wb6bcn
11-28-2002, 07:58 PM
THE DECIBEL
Alexander Graham Bell is the creator of this commonly used, and misunderstood scientific unit. It is a unit that has no constant, but it is easy to calculate the answer. It is just as easy to get the wrong answer. The reason is some of the functions use log(10) x 10 (value in/value out) and others use log(10) x 20 (value in/value out).
The decibel is used to give an easy to handle number to define a vast change in levels of energy. Changes in voltage or linear units like potential energy use log(10) x 20 and power or units derived using the square law use log(10) x 10. In using the db it is important to make use of the sign preceding the value when referencing certain suffixes like the dBm or dBv. I don't mind putting my finger on 1 microvolt (-60 dBv) But 1 megavolt (+60 dBv) is another question.
There are some common well defined units, but most of the time you are dealing with relative units. With the db what you need to know the minimum and maximum value so you can calculate the difference, then use the proper log function to get the result.
The dB is used to calculate change in any energy unit. Like the relationship of 1 horsepower to 1,000,000 horsepower is 60 db change in horsepower, it is a unit of power.
The db has many suffixes. Probably the most common unit is dBvu. This is the unit established by the tape recording industry to establish a standardized reference for recording of tapes. The Vu is for "Volume units" They relate to the density or concentration of magnetic energy on magnetic the tape on the recorded tape. It is the number of nano Webbers per meter on the tape. This number varies with the media. Full track, half track, stereo, four track, etc. Even between reel to reel and cassette or digital to analog are different values.
Bell most likely started with dBspl, That is sound pressure level. 0 dBspl is the threshold of hearing. Most
likely the average level, as some people have very acute levels and others can't hear a shout at 10'.
The ones we as hams may be interested in would be the following:
dBm: dB referenced to a milliwatt. The value most hams may use is 224 millivolts RMS or 290 millivolts peak. This is 0 dBm at 50 ohms.
Other common values are 274 Millivolts RMS or 387 millavolts peak (0 dBm at 75 ohms) and 1 volt peak or .775 volt RMS ( 0 dBm at 600 ohms). The higher the
ohmic value, the higher the voltage for 0 dBm.
dBw: dB referenced to a watt. 1 watt is also +30 dBm or -30 dBk. (0 dBk = 1 kilowatt).
dBv: 0 dBv is 1 volt
dBuv: This is a db referenced to a microvolt. 1 microvolt is -60 dBv.
dBi: This is a db referenced to an isotropic radiator. The isotropic radiator has no set value. It is a theoretical
perfect radiator. It radiates uniformly from any point on a sphere. It is used to show the relationship of radiant
power from one antenna and other radiant devices to others.
dB over half wave: This comparing the relative gain of an antenna compared to the gain 1/2 wave dipole.
dBs: No, this isn't for Direct Broadcast Satellite. It is dB as it relates to signal strength units. It is a reference
as to the absolute signal level at the antenna jack on the receiver. The S units start at 1 and go to 9. They origi-
nally equated to the familiar RST system used in CW contacts. With the RST system S1 is threshold of recep-
tion, and S9 is perfect copy.
Providing no QRN or QRM impaired the quality of reception the S meter indication should relate to specific
values of RF level of the distant station. Since QRM and QRN are seen the receiver as an RF signal they will
indicate on the S meter.
S unit Microvolts dBm
S-9+70 158000 -3
S-9+60 50000 -13
S-9+50 15800 -23
S-9+40 5000 -33
S-9+30 1580 -43
S-9+20 500 -53
S-9+10 158 -63
S-9 50 -73
S-8 25 -79
S-7 12.5 -85
S-6 6.25 -91
S-5 3.125 -97
S-4 1.5625 -103
S-3 .78125 -109
S-2 .390625 -115
S-1 .1953125 -121
There is no S-0
Alexander Graham Bell is the creator of this commonly used, and misunderstood scientific unit. It is a unit that has no constant, but it is easy to calculate the answer. It is just as easy to get the wrong answer. The reason is some of the functions use log(10) x 10 (value in/value out) and others use log(10) x 20 (value in/value out).
The decibel is used to give an easy to handle number to define a vast change in levels of energy. Changes in voltage or linear units like potential energy use log(10) x 20 and power or units derived using the square law use log(10) x 10. In using the db it is important to make use of the sign preceding the value when referencing certain suffixes like the dBm or dBv. I don't mind putting my finger on 1 microvolt (-60 dBv) But 1 megavolt (+60 dBv) is another question.
There are some common well defined units, but most of the time you are dealing with relative units. With the db what you need to know the minimum and maximum value so you can calculate the difference, then use the proper log function to get the result.
The dB is used to calculate change in any energy unit. Like the relationship of 1 horsepower to 1,000,000 horsepower is 60 db change in horsepower, it is a unit of power.
The db has many suffixes. Probably the most common unit is dBvu. This is the unit established by the tape recording industry to establish a standardized reference for recording of tapes. The Vu is for "Volume units" They relate to the density or concentration of magnetic energy on magnetic the tape on the recorded tape. It is the number of nano Webbers per meter on the tape. This number varies with the media. Full track, half track, stereo, four track, etc. Even between reel to reel and cassette or digital to analog are different values.
Bell most likely started with dBspl, That is sound pressure level. 0 dBspl is the threshold of hearing. Most
likely the average level, as some people have very acute levels and others can't hear a shout at 10'.
The ones we as hams may be interested in would be the following:
dBm: dB referenced to a milliwatt. The value most hams may use is 224 millivolts RMS or 290 millivolts peak. This is 0 dBm at 50 ohms.
Other common values are 274 Millivolts RMS or 387 millavolts peak (0 dBm at 75 ohms) and 1 volt peak or .775 volt RMS ( 0 dBm at 600 ohms). The higher the
ohmic value, the higher the voltage for 0 dBm.
dBw: dB referenced to a watt. 1 watt is also +30 dBm or -30 dBk. (0 dBk = 1 kilowatt).
dBv: 0 dBv is 1 volt
dBuv: This is a db referenced to a microvolt. 1 microvolt is -60 dBv.
dBi: This is a db referenced to an isotropic radiator. The isotropic radiator has no set value. It is a theoretical
perfect radiator. It radiates uniformly from any point on a sphere. It is used to show the relationship of radiant
power from one antenna and other radiant devices to others.
dB over half wave: This comparing the relative gain of an antenna compared to the gain 1/2 wave dipole.
dBs: No, this isn't for Direct Broadcast Satellite. It is dB as it relates to signal strength units. It is a reference
as to the absolute signal level at the antenna jack on the receiver. The S units start at 1 and go to 9. They origi-
nally equated to the familiar RST system used in CW contacts. With the RST system S1 is threshold of recep-
tion, and S9 is perfect copy.
Providing no QRN or QRM impaired the quality of reception the S meter indication should relate to specific
values of RF level of the distant station. Since QRM and QRN are seen the receiver as an RF signal they will
indicate on the S meter.
S unit Microvolts dBm
S-9+70 158000 -3
S-9+60 50000 -13
S-9+50 15800 -23
S-9+40 5000 -33
S-9+30 1580 -43
S-9+20 500 -53
S-9+10 158 -63
S-9 50 -73
S-8 25 -79
S-7 12.5 -85
S-6 6.25 -91
S-5 3.125 -97
S-4 1.5625 -103
S-3 .78125 -109
S-2 .390625 -115
S-1 .1953125 -121
There is no S-0