Common RF Terms

Refer to Glossary for additional definitions of common RF terms that may be associated with the use of an S820E Site Master.

3 dB rule: A 3 dB gain means twice (x2) the power. A 3 dB loss means half the power. A system with 40 watts of input power and a 6 dB insertion loss will have only 10 watts of output power.

dB: Decibel, a logarithm ratio of the difference between two values (a logarithm ratio is equal to 10 times). 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.

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 or VSWR limits. DTF is also useful to verify the total length of a coaxial cable assembly.

Impedance: A measure of RF component 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. Although S11 values are expressed as negative numbers, Return Loss values are expressed as positive numbers because 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. A measured value of 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. The 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. A ratio of 1:1 would be a perfect match. A typical cable and antenna system would be around 1.43:1 (VSWR) 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.

Watt: Unit of measure for power.