I bought a variable attenuator like the one in the Keith_Beddoe's linked picture. Turning the dial on this my BT YouView box reports the signal strength is varied between 50 and 100%. The Signal Quality is always reported as 100%. I tried viewing TV at ~50%, 60%, 70%, 80%, 90%, 95% but could not discern any noticeable change in picture quality on any setting. The picture quality on the LG TV using the Freeview aerial is better than the BT YouView through HDMI. I don't see any settings on the LG to adjust the HDMI inputs other than simplelink. The picture looks good via BT YouView and I would have been happy with it had I not seen the superior Freeview quality.