I forgot to mention that you can also enable Wi-Fi scan mode for the iOS AirPort Utility, as follows:
Settings > AirPort Utility > Wi-Fi Scanner
Now when you run the AirPort Utility, you will find a Wi-Fi Scan option in the upper right-hand corner. Select this, and then, choose the scan duration. Select Scan and let the scanner run. Depending on the duration time set, you should get a number of results that also includes the RSSI value.
The only other applications that I am aware of require a dongle and these can get quite pricy.
Problem I am trying to solve is that I sometimes have a stronger signal in the separate building in my back yard from the Airport in the house than from the Airport in that building because of some shoddy splicing done by my Fiber Provider several years ago.
Instead of running a separate Ethernet Cable to that building from the house, they merely spliced into a telephone cable that was already there.
I am trying to convince them to re-do it correctly by running that separate Ethernet Cable.
The cable itself should not directly impact Wi-Fi performance, but can certainly affect overall bandwidth between what is connected at either end of that cable. It was common years back to utilize phone line cable for both voice and data connections. This was "acceptable" for 100 Mbps or less Ethernet connections, but crossover noise was still an issue. You could never achieve Gigabit connections as these types of connections would require using all available wire pairs in the cable.
It is now always recommended that you use separate dedicated cables for data and voice connections. Of course, I am referring to voice connections used by POTS-type land lines, not VoIP.