# What is an antenna amplifier ? How does it add gain?

I don't know what exactly does antenna amplifier mean. Is it possible to increase the gain of a Tx antenna ? Where do I connect the antenna amplifier to it ?

If, for example, the Tx has a power of 20 dBm , antenna has a gain of 10 dBi & antenna amplifier "repeater" has a gain of 60 dB "like common cellular repeaters" , what does this gain mean ? Does it mean the antenna gain will be 10+60=70 dBi ? Or does it mean the Tx power will be 20+60=80 dBm ? I am confused !

Another example for Rx antenna, if we have a receiver with a sensitivity of -114 dBm and Rx antenna has a gain of 10 dBi & we connect it to a 40 dB amplifier, does it mean the total sensitivity will be -114-10-40= -164 dBm ? (please ignore the SNR)

What is the weakest possible signal at Rx antenna to be amplified ?

Last example , just to understand what the rule of repeater and its gain is.

If we use a Tx that has a power of -30 dBm at 1000 m, antenna has a gain of 10 dBi, & at 1000 m distance there is a repeater that has a gain of 60 dB, how do I calculate the total gain in this situation ?

Should the total gain at repeater be 10 "antenna" +60 "repeater" ?

Or should we just calculate the resulting power at repeater instead of its gain ? Do we write (-30 "Tx"+ 60 "repeater")"total power" + 10"antenna" ?

Sorry for my long question, I am a beginner , I want to understand .

Thanks.

An antenna has a gain set by its construction. For instance, a simple wire forming a dipole will have a gain of about 2dB whereas a dish might have a gain of 20dB. The lower the antenna gain the less directional will be the transmitted electromagnetic wave. Higher gain antennas such as dishes have to be pointed in the correct direction for them to be effective but, give you a bigger signal when receiving or concentrate the power output more when transmitting.

You can't affect antenna gain by adding an amplifier.

if "for example" the Tx has a power of 20 dBm , antenna has a gain of 10 dBi & antenna amplifier "repeater" has a gain of 60 dB "like common cellular repeaters" , what does this gain mean ? is it mean the antenna gain will be 10+60=70 dBi ? or it mean the Tx power will be 20+60=80 dBm ? I am confused !

• The antenna gain is fixed by it's construction
• Gain is a word that describes how much something amplifies a signal.
• Tx power is 20dBm irrespective of antenna gain BUT this power is projected into a tight beam on higher gain antennas therefore it is equivalent to an isotropic antenna (transmits the power equally in all directions) fed from 20dBm increased by 10dB = 30dBm.

another example for Rx antenna , if we have receiver has a sensitivity of -114 dBm and Rx antenna has a gain of 10 dBi & we connect it with 40 dB amplifier , should it mean the total sensetivity will be -114-10-40= -164 dBm ?"ignoring SNR please"

It's impossible to ignore SNR - if a receiver has an input sensitivity of -114dBm then pre-amplifying the signal by 40dB will likely increase noise and signal together and this will mean exactly the same SNR and no net benefit.

if we use Tx has a power of -30 dBm at 1000 m, antenna has a gain of 10 dBi & at 1000 m distance there is a repeater has a gain of 60 dB , how to calculate the total gain at this situation ?

You need to draw a sketch of something here - where is the final receiver (and I don't mean the repeater). Maybe something like this but with ALL the information on: -

• For the last part, consider What is a link budget, and how do I make one? which lists some of the factors that go into a proper link budget.
– user
Commented Oct 21, 2014 at 14:55
• For the "Tx power is 20dBm irrespective of antenna gain" bullet point I feel it should be pointed out that the total radiated power remains the same, but the angle across which the power is radiated is affected by the antenna's directionality ("gain").
– user
Commented Oct 21, 2014 at 14:56
• @michael. That's pretty much what I did say dude. Regarding your 1st comment, was that aimed at the guy asking the question? Commented Oct 21, 2014 at 16:25
• ok thanks, but there is some parts I didnt understand . the sketch you got is the exactly what I mean by my question ,but my question about the total budget of this system. the repeater has a gain of 60 dB but the output of only 30 dBm , that is the point I didnt understand. to calculate the total budget should be adding the gain of the repeater & ignoring its output power ? or we should adding only output of repeater and ignoring its gain ? Commented Oct 21, 2014 at 17:00
• @manabduo I like you - you have really forced your self to come to terms with quite difficult stuff. You probably will need for this to sink in and come back to it several times beofre it's finally there so good luck. Commented Oct 21, 2014 at 21:02