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Author Topic: RF problems in new HS stadium install  (Read 7824 times)

Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #10 on: September 10, 2016, 10:17:20 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.
Thanks for the input Kevin. I had no idea about the specific broadcast. Since the inception of the company in 2010, i have installed similar systems in roughly a dozen high schools stadiums and a college stadium all with wireless mics, several of them had radio broadcasts also. This is the first time i have ever had a problem. I have definitely learned a lesson about digging into what is being broadcast in advance. In this case, noone from the school seems to know anything about the broadcasts.
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Mac Kerr

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Re: RF problems in new HS stadium install
« Reply #11 on: September 10, 2016, 11:10:08 pm »

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.

Looking at the spectrum on a spectrum analyzer or or scanner like a TTi or RF explorer will let you see if the Marti is operating as it should, or if it's splashing all over the RF spectrum. If it is broadcasting a clean RF signal knowing how strong that signal is at you receivers and antennas will help you determine if overloading your RF inputs is the problem. If the Marti is defective tell them to get it fixed, if it's not, you may need to find a way to filter your system, or get your antennas farther from the Marti and closer to your transmitters, or get the Marti antenna in the null of your antennas, or get mics that have better input filtering, like Lectrosonics with tracking filters.

Mac
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Scott Holtzman

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Re: RF problems in new HS stadium install
« Reply #12 on: September 11, 2016, 12:39:54 am »

Looking at the spectrum on a spectrum analyzer or or scanner like a TTi or RF explorer will let you see if the Marti is operating as it should, or if it's splashing all over the RF spectrum. If it is broadcasting a clean RF signal knowing how strong that signal is at you receivers and antennas will help you determine if overloading your RF inputs is the problem. If the Marti is defective tell them to get it fixed, if it's not, you may need to find a way to filter your system, or get your antennas farther from the Marti and closer to your transmitters, or get the Marti antenna in the null of your antennas, or get mics that have better input filtering, like Lectrosonics with tracking filters.

Mac


This kind of interference mitigation is really in my wheelhouse.

I have to reiterate that a properly placed filter on the output of the Marti, even if it is not malfunctioning is much easier than trying to reduce the interference at the RX input.  Anything installed on the input is going to have some attenuation of the fundamental or unwanted gain.

A three or four cavity bandpass filter, on the output of the Marti will have sufficient Q for the task and be very compact.  A circulator should also be added to keep any RF from getting back into the output amp of the Marti and mixing.

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Scott AKA "Skyking" Holtzman

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Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #13 on: September 11, 2016, 11:19:17 am »

This kind of interference mitigation is really in my wheelhouse.

I have to reiterate that a properly placed filter on the output of the Marti, even if it is not malfunctioning is much easier than trying to reduce the interference at the RX input.  Anything installed on the input is going to have some attenuation of the fundamental or unwanted gain.

A three or four cavity bandpass filter, on the output of the Marti will have sufficient Q for the task and be very compact.  A circulator should also be added to keep any RF from getting back into the output amp of the Marti and mixing.

 

Thanks again to all for these great responses!   In light of all of your suggestions i think my best strategy here will be:

1) Scan with spectrum analyzer as Mac and many others suggested.  I know that i need to find out what FM frequency the Marti is actually broadcasting and whether the output is clean or not and what its input level is at the AT receiver/antennas.

2) Ask the AM guys if they would be willing to put the bandpass filter on the output of their Marti when they broadcast from that stadium.

3) Back up solution: Try filters on the receiver input if the AM guys are not willing to try the bandpass filter.  Not as good of a solution and may attenuate the desired RF signal.


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Kevin Maxwell

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Re: RF problems in new HS stadium install
« Reply #14 on: September 11, 2016, 06:18:35 pm »

There are many more people on here that know a lot more about wireless and Marti units then I do but I will put my slant on this here. I am sure I am also repeating some of what has already been said but maybe in a slightly different way. Part of the time I was writing this I was on the phone with a fellow I work with who knows a lot more about Radio then me. 

It looked like the Marti model you said it is has thumbwheel setting on the front to tune the transmitter to. So I would start by looking at that. Then you will know where to start scanning for issues. I would start at least a little bit below that frequency (if your RF Explorer goes that low) if not even more. Ideally if you can do it with one thing on at a time, in this case just the Marti you should be able to see what it is doing in relationship to what else is on air. You say that Ė ďMain problem seems to be that there is an AM station WLEC 161.640.Ē This is WLEC 1450 (itís frequency is 1450) in Sandusky, Ohio. And the 161.640 looks like it is the frequency of the Marti transmitter back to the station itself.

