Wireless Wireless Mic Only Audio Dying, Then Lose RF Overnight.

[jehb416]

At my High School, we are running 21 body mics currently: 6 Audio Techncia 3100, 8 AT 310, All in the "D" Band; 4 more AT's in the 500's, and 3 old Sennheisers.

The issue is this:
Mon, Tues, and Wed. dress rehearsals all worked perfectly, then Thurs, Opening Night, Half way through the show, 3/4 of the AT's Audio cut out; every connection, wire, RF, antenna, and battery was still at full. We (Theatre Director/Me) called a professional from the county, who is very good, and he said that everything I did was all he could do. We then called a professional dealer/company in the area, and they came out. With them, we re-addressed all of the mics, and again checked every connection. only issue was a slightly faulty antenna splitter. they replaced it, and gave the system a clean bill of health, and left.

Last night, by the start of the show, only 13 mics were working. They now had half RF and no audio.

I have no idea of what to do next, as there is a final show tonight, and although we were able to run with 13 mics, it is much more difficult with quick changes, and fast re-taping.

Any Ideas?

 

[FMEng]

Did the dealer have all 21 mics operating at the same time when they did a final check out? That would be critical to do. Do they all work with just one transmitter on at a time? What other nearby RF systems might be running in the evening during the performance that isn't during the day? I'm thinking of a high school radio station, hearing assistance system, another venue with mics, or wireless intercom. That they go silent and still show moderate signal level suggests an interference source or intermod problem to me.

 

[jehb416]

Did the dealer have all 21 mics operating at the same time when they did a final check out? That would be critical to do. What other nearby RF systems might be running in the evening during the performance that isn't during the day? I'm thinking of a high school radio station, hearing assistance system, or wireless intercom. That they go silent and still show moderate signal level suggests an interference source to me.

There's no other wireless around but us that I know of. Our HS has no other wireless gear except what we (the theatre)own and use. The dealer set 1 mic, then keeping it on,set the next, then keeping both on the next, and so on. Every mic was on when they performed a final check.

 

[Chris15]

There's no other wireless around but us that I know of.

Is your profile location of DC Metro area accurate?

If so, there will be PLENTY of other wireless around...

 

[jehb416]

The company checked that all of the frequencies checked out when they re-addressed the mice.

 

[FMEng]

Just a wild thought that could be way off, but I'll toss it out there. A-T block D channels are just below the 700 Mhz cellular band. Maybe some of the receivers are getting nailed by a house full of 4G phones. Test the mics again with an empty house, if they work then, we might have the culprit.

Another possibility is that block D encompasses TV channel 44-49. WRC-TV is on channel 48. Low-power WMDO is on channel 47. Either or both of them could be causing problems with receivers that aren't selective. It seems like three channels would be pretty crowded with 21 systems. What I don't have a good explanation for is why the mic performance is varying if it is TV interference. It could be anything from changing propogation to station transmitter trouble.

 

[jehb416]

We had no issues during the empty house dresses, but we also ran beauty and the beast earlier, in December nearly flawlessly, with only 2 issues over the then working 21 body mics. Closing night, Saturday, ran well with using only 12 of 21 mics, despite having numerous changes backstage, as well as less ensemble support. however the issue remains that almost half died essentially over night. We called the dealer back, and they can come on Tuesday to check their work, because their work made it worse. they removed an antenna splitter, and re-routed all of the antennas to get signal. Again However, all was working until the shows, when all would crash again.

 

[avkid]

Somebody really needs to do a RF scan.
I doubt antenna re-routing has much to do with your problems.

 

[mbenonis]

A few questions to start off with:
- What makes/models of antennas are you using
- What kind of coax (antenna) cable? (it's written on the jacket...maybe RG-58, or LMR-240, or LMR-400?) How long?
- What antenna splitters, and how many?
- What systems are wired to what antennas?
- Do you run any wireless in-ear monitors? If so, what kind?
- Any two-way radios? If so, what kind?
- Can you give me a list of ALL frequencies, and which ones had the problem?

I'm with FMEng, sounds to me like some kind of blanket interference problem, based on RF meters still being pegged even though you don't have any audio. It's hard to guess without a more specific location (feel free to send it to me via PM if you prefer to), but I suppose one possibility is that a broadcaster or other entity lit up some sort of experimental or temporary transmission that evening. I kind of doubt LTE phones are the cause here. I can do a check of the FCC database once I have a picture of what the frequencies were that had problems.

Channel 4 (WRC; RF 48) is about 800 kW effective radiated power into 6 MHz, which is about 24 kW into 200 kHz (the approximate bandwidth of a wireless mic receiver). Bottom line is you want to stay away from that TV channel. That said: it doesn't sound to me like TV interference is the problem here. I don't know if WMDO-CA is still on air; looks like they have a Low Power digital signal now but they may still be running their analog transmitter on 47. Regardless I doubt it's causing interference.

I should be able to help more with a bit more information. BTW, I grew up in the area...

 

[FMEng]

When you have the dealer come back again, make sure all electronic systems in the building are going as they would during a show. That means turning on lights, video, intercom, wireless routers, computers, all sound equipment, etc., just as it would be for a show night. Something in the RF domain is changing, and that's what we need to figure out. The source of interference doesn't have to be some kind of wireless device, it could be a wide variety of things radiating in a way they shouldn't. Ask the dealer to bring a spectrum analyzer. If they don't have one, find a consultant who does.

 

[BurkeTheJerk]

If it works with an empty house, but not with a full house, I would be lead to believe it's cell phone interference. I've heard people say, "Cell phones? That's impossible, cell phones can't do anything." but I've both seen and hear it happen.

When we do mic checks in my high school, we have as many cast and crew as possible sit in the auditorium and use their cell phones.

 

[Chris15]

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