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Author Topic: RF problems in new HS stadium install  (Read 7157 times)

Denn Cimo

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RF problems in new HS stadium install
« on: September 09, 2016, 07:48:51 pm »

Hi all, I was hoping I could get some wisdom from the RF experts on the forum.. My company, Sound Solutions of Ohio, LLC. just finished a new stadium sound install in Huron Ohio(44839) We installed a new Audio Technica 3000 series wireless bodypack system for the referee and integrated an older 3000 series Handheld System. Both systems are in C Band (542-565MHz)  We installed a DA49 Wide Band Distro and remoted a pair of wide band paddle antennas to the outside front of the press box using RG8 cabling. The paddles are pointed at the end zones and are about 5 ft apart. We worked with AT to find TV/Intermod Free frequencies. Everything works beautifully, full bars of RF all over the field and no interfence. That is, until game time, when the microphones become unusable.
Main problem seems to be that there is an AM station WLEC 161.640 that does a remote broadcast from the press box.  They are running a Marty SRPT 40A Transmitter to an antenna that is about 15 feet to the left of the pressbox and parallel to our antennas mounted on the front of the pressbox. As soon as he turns the Marty on, the RF bars on the AT receivers go up to 5 or 6 when their corresponding transmitters are not on. When the AT transmitters are turned on, so much noise is coming through that the wireless is unusable. I moved our antennas to the opposite side of the press box to get them as far away as possible from the AM antenna, (about 55 ft) but it did not help. The BCSN guys said the same antenna was interfering with their TV camera also.
Other known sources of RF during the game are as follows:
1:there is also occasionally an FM broadcast KM96 during the games, also using a Marty SRPT 40A
2: In addition, BCSN Erie is using a lectrosonics system to get their audio to their mobile broadcast vehicle. Running 540.500MHz) I am not sure if they are broadcasting a TV station from the vehicle or not. I have a call into them to get more info.
3: The home team coach comm system that is working in the 2.4GHz band.
So I am hoping that some experts will help me to weigh my options here. The only options I can see at this point are:
1)   Try the Audio Technica AEW-DA550C Distro that only operates in C band with hopes that it will filter out the interference. I donít have one of these in stock so I would have to sacrifice job profitability to try this.
2)   Try a different band or different manufacturer of wireless systems. Again sacrificing job profitability, especially if it doesnít solve the problem.
3)   I purchased and received an RF Explorer and software today. Iím a newbie with the product but I figure I could go to the next game and do a scan to see what Iím really dealing with before ordering any additional gear, unfortunately this leaves the customer with no wireless mics for more games.
Please let me know if there is something that I am missing here or maybe someone has seen this sort of interference from an AM broadcast before and has a solution. Thanks in advance.
Denn Cimo
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David Buckley

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Re: RF problems in new HS stadium install
« Reply #1 on: September 10, 2016, 01:37:55 am »

The Marti (not Marty!) is a one box FM transmitter and mixer, used for remote linkups, and has, by wireless microphone standards, a lot of RF output, depending on the exact model, perhaps 50W, and if your wireless mic antennas are near, then your receivers may well be desensed.  There is a matching receiver somewhere else, and then the signal goes in to the station audio chain, eventually appearing at their AM transmission facility, almost certainly by another UHF FM STL link (STL - Studio to transmitter).

You mention 161.640, is that the Marti is on 161.640MHz?  If so, its a long way away from C band stuff, so I'm firstly surprised that the filters already in the wireless mic receivers are not removing that already.

Our remote Marti is on 159.something, and we have no problems at all with radio mics on 500-odd MHz, but... by wireless mic I just mean a receiver with its bugs ears, not paddles and amps and stuff.

So the other thing that occurs to me is perhaps you have too much reception gain on the wireless mic system, which is pushing that out of band signal all over your receivers.

The "standard" solution is filters after your antennas.  Employ an RF expert.