Which RF-Explorer model did you get? If it is capable of going as low as the Marti transmitter I would gather that you would see if the output looks right. But if you can only scan in your wireless mic range then you should see if there is a lot of garbage there. Hopefully the radio people will be cooperative enough that they will meet you at the stadium at a time when you arenít about to have a game so you can work together to see if you can figure out what the problem is.

Others have mentioned you could have problems if you have more gain in your antenna system (then is needed to make up for cable lengths) for your wireless. It causes the front end of the receivers to take in too much and can desensitize the receiver.  I donít know if your paddles use power or not and if there is any gain setting on them. I use Shure antennas that need power from the antenna distro and have gain switches on them. You said you bypassed the distro and it made no difference but did the antennas themselves have the gain setting like I mentioned. You could try putting the quarter wave antennas on the cable in place of the paddles and see what happens. Also check with the antennas completely disconnected and see if this interference is still getting into the receiver. If the receiver is being saturated without any antennas then maybe it is getting into it some other way then over the air. But I just reread the thread and I see that with the antennas disconnected there is nothing being picked up so forget that.

I am assuming that the AM station is using the Marti to send the signal back to their station or transmitter. So you know how far away that is? Do you know what type of antenna they are using? Is there a land line in the press box that they could use instead of the Marti? I understand that the output power of the Marti is adjustable. Do they need it at full power to reach the receiver? Maybe if they could do it with less power it might help reduce your problems.

You mention that there is an FM station that is using a Marti transmitter also. The same exact questions apply to them as to the AM station people.

And just to clarify in case you donít know. The Marti units are a transmitter that is used to get the signal to their station or transmitter. Usually the receive end (antenna and receiver) is on their broadcast antenna site because it gets it up in the air. They receive this signal and rebroadcast it over there station transmitter.  We were doing an event years ago that had a radio station broadcasting the event and we had a tape player for some singers music and it was crystal controlled. The Marti was messing with the playback speed. Luckily there was enough cable to the Martiís antenna that as soon as it happened we realized what the problem was and someone moved the antenna as far away from us as they could and it fixed the problem. Their studio was across the street but the receiver was on their transmitter tower about a mile away.   

I understand that these Marti transmitters arenít used as much as they used to be. There are cellular solutions that give them 2way communications like the Tieline systems. http://www.tieline.com/ our local HS uses the Tieline Report-IT and Bridge-IT system when they do remote games so they can broadcast them on their radio station. And also versions that use the internet for the link.

Please let us know what you eventually find is the cause and solution.
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Scott Holtzman

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Re: RF problems in new HS stadium install
« Reply #15 on: September 11, 2016, 06:33:36 pm »



Thanks again to all for these great responses!   In light of all of your suggestions i think my best strategy here will be:

1) Scan with spectrum analyzer as Mac and many others suggested.  I know that i need to find out what FM frequency the Marti is actually broadcasting and whether the output is clean or not and what its input level is at the AT receiver/antennas.




Clarification, FM (Frequency Modulation) is the type of modulation where the center frequency is deviated to correspond with the input signal.

FM can be used in any frequency range.

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Scott AKA "Skyking" Holtzman

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Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #16 on: September 12, 2016, 09:26:02 am »

Clarification, FM (Frequency Modulation) is the type of modulation where the center frequency is deviated to correspond with the input signal.

FM can be used in any frequency range.

Duh, yes, duly noted. Hey Scott, I sent you a PM
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Denn Cimo

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Re: RF problems in new HS stadium install
« Reply #17 on: September 12, 2016, 09:45:19 am »


It looked like the Marti model you said it is has thumbwheel setting on the front to tune the transmitter to. So I would start by looking at that. Then you will know where to start scanning for issues. I would start at least a little bit below that frequency (if your RF Explorer goes that low) if not even more. Ideally if you can do it with one thing on at a time, in this case just the Marti you should be able to see what it is doing in relationship to what else is on air. You say that Ė ďMain problem seems to be that there is an AM station WLEC 161.640.Ē This is WLEC 1450 (itís frequency is 1450) in Sandusky, Ohio. And the 161.640 looks like it is the frequency of the Marti transmitter back to the station itself.

