# Why there is minus sign in my tablet phone signal strength?

1. Oct 6, 2011

### xugi

I have tablet phone samsung galaxy tab...
when I check the signal strength, it shows -82 dbm...

I thought it is produced by the Signal PA (Power Amplifier) on the PCB...

According to what I know, the signal from the handphone is so weak to reach the nearest terrestrial station on the earth, so it is amplified by the Power Amplifier...

But when it show -82dbm, it means deamplified...

Any correction for this? What is the purpose making it with minus sign?

2. Oct 6, 2011

### davenn

That is most likely showing the received signal strength ( thats is the signal level received from the cell tower transmitter)

thats not a particulatly strong signal, but most receivers would have a sensitivity down to ~ -110 to -120 dBm

interestlingly, I havent seen a cellphone giving an actual signal level report like that
usually just the usual "S" meter 1 - 4 bars etc Tho I am used to working with RF test equip that does give that type of indication

cheers
Dave

3. Oct 6, 2011

### xugi

Oh that is receiving signal...
Yeah it's quite interesting to have the setting in Samsung Galaxy Tab to know the signal strength in numbers...

What do you mean usual "S" meter?

N when it signs minus, does it mean the signal power is decreased when coming to the receiver?

4. Oct 6, 2011

### f95toli

dBm is short for "dB referenced to 1 mW".
Now, whenever something is given in dB, a minus sign simply means that the number which was converted to dB was smaller than 1; simply because 0.1=10^(-1)

Hence, -82 dBm is just another way of writing 8 pW (p=pico)

5. Oct 6, 2011

### xugi

Oh I see, so It means the receiving signal to the receiver is about 8pW...

Does it mean when the 8pW becomes higher then the signal is better to receive data/phone?

6. Oct 7, 2011

### davenn

a -40dBm signal is 40 dB stronger than -80dBm
The level of a transmitter signal drops off very quickly as it moves away from the antenna
have a look in wikipedia under Inverse square law ... the first few lines...

"In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity."

so by the time you get even a few 100 metres away from the antenna, the signal strength has already dropped to a few microWatts
There are formula to work it all out, I wont confuse you with all that at the moment.
But for radio comms activities I do I need to be abe to work those signal levels out at a given frequency so I can work out what is called a "Link Budget" ... briefly....

I have a certain transmitter power out, a loss in the coax cable, gain in the antenna, loss in the path between the TX antenna and the RX antenna coax loss again at the receive end
and then gain in the receiver circuit. I can work out if I am going to successfully hear a signal over a given distance path.

in that last post of mine I commented about receiver sensitivity usually down to ~ -110 to -120 dBm .... that is a VERY weak signal.

"S" meter = Signal strength meter on communications radios, the signal is given in S points
each "S" point on the meter is supposed to be an increase of 6dB. In reality it is never that good .... very few comms receivers have accurately calibrated meters.

In communications, like amateur radio, we give readibility and signal strength reports eg 5 x 9 .... thats readibilty 5 signal strength (S meter reading) of 9
A mobile phone's S meter is usually a bargraph display that you would be familiar with ... 1 bar weak signal, 2 bars medium strength signal, 3 bars full signal ....

Hopefully you find that useful and not too confusing... just ask Q's for clarification :)

cheers
Dave

7. Oct 9, 2011

### xugi

Very complete explanation...
It's cool, means if I know the signal strength, in ideal condition I can estimate the nearest tower surround me!
What an amazing Physics!

Thank you very much to davenn and f95toli :D

8. Dec 2, 2011

### Offline-User

if you have an android phone go to:

setting > about phone > status

to find your signal strength, mine at the time of writing this is "-97dBm" and "8 asu"

ASU is ACTIVE SET UPDATES or the rate at which your phone is able to update its location to the towers/radios.

on a BlackBerry hold down the "ALT" key on your BlackBerry and type the letters NMLL in this order to see your dBm