What does db mean?

[gary350]

What does negative db mean?

Can you drive a car negative miles per hour?

An antenna is -20 db what does that mean?

 

[sagor1]

-20dB means you lose 20dB worth of signal. A negative dB is usually a loss when comparing two ends of something, etc.
-20dB can also be a sensitivity level. That is, you need at least a signal of -20dB below a reference into something to make it work properly.
Most antennas are positive dB gain. Any antenna that has -20dB is basically a dummy load, won't radiate much at all. A 100W signal into a -20dB "antenna" would radiate 1W out.

 

[Nigel Goodwin]

What does negative db mean?

Can you drive a car negative miles per hour?

Yes, you drive it in reverse

An antenna is -20 db what does that mean?

dB's are simply a ratio - positive is a higher output than input (gain), and negative is a lower output than input (attenuation).

dB's by themselves are meaningless, as you need to know what the reference is.

 

[Externet]

dB is ratio. Is not a unit like a litre.

When pointer shows "-10" that means the signal is 10 decibels lower than "0"; which can be whatever the manufacturer of the instrument or its calibration -or not- decides it to be, usually hardly accessible information if at all. "0" db can be 25 Watts; 100Volts, 1 milliwatt... whatever the manufacturer wants to refer to when designed the device with a dB meter.

 

[AnalogKid]

As above, the decibel (one-tenth of a bel), is a ratio, so it is a dimensionless number. The B is capitalized because bel is an abbreviation of a proper name, Bell. Alexander Graham Bell developed the dB as a way to describe audio signal attenuation or amplification, and hearing sensitivity. By definition, the dB is a logarithmic scale based on 10 (rather than e).

It always describes a signal amplitude *with respect to* some reference value (0 dB). Going back to its audio roots, 0 dB equals 1 milliwatt of electrical energy dissipated in 600 ohms. Using Watt's Law, this works out to 0.7746 Vrms, or approx 1.1 Vpeak acros the resistance. Because the reference is 1 milliwatt, this is sometimes written as 0 dBm. The metering system for the industry the signal is in might have a different opinion. For example, in broadcast television in the US in the 70's ("back in my day"), VU meters were calibrated such that 0 dB on the meter equaled a signal amplitude of +8 dBm (+8 dB with respect to the 1 milliwatt reference). This gets into how a VU meter actually works, needle ballistics, terminating impedances, and other borscht.

Decibel - Wikipedia

en.wikipedia.org

Many industries have their own "standard" reference value, and a measurement based on that standars usually is noted by something after the B (dBv, dBV, dBA, dBu, etc.)

Decibel - Wikipedia

en.wikipedia.org

ak

 

[Externet]

Hi.

Alexander Graham Bell developed the dB

I thought it was named after him. A bunch of engineers at a telephony industry came up with a easier way to calculate signal losses in wiring by adding and substracting logarithms instead of multiplying because slide rules were not very friendly and not everyone had one.

0 dB equals 1 milliwatt

dB expresses gain or loss, not a level, value or magnitude. 0dB is no gain, no loss.
1 milliwatt can be expressed as 0dBm if loaded with 600 ohm.
0dB can be referred to anything you want it to be.

 

[JimB]

1 milliwatt can be expressed as 0dBm if loaded with 600 ohm.

Usually true in the audio world.

In the radio world 0dBm is 1mW in 50 Ohm.

As I sit here contemplating what I have just written, it occurs to me that a milli-watt, is a milli-watt, is a milli-watt.
So why should the circuit impedance make a difference?
Most (not all) power measurements are made by measuring the voltage and calibrating the display accordingly.

In a 600 Ohm circuit, 0.774v = 1mW
In a 50 Ohm circuit 0.223v -1mW

JimB

 

[AnalogKid]

dB expresses gain or loss, not a level, value or magnitude. 0dB is no gain, no loss.
1 milliwatt can be expressed as 0dBm if loaded with 600 ohm.

The decibel is a measure of the relative level of a thing - voltage, current, power, frequency-weighted noise, whatever. 0 dB corresponds to a ratio of 1:1 to the reference - it is the reference level of whatever the measure is. OK, rather than saying "0 dBm equals 1 mW / 600 ohms", how about "0 dBm equates to 1 mW / 600 ohms"? Or "0 dBm is defined as 1 mW / 600 ohms"? There has to be a semantically correct way to anchor the scale (ratio) to a real-world value.

0dB can be referred to anything you want it to be.

I know. That's why I included a link to a whole pile of them in alphabetical order.

ak

 

[Externet]

1mW on 75 Ohm, 110 Ohm, 300 Ohm, 8 ohm; whatever impedance, has also its own reference level.

 

[KeepItSimpleStupid]

An antenna is -20 db what does that mean?

I believe it's relative to a simple dipole antenna design. Now, since the antenna has a direction, it has a gain associated with that direction.
See **broken link removed**

So, relative to something. Units are important. People are dumb.

