S820E UG PN: 10580-00343 Rev. H B-9
Measurement Review B-4 Common RF Terms
B-4 Common RF Terms
Refer to Appendix C, “Glossary” for additional definitions of common RF
terms that may be associated with the use of an S820E Site Master.
3dB 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.
0dBm = 1.0milliwatt,
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. S
21
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 S
11
. Although S
11
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.
To Next Page
Loading...
