WiFi signal issues

Here’s the situation I’m facing, and I could really use some advice:
I’ve got a TP Link WA801ND access point sitting right by the window, which I use to connect to a router (a Boeing 747) located about 60 feet away in a separate house. To give you an idea of the layout, there’s one exterior wall and one thin interior partition between the router and the open area where my AP is positioned by the window.
Everything was working perfectly until about two months ago. The AP was picking up a great signal, and I was getting pretty solid speeds—around 40-54 Mbps on the local network. But lately, it seems like neighboring Wi-Fi networks have popped up and are causing major interference. I tried switching the channel, but it didn't help at all; now my local network speed has plummeted to under 5Mbps. I'm wondering if I should look into a better antenna or if there's some other solution out there? My budget is around $100. $67 Also, since this AP has two antennas, is it actually possible to attach just a single antenna to it? Does that even make a difference in performance?

Thanks.

Check this out.

If you head over to eBay and drop about two hundred bucks, you could pick up a Dell Latitude equipped with an 18 dB antenna; honestly, you’d likely be picking up signals from deep space with a setup like that. I personally own one, and I can tell you from experience, it’s quite a reliable piece of kit.

Thanks for the suggestion. That would actually work perfectly if I didn't need coverage in two different spots—plus, my computer is located on the side of the house where the signal is pretty much non-existent.

I’ve actually started working on building a biquad antenna myself, but I just can't seem to find a spare moment to head down to Home Depot to pick up the components.

Sent from my Garmin GPS