Normal 2.4GHz and 5GHz needs only 1 antenna; The more antennas are used for multistream connections... like those "1200Mbps" wlans that combine multiple streams of 2.5 and 5GHz for one connection.
If your PC has that "bad" of a position for wireless, I would suggest you use a USB wireless adapter, that way you can position it way better and higher which in return will improve reception.
802.11b - 11 Mbps (2.4GHz) 802.11a - 54 Mbps (5 GHz) 802.11g - 54 Mbps (2.4GHz) 802.11n - 600 Mbps (2.4GHz and 5 GHz) - 150Mbps typical for network adapters, 300Mbps, 450Mbps, and 600Mbps speeds when bonding channels with some routers 802.11ac - 1300+Mbps (5 GHz) - newer standard that uses wider channels, QAM and spatial streams for higher throughput
but that are theoretic speeds, actual speeds are below that:
802.11b - 2-3 Mbps downstream, up to 5-6 Mbps with some vendor-specific extensions. 802.11g - ~20 Mbps downstream 802.11n - 40-50 Mbps typical, varying greatly depending on configuration, whether it is mixed or N-only network, the number of bonded channels, etc. Specifying a channel, and using 40MHz channels can help achieve 70-80Mbps with some newer routers. Up to 100 Mbps achievable with more expensive commercial equipment with 8x8 arrays, gigabit ports, etc. 802.11ac - 70-100+ Mbps typical, higher speeds (200+ Mbps) possible over short distances without many obstacles, with newer generation 802.11ac routers, and client adapters capable of multiple streams.
That adapter has two antennas because it needs to have 2x2 mimo. Yes it is relevant.
There is something in the wireless n and AC spec called wideband or channels greater than 20 mhz apart.
In 20 mhz (single channel mode) you get the best range but lowest throughput this is why its always prudent to set your 2.4 Ghz channel to 20 mhz wide mode and optimize it for range on a dual band router. then on your 5 ghz channel you will optimize it for throughput aka 40 or 80 or 160 mhz channel width. This helps increase the number of MIMO channels. Given that your adapter is only 2x2 then 40 mhz is enough. However as you increase the number of channels you broadcast on your TX power is divided between the two to keep your maximum EIRP down which in turn decreases range.
The way these channels are bonded is rather complicated. The modulation itself is quite complicated so i left this at laymans terms but you want to know more maybe if there is enough interest ill do a write up in the network section
No its side band modulation with some OFDM and 256-QAM for the most part.. The side band modulation controls the bonded channels while you choose your central frequency or control channel... The modulation handles the rest of the sending of data through the air and I think 512 qam and 1024 qam is coming around with qamtenna technology
Not entirely true depending on the modulation of the router it could be anywhere from 300 to 600 even 867 now but yes this is the general gist however as I said before range goes down the more spatial. Streams you add
If you want to use AC, you'll probably need a repeater. AC's range is bullshit marketing, because 5 GHz doesn't have the penetration that 2.4 GHz does. It's great for open areas, but most households won't benefit from it. Does your home network need gigabit throughput? If you've centralized storage and have multiple clients, then you may benefit from AC. Barring this, your internet connection is always going to be the bottleneck (as long as the telecom oligopoly rules). An N router and adapter should satisfy your needs. Wait for something better to come along.
I'm assuming it's for a desktop since you're buying a card... so I'll just ask... what is the reason you can't hardwire?
Alternatively to getting a better wi-fi card... addressing the underlying problem might be a thing, and in that case you can buy wi-fi repeaters or if you already have a wireless N card that works...
I'll go ahead and say that if a Netgear R7000 isn't giving you full throughput then a repeater is the way to go... D-link makes a good one that's easy to use ~$20-25... Powerline is circumstantial but the best option...
Also, for 60 down/10up AC isn't going to help anything even if you can reach the signal...
Wireless AC can be better than N even if the signal percentage shows as lower. I get higher data transfer rates on my laptop using the 5ghz band on my AP even though the 2.4Ghz reports as a stronger signal.