500mA and up will charge a phone (assuming you're not also doing power-intensive things on it), but you'll probably need 1-2A to power a Pi and a SDR reliably.
Yep! Plus the tuner card itself can use 200mA or so, and if you want to use any other USB accessories they'll be drawing power as well. I unfortunately have too much experience using "phone chargers" that had enough current to run it 90% of the time, but under hard load would cause enough voltage drop on the 5V rail to reboot the Pi.