A thread on "AVForums" titled "TV Signal Interference with HDMI Cable" informed me that HDMI cables can radiate RF at a frequency dependent on the pixel/frame structure of the video being sent through the cable. This appears to definitely be the case in my interference problem (below) and may be the root cause of yours:
PROBLEM: My long, amplified HDMI runs appear to be radiating RF interference to my Universal Remote Powerblaster MRF100B RF Extender. The interference is visible as a quickly but steadily flashing status light on the Powerblaster and keeps the extender from working properly. HDMI cable is "PTC 100ft 24AWG Premium GOLD Series HDMI Amplified Cable" from Amazon. When I unplug the cables the interference stops. When I turn off the source interference stops.
SOLUTION?: I will try grounding (earthing), shielding and/or re-routing these long cables away from Powerblaster to hopefully reduce or eliminate the interference. This is the first problem I've had with these extra-long amplified cables. The digital video they pass, even at 1080p over 100 feet, is immaculate and never breaks up. Hopefully I do not have to replace them. I have been chasing this problem for quite some time but this possible cause makes sense.