A decibel is either a positive or negative change.
Decibel referenced against an isotropic antenna (dBi) - An isotropic antenna doesn't really exist, it is a theoretical antenna which we use as a reference to compare one antenna to another. The isotropic antenna gives out spherical waves which are equal in all directions. Essentially the higher dBi, the higher the gain, and the more acute the angle of coverage.

Decibel referenced against a dipole antenna (dBd) - Dipole antennas do exist and give the classic wireless "doughnut" shape. dBi and dBd can be compared, dBi = dBd + 2.14

Decibel referenced against a milliwatt (dBm) - The arbitrator reference point is 1 milliwatt, so 1 milliwatt = 0dBm, or no change from the reference.

And now the maths, Calculating EIRP (Effective Isotropic Radiated Power):
If the dB is increased by 3, double the Transmit power (Tx)
If the dB is reduced by 3, half the Tx power
If the dB is increased by 10, 10x the Tx power
If the dB is reduced by 10, 1/10 the Tx Power

For example, if a radio transmitter emits a signal at 100mW and an amplifier introduces 3dB gain the signal will double to 200mW. Expanding this if the antenna has 10dB gain then the Tx signal will increase to 2000mW.

EIRP is the amount of signal (or power) leaving the antenna. This consists of connectors, cables, antenna and other factors.

EIRP = Tx Power (dBm) + Antenna Gain (dBi) - Cable Loss (dB)

As a rule of thumb for cable loss 50ft = 3.35dB loss, 100ft = 6.7dB loss.

Here is an example:
Start with Tx Power - +20dBm = 100mW
Add antenna gain - +10dBi = 1000mW
Total power - +30dBm = 1000mW or 1W
Subtract 100ft cable loss (power in half twice) - -6.7dB = 250mW
Total power minus cable loss = 23.3dB = 250mW

Phew that was a long post!