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[Henry Cohen]

I'm pretty sure it's because you'll overload the input amplifier for the cascaded input on the second combiner in the chain. Active combiners often have an amplifier on each input to compensate for the loss caused by the passive part of the combiner (~7 dB for a 4-way combiner), and this amplifier is only rated to take in so much signal.

This is part of the reason. Even if the composite power level is below that of the amplifier's 1dB compression point (P1), the amplifiers in the combiners we typically use are not linear enough to amplify multiple carriers; IM products, as well as harmonics, will result, generally strong enough to be detrimental to the coordination.

[Henry Cohen]

My practice is TX antennas are 4' apart and 8' from any RX antenna.  This spacing includes any wifi and cellular antennas which are always 10'- 20' away from any of my antennas.  Digital transceivers, like Freespeak, Bolero or CrewCom, count as TX antennas.

Whereas this is a very good rule of thumb for the RF power levels we typically use, horizontal antenna spacing actually depends on composite power levels, any filtering in use, frequency separation and vertical spacing separation. So higher RF power levels means for physical separation, and/or more filtering, and or greater frequency separation.

[Russell Ault]

This is part of the reason. Even if the composite power level is below that of the amplifier's 1dB compression point (P1), the amplifiers in the combiners we typically use are not linear enough to amplify multiple carriers; IM products, as well as harmonics, will result, generally strong enough to be detrimental to the coordination.

Right, that makes sense. Is this a linearity/power tradeoff as compared to the amplifiers in antenna distros?

Whereas this is a very good rule of thumb for the RF power levels we typically use, horizontal antenna spacing actually depends on composite power levels, any filtering in use, frequency separation and vertical spacing separation. So higher RF power levels means for physical separation, and/or more filtering, and or greater frequency separation.

Is there a more generic rule of thumb for this sort of thing based on power at the antenna instead of distance? Something along the lines of "an IEM signal should show up in another combiner's output at less than X dBm"?

When dealing with uncombined IEM transmitters, is frequency separation as helpful without additional filtering? It seems like, under these circumstances, separation is only going to cause 3rd-order IMD products to be spewed further afield, but without reducing their magnitude?

-Russ

[Henry Cohen]

Right, that makes sense. Is this a linearity/power tradeoff as compared to the amplifiers in antenna distros?

Not really. It's due more to the signal levels. The average carrier level reaching the input stage of a multi-coupler is less than -40dBm, as compared to +15dBm to +20dBm for outbound (IEM, coms, IFB) signals, or about 1,000,000 times stronger. With an RX multi-coupler the composite power from all carriers and general RF noise rarely exceeds a few dBm (milliwatts), unless you're using a preamp, in which case the multi-coupler can be placed into saturation.

Is there a more generic rule of thumb for this sort of thing based on power at the antenna instead of distance? Something along the lines of "an IEM signal should show up in another combiner's output at less than X dBm"?

No real math rule of thumb due to the number of variables, and a lot due to the characteristics of any given gain stage in multi-couplers, combiners, receivers, and even reflected energy back into combiner output if isolators aren't used. The best rule of thumb as it concerns low power wireless mic/coms/IEMs/ IFBs is as Pete said, ". . . TX antennas are 4' apart and 8' from any RX antenna", and the more the better. That said, anytime you can reduce the unwanted energy by at least 20dB, is a 99% reduction.

When dealing with uncombined IEM transmitters, is frequency separation as helpful without additional filtering? It seems like, under these circumstances, separation is only going to cause 3rd-order IMD products to be spewed further afield, but without reducing their magnitude?

Yes and no. Those IM products can only be generated in a non-linear stage that has decent performance at the two IEM frequencies in question. If one of those frequencies is outside the range of the device of concern, then the resulting IM products will be very weak. That said, filtering and isolators are always beneficial to performance.

[Gian Luca Cavalliini]

No real math rule of thumb due to the number of variables, and a lot due to the characteristics of any given gain stage in multi-couplers, combiners, receivers, and even reflected energy back into combiner output if isolators aren't used. The best rule of thumb as it concerns low power wireless mic/coms/IEMs/ IFBs is as Pete said, ". . . TX antennas are 4' apart and 8' from any RX antenna", and the more the better. That said, anytime you can reduce the unwanted energy by at least 20dB, is a 99% reduction.

Also, IMHO, minimum distance depends from antenna type and relative polarization. Not recommended, but I've used sometimes one receiving horizontal LPDA a few inch ahead of a vertical transmitting LPDA without any problem (guess what, coax too short from rental company...). Just put both vertical and problem arise. If receiving is omni, in the same range of a IEM transmission by Helix, you need to stay as far as possible, also taking care of the directivity of the Helix (Cardioid? Hyper?).