What does db mean?

[Pommie]

So glad I skipped dB stuff when I was a teenager.

Mike.

 

[rjenkinsgb]

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

That seems to be a calculation for the signal out of that antenna, based on it being at your address / location; a combination of the transmitter signal level, distance and path type - line of site or otherwise.

eg. The first one, WJFB, has a 950KW transmitter and is less than 20 miles from you, line-of-site, so a very! strong signal out the antenna.

NM is noise margin, at a guess - the signal is 56dB above noise level.

 

[JimB]

Guys, we have been all through this, twice in the last year:

8 bay TV Antenna question

Assuming that the feed impedance of one bay is 300 ohms, then you would run 300 ohm line to where they join, and when you join them in phase, the impedance would be 150 ohm as you say. If you want 300 ohms at the junction (to use your standard balun), then what you could do is make the last...

www.electro-tech-online.com

TV Station signal strength

I am trying to learn TV station signal strength from the TVFool.com online chart? 30 years ago I dropped cable TV then built an antenna using Amateur Radio antenna handbook. Next I bought 2 factory made antennas CM4228 the 8 bay bow tie antenna and Yagi B91X antenna. I decided CM4228 works...

www.electro-tech-online.com

JimB

 

[gary350]

This TV antenna claims -20dB.

 

[Externet]

Hi. There is a chance that has a loss of 20dB at its back direction (-20dB) but appears more like the minus sign is another very frequent goof from the marketing genius that knows nothing about the product. The forward gain may be +20dB for that parabolic shown. Most of the energy radiated (or collected) is not wasted in other directions, like having a lens/mirror placed near a light bulb.

Similar:

Where the dash 24dB (-24dB ) is not a minus sign.

 

[schmitt trigger]

Can you drive a car at -20 dB? Of course you can! But relative to a value.

Let's say that a highway has a maximum speed of 120 km/hr. Let's set that value to 0dBkh

Let's say that you want to drive slower than the maximum speed, let's say at -20dBkh. In that instance this means you would be driving at 12 km/hr.

Now, let's say you are over-speeding and a highway patrol stops you. You will argue: "Sir, I was only doing +3dBkh !!"
Which means you were actually driving 169 km/hr.

 

[gary350]

20 dB is twice the power of 10 dB that is all dB is.

 

[Nigel Goodwin]

20 dB is twice the power of 10 dB that is all dB is.

No, you haven't understood it at all - and it's also completely different for power and for voltage.

Aerial wise an 8 element yagi (such as I used to use on the 2m amateur band) gives 10dB (ten times) power gain. So you feed it with ten watts, you get 100W out of the aerial. Even better it does the same the other way - so if the other end is using ten watts, you're effectively increasing their power to 100W as well.

If you're BOTH using 8 element yagis, the effective output of both is then 1000W.

 

[KeepItSimpleStupid]

It can be power or Voltage typically. See https://en.wikipedia.org/wiki/Decibel It must have a reference.

 

[JMW]

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.

Actually it is based on an imaginary "isometric" antenna which radiates equally in 3 dimensions. A dipole is good for 2

 

[atferrari]

In spite of your request Externet , it seems the deciBel is not going to be abolished any time soon. Sorry!

 

[schmitt trigger]

Exactly!

The decibel is one of the most useful tools in EE, and anyone working or playing with electronics better be fully proficient with it, just like Ohm's law.

 

[Nigel Goodwin]

Exactly!

The decibel is one of the most useful tools in EE, and anyone working or playing with electronics better be fully proficient with it, just like Ohm's law.

As one of my college teachers always said "only people who don't know what they are doing use decibels"

 

[JimB]

As one of my college teachers always said "only people who don't know what they are doing use decibels"

As you often tell us at times like this.

I once came across a little proverb, went something like this:
"He who knows little, repeats it often"

JimB

 

[gary350]

I understand this chart. Every time dB goes up by 10, power goes up by power of 10. 10 dB is 10 power ratio. What I don't understand is how an antenna manufacture can claim their antenna is, 5dB or 10dB or 20dB?

 

[Nigel Goodwin]

I understand this chart. Every time dB goes up by 10, power goes up by power of 10. 10 dB is 10 power ratio. What I don't understand is how an antenna manufacture can claim their antenna is, 5dB or 10dB or 20dB?

As has been explained endlessly, dB is simply a ratio so it's 10dB with respect to something else.

Generally (with any decent manufacturer) the reference point for aerial is a simple dipole - so the 8 element yagi I mentioned earlier gives 10dB gain over a dipole. As has also been mentioned endlessly, there's nothing 'magic' about gain in an aerial, a vertical dipole is omni-directional, so receives equally well from all directions. By adding (correctly sized and placed) a reflector and directors (i.e. making it into a yagi) the aerial becomes directional - this is what gives you gain, and the more directors the greater the gain - and the narrower the acceptance angle.

Once you get to high frequencies, parabolic dishes become a viable size - and their focusing design means much higher gains, and much narrower acceptance angles. Just as with yagis their gain comes from narrowing the acceptance angle, and the larger the dish the narrower the acceptance angle - which is why large satelite dishes are harder to align than small ones.

 

[Externet]

Be aware the chart you understand above shows that a 10 decibel step corresponds to a 10 times power ratio. But not for volts ratio.

Replace in your mind the word 'gain' with 'magnification' to try another point of view.

Was the word 'negative' on the title of this thread deleted ?

 

[KeepItSimpleStupid]

What I don't understand is how an antenna manufacture can claim their antenna is, 5dB or 10dB or 20dB?

QUOTE: https://www.everythingrf.com/community/what-is-antenna-gain

Antenna gain tells us the power transmitted by an antenna in a specific direction as compared to an isotropic antenna. This specification describes how strong a signal an antenna can send out or receive in a specified direction. Antenna Gain is a more important specification than directivity, as it takes in account all the losses.

Antenna gain = Directivity x Efficiency

See, relative to something.

 

[gary350]

Be aware the chart you understand above shows that a 10 decibel step corresponds to a 10 times power ratio. But not for volts ratio.

Replace in your mind the word 'gain' with 'magnification' to try another point of view.

Was the word 'negative' on the title of this thread deleted ?

Someone on the antenna forum said that the antenna in the photo is -20dB. I think that person made a mistake. He probably ment to say 20dB several people corrected him. So I edited this thread so it is no longer -20dB.

So the claim is 8 element yagi is 10 dB, if I build an 8 element yagi then it will be 10 dB no matter what, size, shape, Hz it is. But everyone says it needs a starting point reference but no one knows the starting point. Just for my own interest I would like to know the gain of an antenna that I build or buy. I bought a factory antenna to compare to my home made 8 bay field strength meter shows both antennas are very near equal field strength meter is 98% on almost all channels. Another interesting thing both antennas get a stronger signal after dark, WHY?

 

[KeepItSimpleStupid]

Many transmitters have to cut their operating power at night.

Radio propagation - Wikipedia

en.wikipedia.org