 

[KeepItSimpleStupid]

it does get to be a mess: https://urgentcomm.com/2009/06/01/good-to-know/

but the cool thing is they add/subtract.

Take a look at this wierd thing. An 18 dB tap with ~1.8dB insertion loss.

The loss is dependent on frequency for one. Coax has a loos per foot with frequency. RG-6 is better than RG-59.

So, if I had 18 db coming in, I'd have 16.2 dB (18-1.8)coming out and I could add another tap and another.

Every tap I use. I lose 1.8 dB. The signal to my set sees 18 dB less than what's coming in.

Large installations might not use a home run approach although now days they are going to IPTV or TV over gigabit or higher Ethernet.
But early hotels would just string a cable and put 1 way taps along it's length. Some of the fancy one, would add a terminator if the TV cable was unplugged.

 

[Nigel Goodwin]

it does get to be a mess: https://urgentcomm.com/2009/06/01/good-to-know/

but the cool thing is they add/subtract.

Take a look at this wierd thing. An 18 dB tap with ~1.8dB insertion loss.

The loss is dependent on frequency for one. Coax has a loos per foot with frequency. RG-6 is better than RG-59.

So, if I had 18 db coming in, I'd have 16.2 dB (18-1.8)coming out and I could add another tap and another.

Every tap I use. I lose 1.8 dB. The signal to my set sees 18 dB less than what's coming in.

Large installations might not use a home run approach although now days they are going to IPTV or TV over gigabit or higher Ethernet.
But early hotels would just string a cable and put 1 way taps along it's length. Some of the fancy one, would add a terminator if the TV cable was unplugged.

That's how old relay systems worked, a high level main line with tap off boxes to individual properties. We used to run two of them years ago - valve based, and absolutely horrible!

 

[Externet]

...So, if I had 18 db coming in...

That is not a signal level 'coming in' That is a ratio.

 

[KeepItSimpleStupid]

That is not a signal level 'coming in' That is a ratio.

I'll bite. meaning your correct. How about 18 dBmV coming in.

 

[Externet]

18 dbmV is 7.94 millivolts. What do you want to do with that signal ?

 

[gary350]

An antenna manufacture advertises their antenna is 20db like that means something??? What does it mean???

TV fool shows my TV stations all in - db what does that mean row for signal Pwr ?

There is another row of NM db what is that?
?

 

[KeepItSimpleStupid]

TV Fool Report: Using The Noise Margin Column - The Cord Cutter Guide

TV Fool has long been the go-to site for seeing what TV signals you can pick up in your area. I went into how to read a tv fool report the easy way in an earlier post. For this post, we’ll get into some of the details if what a report is telling you. Specifically, …

thecordcutterguide.com

 

[Externet]

Kiss : IF you want to pass that 18dBmV = 7.94 mV signal trough the device described in post #11

Applying the signal to the left top port

and exiting the signal trough left bottom port 'out', the insertion loss of the device pictured being 1.8 dB; yields 18 dBmV minus 1.8 dB = 16.2 dBmV for a signal within 5 to 1000 MHz per specifications.

If instead; the signal applied to the top left port 'in' is exiting trough the bottom right tap '18dB', the level takes 1.8 dB loss for insertion of the pictured device and also an attenuation of 18dB from the device internal divider resistive? network for that attenuating port. Summing 1.8dB and 18 dB = 19.8 dB
Now the signal outputted at the right bottom port is the original 18 dBmV minus 19.8dB = minus 1.8dBmV = -1.8 dBmV

-1.8dBmV is 0.812 millivolts.
Then, the original signal of 18 dBmV applied has been attenuated by 19.8 dB trough the pictured device which is the same as from 7.94mV to 0.812mV if within 5 to 1000MHz per specifications.

 

[Externet]

TV fool shows my TV stations all in - db what does that mean row for signal Pwr ?

Hi.
NM (dB) means noise margin expressed in decibels.

A power level expressed in dBm has reference to one milliwatt on a 75 ohm impedance load.
(Zero dBm equals 1 mW.)

The signals shown at your 'tvfool' chart showing around minus 100dBm means 100.0 e-12 milliwatt or 0.0000000001 mW of power reaching your 75 ohm loaded antenna at your location for that distance from the broadcast station.

 

[Externet]

An antenna manufacture advertises their antenna is -20db like that means something??? What does it mean???

Can you show which antenna specifies that -20dB ? It is an unusual figure for an antenna. *

If the antenna shows specification of a more common figure; say 4 dBi or 4dBd, I can explain 4dBi is the gain that antenna has compared to an isotropic theorical radiator. Or, 4dBd gain when compared to a dipole antenna.

The gain of the antenna is like a lens placed in front of light : concentrates the light arrived (or emitted) onto a higher density beam in one direction at expense of not concentrating it in other directions.

* But an antenna assembly can have a -20dB (loss) Example : The antenna has a 'normal' +3dB gain and is connected to a very long coaxial feed line that presents 23dB of signal loss.
+3dB gain minus 23dB of loss yields -20 dB