--- In Repeater-Builder@yahoogroups.com, "Eric Lemmon" <[EMAIL PROTECTED]> wrote: "I think you may have overlooked the fact that desense was observed even when a dummy load was connected directly to the output of the duplexer. That pretty much eliminates anything external to the repeater itself, doesn't it?"
--------------------------------------------------------------------- - I don't think I overlooked anything. I suggest that you re-read his posts. He said that he has 1 dB of desensitization when the system is tested on a dummy load, in excess of 15 dB of desensitization when running on one antenna and an improvement of about 6 dB when running on split antennas. I called his desensitization on the load "minimal" I don't see how you can call 1 dB equivalent to 15 dB and put the blame on the repeater equipment. Actually, there is one fact that is important, that there is still on the order of 9 dB of desensitization when running on split antennas. This alone strongly suggests that the repeater transmit RF is illuminating a passive IM source. [or is incident upon and mixing at a source that is illuminated by the DTV signal] Passive IM [PIM] can occur in many places. If the signal level of the interferer is high enough, it could occur in your antenna, in your coax, or in one of your connectors. Each could have a contribution. Deciding where is is occuring is the issue. Within your system or external? The industry is beginning to develop practices to deal with PIM. Andrew has published some information regarding PIM in transmission lines as has Amphenol with respect to PIM in connectors. See also "Intermodulation in Coaxial Connectors", RF Design, September 1996. In this case, there are several possibilities to consider. If the spectrum analyzer can show the character of the noise that is causing the desensitization, and if that interference appears similar to to the DTV signal, then the task is to look for ways to identify its mix point and to defeat it. [I suggest that the source of degradation is a noise like interferer because it appears from the previous posts that the proper tests have been conducted to rule out a narrowband interferer.] I would also suggest swapping antennas when running split. If the results differ - it could give some guidance as to the proximity of the mix point to one or the other. Another test to run would be to measure the absolute level of the DTV signal at the transmission line to each of the two antennas. This will tell you how strong the signal is near your antennas. Remember that the wideband character of the DTV signal is going to affect the way that you measure it.
