Mobile phone LTE interference with 823-832MHz radio mics

[c.cam108]

A few months back our church purchased a set of Thomann radio mics in the 823-832MHz band. They work great as long as there is no one in the room!

As soon as there are more than a small number of people in the church hall, we get terrible interference that completely overwhelms the audio from the mics. It appears that this is from mobile phones in LTE band 20 (837MHz).

Has anyone else come across this and been able to overcome it? Currently we are sitting with £1600 of unusable equipment and Thomann are refusing a return.

[cedd]

Never had issues in this band before.

What antenna distribution are you using, and what aerials? Do these mics have pilot tone and is it enabled? 

Receiver desense might be an issue, but otherwise 5MHz is a fair old way away really. 
What individual frequencies are you using? 

[DavePallant]

Are the antenna/receivers far from the transmitters? Are you able to try operating the receivers alongside and close to the area where the mics are used to see if it is a combination of distance and interference? With supplied antennas on the back of the receivers the system might be running out of steam after 30 or 40 feet. If having the receivers close helps then that might indicate a couple of antennas and amplifiers could be used. The receivers might allow two antennas to be daisy chained down several receivers as opposed to just using the rabbit ears on the back of each unit.

Which Thomann systems did you get? I'm assuming something like the t.bone Free solo twin.

Similar to cedd above I've had less problems with my 823-832MHz sennheiser wireless channels than my 606-612MHz "GB" systems in terms of interference.

[Stuart91]

Hi Colin, 

I wonder if there's anything else could be interfering with the receivers which is turned on during the service, but not at other times? 

To give you one example, I had a church not too far from you with a Sennheiser G2 receiver which had begun dropping out consistently. It turned out that they had an HDMI splitter sitting directly underneath it, as soon as the splitter was turned off the problem went away. 

Something similar happened at an event where the customer had a Midas MR18 sitting on top of one of our RF racks. The PSU in the mixer was sitting directly beside the front-mounted twig antenna. Moving them apart by a foot or two was enough to make some problems disappear. 

[kgallen]

Replied to your post on Reddit, but BR is my "usual space" so I'll copy my response here too now I've seen your post:

I’m in the UK and use Sennheiser EW300G2 in 830-832 (with a license), get 4 channels and have good results in a local hall with audiences up to about 120. I’m using the standard antenna (not paddles) but the receivers are quite close to the stage (as well as the audience!). Actors have beltpacks. BUT I’m in a small town not a large city.

Edited December 19, 2023 by kgallen

[sunray]

7 hours ago, c.cam108 said:

A few months back our church purchased a set of Thomann radio mics in the 823-832MHz band. They work great as long as there is no one in the room!

As soon as there are more than a small number of people in the church hall, we get terrible interference that completely overwhelms the audio from the mics. It appears that this is from mobile phones in LTE band 20 (837MHz).

Has anyone else come across this and been able to overcome it? Currently we are sitting with £1600 of unusable equipment and Thomann are refusing a return.

Unless I have it wrong or there have been changes I was under the impression LTE 20 is below CH38 band, more like 800MHz.

I was given some cheap CH65 kit (I feel I remember the name of 'M Audio' on the kit) for the same interference reason but moving the 3 channels up to the top end and channel mapping them seemed to rid the problem. I used them several times without issue until receivers were right near a phone mast, like 50m, and they were swamped with full bars, tone locked mics would light the busy light  but there was a fairly contsant hiss with just ocassional sylables of audio. Trantec S4.16 CH 38 or 70 (no idea which, may have been both) no problems.

As DavePallant mentions we  also encounter interferance on CH 38 sometimes, in our case on Trantec S5000's from a fairly local CH39 TV transmitter but to date has not affected  CH38 S4.16 yet.

[c.cam108]

Thanks for the replies everyone. I've ordered an active paddle aerial (could only get one just now so will need to wait to get a second) and will try getting it closer to the stage if possible. I've also picked up an SDR dongle that's been on my wishlist for a while and this has given me an excuse to finally buy. Hopefully I can get a proper look at the spectrum and see what's going on.

 

On 12/19/2023 at 4:03 PM, cedd said:

Never had issues in this band before.

What antenna distribution are you using, and what aerials? Do these mics have pilot tone and is it enabled? 

Receiver desense might be an issue, but otherwise 5MHz is a fair old way away really. 
What individual frequencies are you using? 

We're using the Free Solo antenna splitter with the included omni aerials. https://www.thomann.de/gb/the_t.bone_free_solo_antenna_splitter.htm

Not sure which frequencies we're on and can't check until Sunday but they're spread quite well across the band.

