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Wireless microphones in close proximity to high power (cellular) transmitters


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Does anyone here have insight into operating wireless systems in close proximity to high power transmitters?  Or any fanatical RF gurus willing to chime in about the potential risks?

 

I am a staff audio engineer at a university, and I have become the de-facto RF coordinator.  I keep track of over 100 UHF wireless systems spread across our 225 acre campus.  I have been asked about the impact of potential telecommunication equipment installation (e.g. cellular antennas) on buildings.  The information I have does not specify frequencies or technologies, so I have to assume "telecommunications equipment" could include traditional cellular 1GHz and up, 600-700MHz band cellular, and even TV whitespace devices (or even other technologies of which I am not yet aware).

 

My first inclinations are:

* Cellular > 1GHz would generally not be a problem since it pretty far away from UHF (and we don't operate any 941 band devices).

* Cellular in the 600MHz and 700MHz bands might require us to be more careful about filtering on our antennas, particularly where we have wideband multicouplers in use.

* TVBD or white space devices would be a big problem since they would be right on top of us.

 

The part I am not as well versed in, is intermodulation inside the transmitters themselves where a high powered transmitter might be within a couple hundred feet of a location.  A quick estimate with the inverse square law leads me to believe that if a 50mW transmitter could generate problematic intermodulation in another transmitter at 3ft, then a (typical?) 100W transmitter would be a similar problem at 136ft.  Real world issues like directional antennas, intervening structures, etc. could help with that but that seems like it actually could be a potential issue.

 

Thanks for reading and for any insights you're willing to share.

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Hi Matthew,

You have good insight into the potential problems. There isn't much I can add, even as a designer of the systems. One area I can speak to, is that there are a few good multicouplers out there but many more that are, shall we say,  under designed. Look for third order intercept specifications; if the product doesn't specify that number, look elsewhere. For instance, our service department has had too many cases where an inadequate multicoupler distrorted the AM component of digital modulation transmitters, leading to very short range.

Best Regards,

Larry Fisher

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I find that a major key to successful frequency coordination and identifying competing RF devices not under your control is to scan your environment. One of my clients is a major hospital. Recording sound in labs, patient imaging rooms, etc. was very problematic until I started using apps like RF Analyzer.

 

At the end of 2021 I upgraded to TXAdvance. It combines the functionality of RF Analyzer and Frequency Finder. I cannot heap enough praise on this app. You program a database with a location name and your RF devices, It scans the environment for RF sources using $30 Software Defined Radio dongle that plugs into your USB equipped phone or tablet (use one of your wireless mic antennas or buy a $5 telescoping screw on antenna), It reveals 2T30, 3T30, 2T50 and 2T70 intermod issues, it can Auto select the best freqs to use for your wireless mics from 200Mhz to 700Mhz and it can perform a Live Scan with peak hold so you can identify intermittent RF sources. It has additional features but these are the major ones I use in every new environment.

 

(Disclosure: I was so impressed with the app I joined the beta team. The software engineer continues to make improvements and add features. I have not nor will I receive monetary compensation from the company for my endorsement other than access to the app.)

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Thanks, @PMC

Since my location doesn't change I only scan periodically to look for changes in the environment.  I use Shure Wireless Workbench for coordination since it supports lots of different manufacturers out of the box.  I don't think I could get by without WWB's zone features.  I can define zones for different buildings, etc.  I try to make it so our portable systems and media groups can power their stuff on any time, anywhere on campus and "just work" without changing anything, but that small systems far away from each other can even use duplicate frequencies if need be.  Things have been quite reliable.  That's why I want to make sure any additional telecom equipment doesn't ruin my life :)

 

 

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