So, by extension, combining 2 Sennhesiser IEM TX combiners into 1 antenna with a passive combiner should be avoided.
Not necessarily. A passive combiner with decent isolation specs (around 30dB) can work very well in this application, if your system can afford the loss of 3dB per two-way coupling.
It helps to think of TX combining in terms of proper gain staging and component isolation. Most high quality active IEM/RFPL combiners have a signal path that looks roughly like this:
Input> isolator> amplifier> isolator> passive combiner> isolator> output. Each design may have fewer isolators than shown here, and may include other components such as filters or AGC circuits, and the designs vary due to cost, size, and weight considerations.
An overlying design consideration for off the shelf wideband active combining systems is the concept that each carrier is individually amplified to compensate for all losses incurred by subsequent passive stages. This is partly because amplifiers with high enough linearity to handle multiple carriers are prohibitively expensive.
The best active combiners have high isolation between all of their internal signal paths and all of their external ports. When you add a passive combiner after a typical IEM active combiner, isolation between the active components is maintained.
The amps in our active combiners can't handle an input with multiple carriers, such as BTR-800's combined 2ch output, without generating excessive IMD products. Additional noise may also be generated by the transmitters when they are poorly isolated, and this noise is subsequently amplified by the active combiner.
One more note on combining BTR-800; turning one base station transmitter off via the front panel controls doesn't work when you intend to combine the output. All this does is unlock the PLL tuning and you end up with RF noise problems. To do this properly, you must open the chassis and disable the transmitter via the slide switches on the PCB.
Sent from my mobile phone. Please excuse the inevitable spelling and grammatical errors.