Which RF-Explorer model did you get? If it is capable of going as low as the Marti transmitter I would gather that you would see if the output looks right. But if you can only scan in your wireless mic range then you should see if there is a lot of garbage there. Hopefully the radio people will be cooperative enough that they will meet you at the stadium at a time when you arenít about to have a game so you can work together to see if you can figure out what the problem is.

Others have mentioned you could have problems if you have more gain in your antenna system (then is needed to make up for cable lengths) for your wireless. It causes the front end of the receivers to take in too much and can desensitize the receiver.  I donít know if your paddles use power or not and if there is any gain setting on them. I use Shure antennas that need power from the antenna distro and have gain switches on them. You said you bypassed the distro and it made no difference but did the antennas themselves have the gain setting like I mentioned. You could try putting the quarter wave antennas on the cable in place of the paddles and see what happens. Also check with the antennas completely disconnected and see if this interference is still getting into the receiver. If the receiver is being saturated without any antennas then maybe it is getting into it some other way then over the air. But I just reread the thread and I see that with the antennas disconnected there is nothing being picked up so forget that.

I am assuming that the AM station is using the Marti to send the signal back to their station or transmitter. So you know how far away that is? Do you know what type of antenna they are using? Is there a land line in the press box that they could use instead of the Marti? I understand that the output power of the Marti is adjustable. Do they need it at full power to reach the receiver? Maybe if they could do it with less power it might help reduce your problems.

You mention that there is an FM station that is using a Marti transmitter also. The same exact questions apply to them as to the AM station people.

And just to clarify in case you donít know. The Marti units are a transmitter that is used to get the signal to their station or transmitter. Usually the receive end (antenna and receiver) is on their broadcast antenna site because it gets it up in the air. They receive this signal and rebroadcast it over there station transmitter.  We were doing an event years ago that had a radio station broadcasting the event and we had a tape player for some singers music and it was crystal controlled. The Marti was messing with the playback speed. Luckily there was enough cable to the Martiís antenna that as soon as it happened we realized what the problem was and someone moved the antenna as far away from us as they could and it fixed the problem. Their studio was across the street but the receiver was on their transmitter tower about a mile away.   

Please let us know what you eventually find is the cause and solution.

I got the 3G combo RF explorer. So it will go down to 15MHz I believe. I spent about 8 hours over the weekend trying to get it to work with my laptop and had no luck, the laptop will just not recognize the device. (but that's a whole other problem.)

The paddles have no power on them or amplification. About the quarter wave antennas, I thought that quarter waves need a ground plane to work. I do think I have a pair of C band half waves rubber duckies, I will take those with me to the game Friday night.

The AM station in Sandusky, I believe is about 10 miles from Huron as the crow flies. I will have to ask the AM guy if he has experimented with different power settings. He does remote broadcasts from several HS stadiums so if I had to guess, he probably just operates the thing at full power.

That is good info about using a land line for the broadcast, I will investigate that possibility. Thanks for the links!
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Pete Erskine

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Re: RF problems in new HS stadium install
« Reply #18 on: September 12, 2016, 09:53:32 am »

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.

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.

1.  Are your antennas amplified?  if so try passive.

2.  Move your RX antennas further away.  Ideally locate the RX at field level with the antennas but even a <200' low loss (LMR400) antenna cable to the field would be better.

As a test try the RX on the field with just the whips and see if the Martty affects the RX level meters at all.

3.  Definitely ask them what the freq if their marti is.  A simple hi pass or low pass might help if they are at least 100 mHz away
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Jason Glass

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Re: RF problems in new HS stadium install
« Reply #19 on: September 12, 2016, 01:13:19 pm »

STL (studio-transmitter links) like the Marti are licensed operations with strict power limits, frequency assignment, and transmitter location. They should be used with tuned, highly directional antennas (usually Yagi type) pointing directly at the receiver site. If the operator is using an omni antenna and blasting full power to reach the receiver, this must be corrected or your problems will continue.

Sent from my mobile phone. Please excuse the inevitable spelling and grammatical errors.

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Re: RF problems in new HS stadium install
¬ę Reply #19 on: September 12, 2016, 01:13:19 pm ¬Ľ


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