The USB on my raspberry pi has 1 x 5dBi antenna. I have a second one from an old router. Can I just connect the ground shielding and signal to the existing and and signal connections. Would it be beneficial? 2x 5dBi antennas, would that make it 10dBi or just 2 x 5dBi .
I am trying to get a little more signal strength on a n300Mbps WiFi controlled vehicle. A little more range or stronger signal when it goes behind vehicles and block walls.
Thanks for the insight.
Ed
Not beneficial
There’s a couple of ways to increase signal, such as increasing total transmission power, making the antenna direct more power towards the receiving end (and less the other ways), and increasing the antenna efficiency. Adding a 2nd 5dBi antenna, however, doesnt do either of these things. What you would basically do is create one larger antenna that’s composed of the two individual antennas.
A larger antenna may suggest better signal, but that completely depends on the geometry of the antenna. By just slapping together two random antennas, you are almost guarantueed to decrease the efficiency compared to one of the two antennas, and probably also reflecting more power back to the amplifier due to higher impedance mismatch (thus reducing total transmission power and potentially damaging the amplifier). Simply because the geometry of the single antennas was pretty optimal, and the geometry of the combined antenna is likely rather bad. As for the directivity, it will probably turn out rather wacky, eg having good signal in some directions but running right into a “blind spot” after a right turn of your vehicle.
So what to do instead? Some ideas:
increase TX power: most routers have this options, but I dont think usb dongles allow this usually; does somebody know for sure? A different USB dongle may use more power.
better placement of the dongle, further away from noise sources (motors maybe) and metal
get/make a directional antenna… You would probably need an extra servo then to keep it pointed the right way then. Would look cool!
EDIT: Just thought of the short wavelength of WIFI. Theoretically, two antennas could be beneficial for reception because signal can vary on very short distances, see antenna-theory.com. Transmission would still be problematic though, because of the reasons described above, and transmission from the dongle to the router is likely a bigger issue than the reverse anyway.
WiFi channel
If you havent tried already, you can also try a different WiFi channel (can be choosen in the router), sometimes gives a small benefit as well 
With a good router you would
With a good router you would get signal even outside home. Mine has a range of about 50m, with 3walls.
Don’t enclose the robot with metal. Two wifi adapters may not give any benefit, Raspi will use one at once, while giving power to both. Search for an adapter with a full antenna.
