Bob Dolph Answers Questions on Perimeter Security, ‘Ghost Images’

Q: I have a residential installation in which the customer wants me to apply some perimeter security to the skylights. Any suggestions?
A: Completely protecting a residential skylight can be a challenge. First, you want to make sure the entire opening is protected. Many skylights are very easy to unscrew and remove. I can think of a couple of methods you might want to try:

  • One obvious method would be to use an alarm screen over the opening. These can be ordered from companies like Maxwell Manufacturing (
  • The customer may not want a screen covering the skylight, so another method would be to use a curtain PIR looking across the opening area.
  • Another device that you might want to try is a multi-head photobeam curtain such as the “Enforcer” from SECO-LARM. Check them out at

Q: I have a CCTV camera installation in which the picture at the monitor has “ghost images.” I have put a test monitor at the camera and the picture is fine. I have checked the coax for correct type (RG-59U) and the BNC connectors look good and tight. What am I missing?
A: Sometimes when we are in a hurry, we can miss the obvious and simple things. Let me offer one suggestion that may help.
First, the ghosting is a sign that you have a termination problem and the signal is reflecting back on to itself. You may have remembered that on some monitors you have to manually select the 75-ohm termination switch if the monitor is the last device in the video line. I would check this, it is easy to overlook when busy installing. We can all miss any easy one now and then.

A Special Tip for Our Online Q&A Readers: Those that are interested in some other great TV troubleshooting tips might want to also check out some great CCTV tech information at my good friend Charlie Pierce’s LTC site:


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