On 12/19/2023 at 6:45 PM, DavePallant said:

Are the antenna/receivers far from the transmitters? Are you able to try operating the receivers alongside and close to the area where the mics are used to see if it is a combination of distance and interference? With supplied antennas on the back of the receivers the system might be running out of steam after 30 or 40 feet. If having the receivers close helps then that might indicate a couple of antennas and amplifiers could be used. The receivers might allow two antennas to be daisy chained down several receivers as opposed to just using the rabbit ears on the back of each unit.

Which Thomann systems did you get? I'm assuming something like the t.bone Free solo twin.

Similar to cedd above I've had less problems with my 823-832MHz sennheiser wireless channels than my 606-612MHz "GB" systems in terms of interference.

3x Free Solo twin receivers with the antenna splitter.

We've got about 20m from the stage to FoH. Aerials are on 5m cables and we've tried a few different positions away from the rack - higher up, along the wall towards the stage - with some improvement.

On 12/19/2023 at 7:18 PM, Stuart91 said:

Hi Colin, 

I wonder if there's anything else could be interfering with the receivers which is turned on during the service, but not at other times? 

To give you one example, I had a church not too far from you with a Sennheiser G2 receiver which had begun dropping out consistently. It turned out that they had an HDMI splitter sitting directly underneath it, as soon as the splitter was turned off the problem went away. 

Something similar happened at an event where the customer had a Midas MR18 sitting on top of one of our RF racks. The PSU in the mixer was sitting directly beside the front-mounted twig antenna. Moving them apart by a foot or two was enough to make some problems disappear. 

This did cross my mind. I had a sudden thought last night that we stream on 4G because our broadband is awful, so it could be the proximity of the 4G router. There's also a rats nest of cheap HDMI and SDI distribution just under the table from the mics. Problem is that we can't replicate the issue at any other time other than during Sunday services so we can't try turning stuff off easily!

On 12/19/2023 at 10:33 PM, sunray said:

Unless I have it wrong or there have been changes I was under the impression LTE 20 is below CH38 band, more like 800MHz.

I was given some cheap CH65 kit (I feel I remember the name of 'M Audio' on the kit) for the same interference reason but moving the 3 channels up to the top end and channel mapping them seemed to rid the problem. I used them several times without issue until receivers were right near a phone mast, like 50m, and they were swamped with full bars, tone locked mics would light the busy light  but there was a fairly contsant hiss with just ocassional sylables of audio. Trantec S4.16 CH 38 or 70 (no idea which, may have been both) no problems.

As DavePallant mentions we  also encounter interferance on CH 38 sometimes, in our case on Trantec S5000's from a fairly local CH39 TV transmitter but to date has not affected  CH38 S4.16 yet.

The LTE downlink (mast to phone) is around 790MHz I believe, but the uplink frequencies are in the 830MHz region.

Sounds like exactly the same problem as us. Not quite as close, but there are 4 band 20 masts within a mile of us in all directions.

[sunray]

24 minutes ago, c.cam108 said:

The LTE downlink (mast to phone) is around 790MHz I believe, but the uplink frequencies are in the 830MHz region.

Sounds like exactly the same problem as us. Not quite as close, but there are 4 band 20 masts within a mile of us in all directions.

Frequencies makes sense.

That was at a village fete at the village hall and sports field, the mast is within their property as a means of financing their facilities.

I've identified the system I had was a W Audio quad receiver and I believe it was only £200 for the whole system of 4 mics so an 'economical' system. I sold it on ebay and received several messages about only 3 mics holding the price down.

[Robin D]

I had similar in our local church (Dating to the 8th century) for a funeral of a village and church stalwart that was being streamed for overseas family. Someone else was handling the video, I just had to give him, and those sitting in the overspill marquee outside, the audio feed. The vicar wanted to move about so radio mic was the obvious answer. All worked great until they turned the organ on (a very old electric organ in a beautiful walnut cabinet). Terrible interference as soon as the vicar walked anywhere near the organ which is placed to one side of the aisle. Given the timescale... the coffin was already being carried in lead by the vicar. It was a couple of quickly placed Behringer C2 cardiods (No sneering please, they are cracking little units and great value for money.) plugged straight into the desk. I had taken them down to the church as originally told they would have a choir, but that didn't happen so I hadn't rigged them. Fortunately I had already run in a convenient snake from near the alter for the pulpit mic, through the priest door, back in through a tower window to the desk otherwise I would have been well and truly stuffed. 

Proves the old adage that you can test things in isolation, but environmental factors can still get in the way!

[dbuckley]

On 12/19/2023 at 7:18 PM, Stuart91 said:

...they had an HDMI splitter....

 

HDMI.  Yeah.  HDMI over Cat5 boxes can be really terrible, they act as a source of broadband noise, and the frequencies involved change with resolution.  The Cat5 cable acts as a very effective antenna for this form of interference.