Poor old Yellow is really getting a bad press. 
It does seem to be true that the antenna is relatively low powered, but a stronger one will not âfixâ the network, it will just mask the networkâs weaknesses - you can practically guarantee problems down the line.
Zigbee is not wi-fi; the signal is supposed to be weak (otherwise you wouldnât be able to get all those cheap battery-powered gadgets). Itâs supposed to be bounced backwards and forwards between lots and lots of routers, very close together. Sometimes it will travel quite a long way, but three to four metres and a wooden wall could easily be too far, even for a larger antenna.
I havenât tried the Yellow myself, but I would expect interference to be a bigger problem, with the coordinator jammed into a box with lots of other stuff. Get a dongle (with an external antenna if you like) and connect it on the end of a long cable through a powered USB hub.