I bought a 2018 MacBook Air (16 GB RAM 512 GB SSD) yesterday, and I'm experiencing Wi-Fi issues as well. I still have an old 2012 Mac Mini, and the signal is sees from my APs is stronger and even the access speed is sometimes higher than what the 2018 MacBook Air has sitting right next to the 2012 Mac Mini. I also have a Late 2014 iMac Retina, and when I place the 2018 MacBook Air right next to the iMac, the signal strength on the iMac is about the same, but the TX rate on the 5GHz (with 80MHz channel width) is 1,300 Mbps, whereas the 2018 MacBook Air shows access shows the TX Rate of 866 Mbps, and they are sitting right next to each other.
Today I went to the Apple store and compared my 2018 MacBook Air with the ones on display at the local Apple store. The RSSI, the Noise levels, and the TX Rate of my 2018 MacBook Air vs the ones on display at the Apple store were very similar. However, when I placed the 2018 MacBook Air on display at the Apple store right next to the 2018 15" MacBook Pro also on display the Apple store, the TX Rate on the 15" MacBook Pro was twice the TX Rate on the 2018 MacBook Air. The speed test (speedtest.net) showed about 30% better download speeds on the 15" MacBook Pro than on the 2018 MacBook Air, and I placed both machines next to each other.
The biggest complaint I currently have with the MacBook Air is that it disconnects from my home Wi-Fi (5GHz) after being connected for 20-30 minutes and sits disconnected even though the setting to automatically connect to my home Wi-Fi is selected in the Network settings. The only way it reconnects is when I manually select my home Wi-Fi. This happens at least in one room of the house where the 2012 Mac Mini is located, and that happens to be my office. I haven't hat the 2018 MacBook Air long enough yet to know if this also happens in other areas of my home. The Wi-Fi in my home is *enterprise-class* Ubiquiti APs (UAP-IW-HDs). I have two APs, and even though these APs were released about 4 months ago, and the firmware was initially buggy, I've worked with the Ubiquiti engineers to report the bugs, and most of the bugs have been squashed by now (end of 2018). The interesting thing is that my Late 2012 Mac Mini locks onto the 5GHz signal and keeps it without disconnecting, being able to download of 135 Mbps (when measured with speedtest.net from the Internet). My Internet bandwidth is 135 Mbps, and I haven't run iperf tests yet to see what the maximum download speed is on my Wi-Fi internally from one host to another, but I know that it is at least 135 Mbps from my office. And the 2012 Mac Mini doesn't even have 802.11ac (it has 802.11n with both 2.4 GHz and 5 GHz). So, my home Wi-Fi signal quality is good and. I attribute the frequent disconnects of the 2018 MacBook Air to the MacBook Air itself and not to any other factors.
I can definitely see an issue here. It may be the inferior antenna design or the buggy Wi-Fi driver, or both on the 2018 MacBook Air. I can definitely say for sure now that both the Late 2014 Retina iMac and the new 2018 15" MacBook Pro have much superior Wi-Fi connectivity to the 2018 MacBook Air.
I've also noticed in the Apple store that the 2018 13" MacBook Pro has the Wi-Fi connectivity inferior to the 2018 15" MacBook Pro, but it still seems to be better than the Wi-Fi connectivity on the 2018 MacBook Air.
It only took me a few hours to diagnose this problem and determine an issue with the Wi-Fi signal on the 2018 MacBook Air. How is it possible that Apple released this model without doing this little testing to see that the Wi-Fi has an issue? There's a small chance that my 2018 MacBook Air is defective, so I will set up a replacement with the vendor from whom I bought it, but I'm not holding my breath that the replacement machine will be any better.
One more thing about the 2018 MacBook Air. I feel electrical current on the aluminum chassis under my wrists on the palm rests when I plug the MacBook Air in to the mains with the included USB-C adapter, which has a 2-prong plug. I've googled this issue, and learned that if I plug the Apple extension cord (which Apple doesn't ship anymore with the Mac laptops, but you can still buy the extension cord from Apple), then the issue goes away. This seems to be an issue with the Y-capasitor leaking voltage onto the cassis, and the Apple extension cord, which has a three-prong connector with a grounding prong) resolves this issue. The issue only seems to be happening when the MacBook Air is at 50% charge or less and is charging. When it's fully charged, I can't sense the current on the aluminum body. This is another unbelievable thing to me that Apple knows of the issue and doesn't redesign the 2-prong plug to make it a 3-prong plug with a grounding connector in it.