# RF link budget and antenna efficiency

Thread Starter

#### bug13

Joined Feb 13, 2012
1,995
Hi guys

I have a question about link budget and efficiency.

Say I have two transceivers, and have a link budget of 100dBm, both antennas have 100% efficiency. If I replaced one antenna with another antenna which is 50% efficient.

How much link budget do I have now, 50dBm or 97dBm?

Thanks guys!

#### mrblucor

Joined May 30, 2008
1
Bug13,

I think you meant to say that you have a link budget of 100 dB. 100 dBm = 10 Million Watts!!!
dB is a relative measure of power. For instance, if I cut the power in half, I'm down 3 dB.
If I double the power, that would be +3 dB.

-3 dB = half, +3 dB = double
10 dB is a factor of ten.
20 dB is a factor of 100
30 dB is a factor of 1000 etc.

-JB

#### kubeek

Joined Sep 20, 2005
5,757
What does it mean for an antenna to be 50% efficient? Surely it does not mean that it cuts an arbitrary number of dB in half (which it even doesn't know about), so the latter is most likely the right answer.

#### Kermit2

Joined Feb 5, 2010
4,162
Since 50% of something is one half whatever 100% is.

Then you would be down 3db.

Thread Starter

#### bug13

Joined Feb 13, 2012
1,995
Bug13,

I think you meant to say that you have a link budget of 100 dB. 100 dBm = 10 Million Watts!!!
dB is a relative measure of power. For instance, if I cut the power in half, I'm down 3 dB.
If I double the power, that would be +3 dB.

-3 dB = half, +3 dB = double
10 dB is a factor of ten.
20 dB is a factor of 100
30 dB is a factor of 1000 etc.

-JB
Hi JB

Thanks for correcting me, yes it should be dB

Thread Starter

#### bug13

Joined Feb 13, 2012
1,995
What does it mean for an antenna to be 50% efficient? Surely it does not mean that it cuts an arbitrary number of dB in half (which it even doesn't know about), so the latter is most likely the right answer.
I think I should have said 50% radiation efficiency.

Thread Starter

#### bug13

Joined Feb 13, 2012
1,995
Since 50% of something is one half whatever 100% is.

Then you would be down 3db.
Thanks for confirming this.

#### bertus

Joined Apr 5, 2008
21,546
Hello,

The link budget is the difference between the output level of the transmitter and the sensitivity of the receiver.
When the transmitter has an output power of +3dBm and the reciever has a sentitivity of -97dBm the linkbudget is 100dB.
The output power of the transmitter and the sensitivity of the receiver should be given in their datasheets.
When you use an antenna with less gain, the linkbudget will be lower by that gain calculated in dB's.

Bertus