the first thing what does gain mean

Gain is the power amplification of a system usually expressed in decibels.

Link budget, as the name implies is the power budget from one end of the link to the other and should take account of: -

- Tx amplifier power lost in feeder to antenna
- VSWR effects due to mismatched coax/amp/antenna
- Antenna losses (especially if not bang on the optimum frequency)
- Antenna gain (aka directivity) - usually a minimum of 1.7dB per antenna
- Localized losses in antenna structure (usually low by design)
- The actual free-space transmission loss of power versus frequency and distance
- The extra losses due to fade and the fact the earth isn't free-space
- The receive antenna gain
- Rx mismatches and feeder losses to receiver

Lastly, you should understand that if transmitting anything you are occupying a finite bandwidth in the spectrum and that bandwidth comes with a cost - noise. Let's begin with this. The minimum power in dBm that a receiver needs to generally operate with a low bit error rate is: -

dBm = -154dBm + \$10\space log_{10}(data\space rate)\$

So if you are transmitting 1Mbps the minimal signal you'll need at your receiver input terminals is -154dBm + 60 dBm = -94 dBm.

This accounts for your receiver being at normal ambient temperatures of 300k.

Assuming you can put figures on cable losses and antenna gains and that mismatches are minimized the basic free space link loss equation is: -

Link Loss (dB) = 32.4 + 20\$log_{10}\$(F) + 20\$log_{10}\$(d) where F is in MHz and d is kilometres

This tells you how many dB are lost at a particular frequency over a certain distance in free-space (not earth). Usually, RF guys then assume it's going to be at least 20dB worse than that (margin for fading) and that in a highly populated area it will be possibly 20dB worse again.

It all sounds a little haphazard but with care and attention to detail these equations produce very realizable systems.

I'm not going to comment on your figures because

let say we have 100 watt "50 dBm" transmitter connected to an
amplifier has 70 dB gain and output power of 400 watt "56 dBm"
connected to antenna has a gain of 15 dBi

.... makes no sense to me. I don't understand what you are trying to say.

See also other answers on this Long range (~15 km) low baud-rate wireless communication in a mountain environment (no LOS) and How to know (or estimate) the range of a transceiver?

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: -

## Best Answer

The amplifier gain is the ratio of output power to input power. For example, a 20 dB power gain means that the output power will be 100 times the input power.

On the other hand, the power rating is the maximum output power the amplifier can handle. If that is 15 dBm, then the maximum output power is 32 mW (with a given distortion).

If you are connecting amplifiers in

series, then the total gain (in dB) is the sum of all the individual gains. And you have to make sure that the signal level at the output of each amplifier does not exceed its power rating.