3 dB rule: A 3 dB gain means twice (x2) the power. A 3 dB loss means half the power. For example, a system with 40 watts of input power and a 6 dB insertion loss will only have 10 watts of output power.
dB: Decibel, a logarithm (equal to 10 times) ratio of the difference between two values. The Site Master uses dB to measure the ratio of sent signal energy to reflected signal energy.
Common values of dB to ratios: 0 dB = 1:1, 10 dB = 10:1, 20 dB = 100:1, 30 dB = 1,000:1, -30 dB = 0.001:1. or (1/1000):1.
dBm: An absolute measurement of power relative to 1 milliwatt. 0 dBm = 1.0 milliwatt, 10 dBm = 10 milliwatt, 30 dBm = (1 mW x 1,000) = 1 watt.
DTF (Distance to Fault): Measures the location and reflection size of impedance mismatches. This is typically a diagnostic measurement, not a pass/fail judgment measurement. DTF is used to identify and locate faults within an antenna system when the system is failing to meet the specified return loss/VSWR limits. DTF is also useful to verify the total length of a coaxial cable assembly.
Impedance: A measure of an RF components electrical resistance. Measured in ohms (Ω). In most cable and antenna systems the standard impedance is 50 Ω.
Insertion Loss (Cable Loss): Measures the total amount of signal energy absorbed (lost) by the cable assembly. Measured in dB. S21 is another name for this measurement. This is often a pass/fail measurement.
Return Loss: Measurement in dB of reflected energy caused by impedance mismatch. May also be referred to as S11. S11 values are expressed as negative numbers, but Return Loss values are expressed as positive numbers since by definition the “Loss” expression implies a negative sign. The higher the value, the better the impedance match (think of a large negative number being less than a smaller negative number). 40 dB is nearly ideal. Only 0.01% of the total transmitted power is reflected if the Return Loss measurement value is 40 dB. 0 dB would be a complete reflection, or stated another way, 100% of the transmitted power is reflected back. Return Loss is typically a pass/fail measurement.
RF (Radio Frequency): Frequency of radio sine waves. RF range is 3 kHz to 300 GHz.
VSWR (Voltage Standing Wave Ratio): Another method to measure reflected energy caused by impedance mismatch. Expressed as a ratio of X:1. VSWR measures the voltage peaks and valleys. 1:1 would be a perfect match. A typical cable and antenna system would be around 1.43:1 or 15 dB Return Loss. The Site Master can measure either Return Loss or VSWR. Some carriers require that Return Loss is measured in VSWR. This is typically a pass/fail measurement.