Do Multicouplers (Antenna Distros) Prevent Intermod?

I recently heard someone mention on a forum that if you do not use an antenna distribution unit (multicoupler) with your wireless microphones, then you would be facing intermodulation problems. This post will hopefully dispel that belief and explain why antenna distros are useful, and what they can and can’t do,

Antenna distro units are composed of two things, typically. First, the antenna distro unit amplifies the signal with a (typically wideband) RF amplifier by about 7 dB. Second, the unit splits it with a (close to) ideal splitter. The net result is that the signal at the output ports is pretty close in power to the signal coming into the input port.

Now, here comes the problem. When you run the signal through an amplifier, the amplifier will generate intermod products. Period. The good news is that if the amplifier is well designed these products will be very weak and hence will not cause much in the way of issues. If your frequencies are coordinated, none of the third-order (most powerful) products (IM3) will fall on any of your operating frequencies anyway. But, if any one signal is hot (a relative term), then any IM3 product involving that guy will also be increased in amplitude. The amplifier also adds noise to the signal, but if you did things right that’s not an issue.

So, the bottom line is that antenna distro systems do nothing to *prevent* intermod. This is not to say they’re useless by any stretch, though. What they do for you is let you share a (good) pair of antennas while keeping the signal at roughly unity gain, and help to reduce local oscillator leakage from one receiver to another*. They also often provide power distribution, which is valuable when you have >12 systems in a rack.

On a side note, when you have a high density of transmitters on stage, and they’re running high power (>10 mW or so), the transmitters themselves can create intermod! Remember that a transmitter is also a receiver, so while the signal from your power amplifier is going out of the antenna (most of it, at least…but some will go back in too because the antenna is imperfect!), signals from all around it are also going *in* to the PA. If the transmitter does not have a way to separate those signals and get rid of them, then they will leak into the PA and mix with the generated signal, creating intermod products. These products are potentially amplified, and transmitted over the air. No amount of receiver filtering will help you at this point.

The solution to this issue is to run the lowest power necessary for a solid link. Another tip is when your mics are lined up waiting for performers, put them all on a metal table and put a coffee can over each mic. This will reduce crosstalk and hence, intermod.

*In a receiver, there are signal generators (local oscillators) that are used to bring the signal down to a frequency that can be demodulated. These signals can leak out of the receiver port and, if an antenna is directly connected, they can actually be transmitted. If a bunch of antennas are located in close proximity, then it’s quite possible that another receiver will see that signal and it’ll get added to the mix and potentially cause direct interference or more intermodulation products.

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