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Scott Holtzman

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Re: RF problems in new HS stadium install
« Reply #2 on: September 10, 2016, 02:33:54 am »

The Marti (not Marty!) is a one box FM transmitter and mixer, used for remote linkups, and has, by wireless microphone standards, a lot of RF output, depending on the exact model, perhaps 50W, and if your wireless mic antennas are near, then your receivers may well be desensed.  There is a matching receiver somewhere else, and then the signal goes in to the station audio chain, eventually appearing at their AM transmission facility, almost certainly by another UHF FM STL link (STL - Studio to transmitter).

You mention 161.640, is that the Marti is on 161.640MHz?  If so, its a long way away from C band stuff, so I'm firstly surprised that the filters already in the wireless mic receivers are not removing that already.

Our remote Marti is on 159.something, and we have no problems at all with radio mics on 500-odd MHz, but... by wireless mic I just mean a receiver with its bugs ears, not paddles and amps and stuff.

So the other thing that occurs to me is perhaps you have too much reception gain on the wireless mic system, which is pushing that out of band signal all over your receivers.

The "standard" solution is filters after your antennas.  Employ an RF expert.




Sounds to me like you have an intermod hit on frequency with the Marti unit.  A spectrum analyzer would confirm this.

If so filtration on the receivers is useless in this case.

If the intermod is mixing in the marti a cavity and circulator on it's output would clear this up.  If there is a community repeater or some other land mobile asset participating in the mix then the remediation could become more complex.

Is the interference constant when the Marti is radiating? 

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Scott AKA "Skyking" Holtzman

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Dan Currie

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Re: RF problems in new HS stadium install
« Reply #3 on: September 10, 2016, 10:19:57 am »

As soon as he turns the Marty on, the RF bars on the AT receivers go up to 5 or 6 when their corresponding transmitters are not on.

The Marti could be raising the noise floor a considerable amount.  If that's the case filters on your receiver inputs won't help because if that's the case it would be a signal to noise issue.  You're headed in the right direction running a scan of the environment with/without the Marti.  Are you able to get some face time with the the unit before the next game?  Is the ac ground pin still there?  Is the Lectrosonics at 540.500mhz having any issues when the Marti is on? 

  If the Marti is creating that much noise it would need to be serviced as it is causing harmful interference to yourself and probably every DTV in the vicinity.   
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Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #4 on: September 10, 2016, 02:19:03 pm »

Sounds to me like you have an intermod hit on frequency with the Marti unit.  A spectrum analyzer would confirm this.

If so filtration on the receivers is useless in this case.

If the intermod is mixing in the marti a cavity and circulator on it's output would clear this up.  If there is a community repeater or some other land mobile asset participating in the mix then the remediation could become more complex.

Is the interference constant when the Marti is radiating?

Thanks so much for the reply! The effect of the Marti is as follows. The RF meter goes up immediately when the Marti is turned on, but it is not constant. It will clear for a few minutes, then go to anywhere from 5-8 bars, then flash up and down, then clear again for a few minutes.  When the AT transmitter was turned on during the 1st game it would just get drop outs and crackling noise. The 2nd game it was completely unusable and the receiver was putting out AF noise the entire time it's transmitter was on.
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Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #5 on: September 10, 2016, 02:30:36 pm »

The Marti could be raising the noise floor a considerable amount.  If that's the case filters on your receiver inputs won't help because if that's the case it would be a signal to noise issue.  You're headed in the right direction running a scan of the environment with/without the Marti.  Are you able to get some face time with the the unit before the next game?  Is the ac ground pin still there?  Is the Lectrosonics at 540.500mhz having any issues when the Marti is on? 

  If the Marti is creating that much noise it would need to be serviced as it is causing harmful interference to yourself and probably every DTV in the vicinity.   

