Q: Poor wifi performance - poor SNR

Hi, I'm sure this is not really an iMac or Apple problem but I hope that by posting here I get some useful advise or pointers.

Using the Wireless Diagnostics tools in Mavericks I can see that the Signal Noise Ratio (SNR) on my home wifi never gets above 19, my research tells me that above 25 is good and 41 is excellent could someone please tell me whether I am using the tool properly and do these results explain why I experience a lot of wireless network problems with my iMac and other Apple wifi devices in the house.

Thank you in advance for any help