What happens are that a scanner uses several bandpass filters, one for each band, and when one scanner use a 108-136MHz filter that are more or less 50 Ohm within its band, the other scanner could be monitoring a 400MHz signal using a 380-450MHz filter. The 108-136MHz filter at 400MHz could either present a very high impedance, an open coax, or a very low impedance, a shorted coax depending of its design. In both cases it works as a notch filter for some frequencies and their multiples or as a bandpass.
You can easily hear this if you monitor a steady weak analog signal with one scanner and let the other scanner scan over its different frequency bands. To isolate this problem you can use a splitter, a $5 CATV model can do, and will have some 20dB isolation between scanners but also a 3dB loss to the antenna signal.
Sometimes the T connectors 0dB loss will compensate for the interaction between two scanners load from bandpass filters but the best solution are to get a $25 amplifier to overcome the splitter loss, and if placed at the antenna you also compensate for coax loss and the antennas different impedance load to the coax. Using a low noise amp also improve weak signal reception. If using a PGA103+ based amp it will also hold up to strong signal overload better that a scanner and you can also use FM broadcast filter at the splitter and the gain from the amp will compensate for the loss in the filter. Attenuation before the splitter is needed and a variable 0-20dB are the best one to use to dial in the perfect amount of total gain.
/Ubbe
