A very simple principle you will agree with:
You can not 'repeat' [or amplify as most desire] a weak signal.
I am sure in all of your years of doing online technical support that its a familiar scenario of "why isn't my repeater working as desired" due to placement. This is also why you place a repeater, usually, somewhere between the source wifi and destination receiver not at the receiver location which simply replicates the initial receiver problem of lack of bars/connection.
Think about it. 15 meters is appox. 85 feet. The OP wants to put an antenna 85 feet from the pc to pick up the router wifi signal in the other apartment.
If it takes that distance to get a good signal you sure as heck aren't going to repeat it in the apt
So the concern is a repeater would not work in this situation. The reason is if you need to move the PC 15m closer to the router's wifi signal what putting an antenna on the pc's wifi card is all about] it means you don't have a good enough signal to repeat.
Here is where your lack of understanding in wireless networks shows. The average wireless router broadcasts out an omnidirectional signal 100 meters or 328ft from the center point of the router.
This means that it broadcast 328ft to the north, south, east, west. Now you, of course, will have to deal with other factors such as co-channel interference or channel bleed, absorption, reflection, refraction, diffraction, and scattering. This is why site surveys are done to address these other factors.
A repeater will also broadcast 100 meters or 328 ft taking the signal from the router cleaning it up and rebroadcasting it. Also your math my friend is wrong the distance is 15 meters or 50 feet. The industry makes repeaters aka range extenders for the home just for this purpose. There is no need to go and purchase a $2000 piece of equipment like you recommended to another member.
A wireless repeater
is well within the range to easily resolve this problem and they can be purchased for under $50 unlike $2000.
Edited by arlattimor, 01 October 2017 - 10:42 PM.
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Network Security Engineer