LOL, thanks @Wayne2
Couple things to check first,
- what firmware are you running on the A5x and the C5x respectively?
- Are you using AGC on the A5x? (Under Wireless >> MAC & PHY >> Wireless PHY Configuration AGC Mode
AGC basically acts like the Squelch control on old CB radios, if your signal is below a certain threshold then it’s ignored by the AP. If you are running AGC Mode as Auto there should be a Switch underneath it labeled Client Association Assist. Turn that on and it should enable you to connect with the weaker client. If you are running AGC Mode as Manual, you will need to adjust the sensitivity down so that weaker clients can associate.
From there, I guess we start getting down into the weeds.
Firstly, -70 isn’t all that great of a connection. I have a few people down that low in a pretty rural environment and I regret it. Noise will always be an issue and you have barely any room for environmental issues that will effect the signal negatively and ruin your SNR. (Which at -70 will be a rare case that it will be high enough to get a very high MCS rate)
Secondly. I love RF Elements, but 60 degree horns don’t have a boatload of gain. When you start going longer distances, every dB of gain you can get in your antenna will make a difference.
That said, -70 seems low for 3 miles, what size of antenna do you have on the C5x?