# Just I want to understand gain, power, link budget

I am a beginner, I am confused about calculating the nessecary gain and power and sensitvity to overcome path loss at certain distances.

the first thing what does gain mean ? I am not talkink about identification , but how to use this term ?

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 .

simply how to calculate the total budget ? is just adding gain to last output power to antenna gain right ?

or we just adding the total gain of amplifier and antenna and ignoring power ?

• if we add for example 50 db amplifier to receiver , should be adding this gain to calculation of total budget ? "ignoring the noise " – man abduo Oct 20 '14 at 19:54

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: -

1. Tx amplifier power lost in feeder to antenna
2. VSWR effects due to mismatched coax/amp/antenna
3. Antenna losses (especially if not bang on the optimum frequency)
4. Antenna gain (aka directivity) - usually a minimum of 1.7dB per antenna
5. Localized losses in antenna structure (usually low by design)
6. The actual free-space transmission loss of power versus frequency and distance
7. The extra losses due to fade and the fact the earth isn't free-space
9. 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.

• I talking about a system has a gain of 70 db and power of 50 dbm connected to 15 dbi antenna "just example" if I want to calculate total budget, can I say the budget = 70 "system gain" + 50 "system power" + 15 "antenna gain" ? – man abduo Oct 20 '14 at 20:04
• A 50dBm power connected to a 15dBi gain antenna pumps out 50dBm but the energy is concentrated in one direction therefore it looks like a power output of 65dBm transmitted by an omni-directional antenna (aka isotropic antenna). The "i" in isotropic is the same "i" used in the term "dBi" The 15dBi antenna has "gain" compared to the theoretical (and non-existant) isotropic antenna. – Andy aka Oct 20 '14 at 20:08
• there is another 70 db gain from amplifier , should we add it to the 65 dBm ? that is my qustion – man abduo Oct 20 '14 at 20:11
• Sure, 50dBm becomes 120dBm but that is 1 billion watts - maybe you have an error somewhere? This is why I couldn't comment on that part of your question. – Andy aka Oct 20 '14 at 20:13
• I talking about something like that everythingrf.com/products/microwave-rf-amplifiers/rohde-schwarz/… has a gain of 65 db and power of 70 dbm !! – man abduo Oct 20 '14 at 20:19