BLE USB receiver and antenna for a custom gateway (RPi2)
I’m looking for experiences and / or recommendations of BLE USB receivers and antennas if anybody has any to share.
Which BLE USB receiver and antenna would be best (or good enough) for ensuring reception of weaker BLE signals?
AFAIU high sensitivity has been one of the CTQs of Ruuvi Gateway and one of the key components from that point of view is the antenna. And based on the Ruuvi Gateway thread, it seems the gateway will have an detachable (SMA?) cloverleaf antenna so (having no electronics or RF background) I presume that would be a good option.
Would the Ruuvi Gateway antenna be available for purchase separately? Even though it would be the easiest solution to simply buy a Ruuvi Gateway (once it’s available), that’s not my primary option as building the RPi2 based node (which is a part of a larger system) is a hobby 
Of course any other antenna that would be a good fit would be interesting as well.
My use case is that I’m planning to have my RPi2 listen to RuuviTags as it’s already collecting some other measurements as well. RPi2 doesn’t have built-in Bluetooth so I was thinking to buy a USB BT adapter. My RuuviTags will be scattered around a wooden two-story house plus an attic and a cellar. The floor between the first floor and the cellar is made of concrete slabs. The receiver should hence ideally be able to pick up signals as well as possible from all directions (360 degrees both horizontally and vertically) and the weakest signal probably comes from the cellar.
My laptop (with its internal antenna) that I’ve used for testing things so far hasn’t virtually ever picked up the signal from the cellar while my Samsung S6 Lite tablet running the Ruuvi Station app does hear it although only every few minutes so packet loss is quite significant. Ruuvi Station reports signal strength of -95 to -85 dBm.
AFAIS there seem to be quite few BLE 4.2+ capable USB dongles equipped with an SMA connector available on the market. LM842-8425 would seem to fulfill the criteria, but its availability seems limited as I’ve only found it at TOP Electronics. Price is 25,95 + VAT + shipping (25 € IIRC).
AFAIK newer RPis have built-in Bluetooth, but I doubt that the internal antenna performs too well. And my RPi2 is still perfectly sufficient for running my custom distro.
The background is that I run a Yocto based distro on my RPi2 with collectd collecting data from a groundheat pump and HVAC unit and sending it over MQTT to my “cloud” that runs (surprise, surprise) InfluxDB and Grafana. I wrote a RuuviTag plugin for collectd (as one didnt’ seem to exist) and have been testing that on my laptop, but I’d need some kind of BLE receiving capability for RPi2 for doing some longer-term testing in order to be able to publish the plugin. And also to actually deploy my RuuviTags in “production” instead of just testing.