RF Guru's: I need your help, please.

B

Brian Ingwell

Freshman
Jul 23, 2011
24
0
1
Wisconsin
Hi All,

Had something very strange happen today. Setup in a casino expo hall. 16 channels of Shure ULX-D on quad receivers. G50 band (470-532 Mhz). Turn on the receivers - all showing 3-4 bars of signal strength with no transmitters turned on. I'm on the recommended Group, so I do a frequency scan. NO available frequencies. Do a Group scan, NO available frequencies.

I broke out Wireless Workbench and did a frequency scan. There is a solid block of wide band RF across the whole G50 band at about -70 dBm. Also showing two expected TV channels in the band at appropriate frequencies at about -55 dBm. This was happening with the whip antennas, although no change after replace those with the Shure wide-band paddles.

Moved the receivers around the room - no change. Called Shure tech support. They indicated noise floor should be around -100 to -110 dBm. He suggested it could be a cell phone repeater in the building that's gone haywire.

What I've done thus far:
Spoke to the casino. They claim to have no RF jamming devices in use.
They turned off Wi-Fi in the room. No change.
They do have cell phone repeaters and 2 way radio repeaters in use, but claim the control software for each would indicate if any repeater was putting out spurious RF where it wasn't supposed to.

I have limited RF knowledge, but my main concern is if anyone has ever encountered something similar and what could potentially be causing it. The ULX-D transmitters do still work at a fairly good distance from the antennas, however the high level of stray RF has me very concerned as RF dropouts on this show will be severely frowned upon.

Rehearsals tomorrow and Sunday and show Monday. Please let me know if you have any ideas.

Thanks much,
Brian
 
Re: RF Guru's: I need your help, please.

If you have the antenna from a powerful UHF 2 way radio repeater in the same room you could be overloading the frontend of the receivers so badly you see something like this. Does the level change if you put on the paddles and point them in different direction or move them to the other end of the room? Another possibility is a big LED installation spewing out broadband noise.
Do you have access to a set of passive filters to put right after the (passive, hopefully) Shure antennas? Something like Lectro PF25: Lectrosonics.com - #PF25 (Specify Block) or Sennheiser AB1036.
 
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Re: RF Guru's: I need your help, please.

Hi Brian,

Are you in one of the following cities? You may be near one of their municipal services repeaters.

Market Channels Market Channels
Boston 14, 16 Miami 14
Chicago 14, 15 New York 14, 15, 16
Cleveland 14, 15 Philadelphia 19, 20
Dallas 16 Pittsburgh 14, 18
Detroit 15, 16 San Francisco 16, 17
Houston 17 Washington DC 17, 18
Los Angeles 14, 15, 16, 20

Admittedly, it would be highly unusual to have a local public safety radio system repeater behaving that way. You would more likely see lots of narrow, 12.5KHz spikes in the range.

Look for a nearby sodium vapor light with a bad ballast or a neon lighting charger. I have seen them blast high power signal across large parts of the UHF spectrum.
 
Re: RF Guru's: I need your help, please.

Jens,

Thanks for the info. I will have to pursue the venue people more for the location of the repeater antennas. I didn't think to move the paddles around and check noise levels. I'll do that tomorrow.

I don't have a set of passive filters. We're using the Shure wide-band paddles. What do the passive filters do exactly?

Thanks,

Brian
 
Re: RF Guru's: I need your help, please.

Jason,

I'm in Milwaukee, so no municipal service repeater, at least according to that list. I'm definitely going to explore the possibility of a bad sodium lamp or neon.

Thanks,

Brian
 
Re: RF Guru's: I need your help, please.

Brian Ingwell said:
Jens,

Thanks for the info. I will have to pursue the venue people more for the location of the repeater antennas. I didn't think to move the paddles around and check noise levels. I'll do that tomorrow.

I don't have a set of passive filters. We're using the Shure wide-band paddles. What do the passive filters do exactly?

Thanks,

Brian
Click to expand...
The passive filters make sure that only the RF in the passband you need will get to the receivers. This way strong out of band signals (2 way radio, IEM transmitters etc.) won't make it to the receiver. Think of it a bit like a crossover, it removes unwanted energy from the signal. They wil not help you if the problem is broadband noise, in this case your only options are to remove the source of the noise, or optimizing your antenna positions to get maxium S/N ratio.
 
Re: RF Guru's: I need your help, please.

Jens Bacher said:
The passive filters make sure that only the RF in the passband you need will get to the receivers. This way strong out of band signals (2 way radio, IEM transmitters etc.) won't make it to the receiver. Think of it a bit like a crossover, it removes unwanted energy from the signal. They wil not help you if the problem is broadband noise, in this case your only options are to remove the source of the noise, or optimizing your antenna positions to get maxium S/N ratio.
Click to expand...

That makes sense. Thanks, Jens.

Brian
 
Re: RF Guru's: I need your help, please.

Brian Ingwell said:
Jens,

Thanks for the info. I will have to pursue the venue people more for the location of the repeater antennas. I didn't think to move the paddles around and check noise levels. I'll do that tomorrow.
Brian
Click to expand...

The paddles are directional, so think of them like a cardioid mic, you want to point it towards what you want to pickup, but you can also use the placement/aiming to put your unwanted noise source in the null of the pickup pattern. Free noise rejection (for one source in one direction) with zero performance hit (other than cabling to get the antennas where you need them)

Jason
 
Re: RF Guru's: I need your help, please.

Thanks to everyone for all their suggestions and advice.

Yesterday we tracked down the problem. It appears to be the lighting in the room. They have large round metal fixtures, reminiscent of sodium vapor fixtures, but these are fluorescent with 6-8 smaller, CFL looking tubes that are arranged in a circle within the fixture.

I'm assuming there was a bad ballast in one, because once we shut off that lighting, the problem went away.

Again, thanks for the help!

Regards,
Brian
 
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