On Oct 22, 2007, at 7:19 PM, Ron Wright wrote: > I am not sure why an isolator would cause intermod.
The fact that they create harmonics unless filtering is done "downstream" of the output port going toward the antenna, means that the opportunities for external mixing are increased greatly by however many harmonics are strong enough out at the antenna and whatever is in its near-field. > In the past commerical sites would often require an isolator for > this reason with strong transmitters close by. In better repeater > equipment an isolator was built in. Most commercial sites require the isolator to keep the mixing/IM from happening in ANY PA's on the site, since mixing in an active junction is likely to be amplifed and much worse than passive IM external to the systems. The addition of an isolator raises the opportunity to externally mix if it's not filtered, however. Or perhaps mix in the ONE site tenant's transmitter that doesn't have an isolator. Best practice with isolators includes adding bandpass filtering beyond them to alleviate those harmonics... or shall we call them, "external mixing opportunities". It's all trade-offs... isolators do a specific job well (keeping unwanted RF out of active junctions in PA's), but add a new (far weaker/less likely) problem via the harmonics generated. When you add in the other "side-benefits" of an isolator (PA protection against mis-matched/missing antenna with the correct load attached to the load port), they're adding more value than the harmonic problem creates... but the harmonics still need to be dealt with, in many site-specific scenarios. -- Nate Duehr [EMAIL PROTECTED]