Thanks so much for your reply!
I also suspect a bad noise floor issue with that Marti.  When I was asking him questions, the AM commentator told me that the station owned a rack full of AT wireless systems that are unusable when they used that Marti unit. He seemed to be saying it was a problem with the Audio Technica systems.  I can get there early and take a scan. He gets there about an hour and 15 minutes before the game starts. I might have about 15 minutes to scan with just the Marti on before everyone else turns their transmitters on. The cable guys say there is no interference on their Lectrosonics at 540.500, however they have quarter wave rubber duckies and are farther away from the AM antenna. Thanks again
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Kevin Maxwell

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Re: RF problems in new HS stadium install
« Reply #6 on: September 10, 2016, 04:01:56 pm »

I looked up the Marti SRPT-40A and it looks like it is a 40watt output unit that operates from 430 MHz to 480 MHz. You mention that the antenna for this is on the press box. I have to assume that they are broadcasting on the Marti transmitter back to their studio or transmitter. Did you know or were you told that these stations broadcast the games? If you were did you in any way coordinate or at least talk to these people? It is very likely with a transmitter that strong and that close is going to just over saturate the front end of your receivers. The only solution would probably be to convince these people to send it down a landline or as I understand it being done lately is over a cell phone signal. This is assuming that you havenít added unnecessary gain in your antenna system making it more susceptible to this problem.   
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David Buckley

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Re: RF problems in new HS stadium install
« Reply #7 on: September 10, 2016, 05:38:21 pm »

I looked up the Marti SRPT-40A and it looks like it is a 40watt output unit that operates from 430 MHz to 480 MHz.

Frequency depends on the exact model, there are several available, the entire range of units covers from 135 MHz to 965, specs here.
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Mike Caldwell

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Re: RF problems in new HS stadium install
« Reply #8 on: September 10, 2016, 07:02:03 pm »

Have you tried a using the AT receivers frequency scan mode to see if it can find an open frequency, you may need to try scanning in several of the frequency groups.
On the hand the receivers may be so saturated with RF from the Marti that they may never be able to find a clear channel.

Try using the 1/4 wave antennas that come with the receivers on the DA49's inputs for a test, also try it with just the 1/4 wave antennas connected directly to the receivers.

Here's a few really long shot ideas, but strange things can happen in a strong RF field.

Is the stadium sound system connected in any way to the radio broadcast system?
Is pin 1 tied to any of the shells on the XLR connectors?
How are you connection the  receivers to the sound system, XLR or 1/4 inch?

Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #9 on: September 10, 2016, 10:03:32 pm »

Have you tried a using the AT receivers frequency scan mode to see if it can find an open frequency, you may need to try scanning in several of the frequency groups.
On the hand the receivers may be so saturated with RF from the Marti that they may never be able to find a clear channel.

Try using the 1/4 wave antennas that come with the receivers on the DA49's inputs for a test, also try it with just the 1/4 wave antennas connected directly to the receivers.

Here's a few really long shot ideas, but strange things can happen in a strong RF field.

Is the stadium sound system connected in any way to the radio broadcast system?


Is pin 1 tied to any of the shells on the XLR connectors?
How are you connection the  receivers to the sound system, XLR or 1/4 inch?

Thanks so much for the input.  At the first game when i arrived on site the Marti was already turned on. As soon as i observed the RF interference on the meters, i tried to do a scan, i tried a scan in all 9 groups and the receivers could not find a single clear frequency. So i think you are correct the receivers are too saturated with RF.

I did not try the quarter wave antennas, mainly because the receivers are in a metal wall swing rack and i also did not have the quarter waves with me anymore.
I can try that next game. I did try bypassing the distro and going antennas straight into the receiver but that made no difference.

The only sound system connection to the radio broadcast system that i can think of would be through the 120VAC circuit. The first game the guy plugged the Marti  into the same 120VAC outlet as the sound system. The second game i asked him to plug into a different outlet but it was unfortunately was still on the same circuit.
The thought crossed my mind that it might be a chassis to chassis sort of thing happening. However, when i unplug the antennas the interference goes away, so im pretty sure its getting to the receiver through the antenna.

All of the connections are balanced,(using the XLR's on the receiver) Pin 1 is not tied to ground.

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Re: RF problems in new HS stadium install
¬ę Reply #9 on: September 10, 2016, 10:03:32 pm ¬Ľ


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