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Author Topic: wireless frequencies up near the 700mghz range  (Read 8093 times)

David Parker

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wireless frequencies up near the 700mghz range
« on: May 06, 2011, 07:44:32 am »

my sennheiser wireless has available frequencies right up to the no-mans-land 700mghz range. If I try to use those upper frequencies, I have all kinds of interference. Has anyone else noticed this problem? As I recall, it was low 680's that was having interference.
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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #1 on: May 06, 2011, 09:21:54 am »

my sennheiser wireless has available frequencies right up to the no-mans-land 700mghz range. If I try to use those upper frequencies, I have all kinds of interference. Has anyone else noticed this problem? As I recall, it was low 680's that was having interference.

David,
I apologize if you are already aware of this but your post did not say you had co-ordinated anything.
Interference is going to be the result of other transmitters operating where you are trying to operate.  Have you checked to see what TV stations are broadcasting in your area so that you can avoid those blocks? 
Have you checked to see how far their power level is down at your particular location if you have to fit into a "used" block?
After problems with TV interference you could also be experiencing difficulty with other UHF devices such as radio's or intercoms.


Where are you located (zip code)?  We can get a pretty good idea of how crowded the spectrum is from that.  To really look at your venue location you would need to look it up on the FCC's database and check their contours for stations in your area.

Lee Buckalew
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David Parker

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Re: wireless frequencies up near the 700mghz range
« Reply #2 on: May 06, 2011, 09:42:30 am »

David,
I apologize if you are already aware of this but your post did not say you had co-ordinated anything.
Interference is going to be the result of other transmitters operating where you are trying to operate.  Have you checked to see what TV stations are broadcasting in your area so that you can avoid those blocks? 
Have you checked to see how far their power level is down at your particular location if you have to fit into a "used" block?
After problems with TV interference you could also be experiencing difficulty with other UHF devices such as radio's or intercoms.


Where are you located (zip code)?  We can get a pretty good idea of how crowded the spectrum is from that.  To really look at your venue location you would need to look it up on the FCC's database and check their contours for stations in your area.

Lee Buckalew
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no, I haven't co-ordinated anything, this was my first stab at the problem. It's not really a problem, I can use frequencies lower in the spectrum with no problem. I was of the opinion that below 698mghz we had free run. I was looking for info and your post will help.
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David Parker

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Re: wireless frequencies up near the 700mghz range
« Reply #3 on: May 06, 2011, 09:49:14 am »

David,
I apologize if you are already aware of this but your post did not say you had co-ordinated anything.
Interference is going to be the result of other transmitters operating where you are trying to operate.  Have you checked to see what TV stations are broadcasting in your area so that you can avoid those blocks? 
Have you checked to see how far their power level is down at your particular location if you have to fit into a "used" block?
After problems with TV interference you could also be experiencing difficulty with other UHF devices such as radio's or intercoms.


Where are you located (zip code)?  We can get a pretty good idea of how crowded the spectrum is from that.  To really look at your venue location you would need to look it up on the FCC's database and check their contours for stations in your area.

Lee Buckalew
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I checked the sennheiser zip code chart and 692-704 shows to be vacant in the houston area. My problem was in this range. Hmm?
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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #4 on: May 06, 2011, 09:56:26 am »

no, I haven't co-ordinated anything, this was my first stab at the problem. It's not really a problem, I can use frequencies lower in the spectrum with no problem. I was of the opinion that below 698mghz we had free run. I was looking for info and your post will help.

There is no "free run" for wireless mics.  Both licensed and unlicensed wireless mic users must work around TV channels since wireless mics are considered to be secondary users of that frequency spectrum. This is not new, it is the same situation that we have had for decades, we just have less space available so problems may show up more frequently. 
There are differences in regulations for licensed and unlicensed operation, I am guessing from your reply you are unlicensed.  As an unlicensed operator you are required to accept any outside interference with no recourse and cause no interference to licensed operators. 

If you want to operate in the most interference place possible you need to check on local TV use and other possible sources of interference.  TVBD's will be coming online around Christmas time, this will create some additional potential for intermittent interference. 

There are many posts in the forums discussing this.  Do a search for Henry Cohen.  He has contributed to nearly all wireless discussions here and is a tremendous resource.  Look through the various discussions and I think you'll be able to answer many of your questions.

Lee Buckalew
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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #5 on: May 06, 2011, 10:06:25 am »

I checked the sennheiser zip code chart and 692-704 shows to be vacant in the houston area. My problem was in this range. Hmm?

The Sennheiser site does link directly to the FCC database but it will only show you TV stations.  There are other possible sources of interference that could be licensed users or could be local problems.
Is that one wireless the only wireless operating in the building?
Is it possible that a second transmitter is on somewhere nearby?
Are there wireless intercoms?
Have you actually scanned the area to see what the interfering frequency signature looks like?  Sometimes you can tell what it is by the shape of the signature on a scope or scanner.

Lee Buckalew
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David Parker

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Re: wireless frequencies up near the 700mghz range
« Reply #6 on: May 06, 2011, 10:19:05 am »

The Sennheiser site does link directly to the FCC database but it will only show you TV stations.  There are other possible sources of interference that could be licensed users or could be local problems.
Is that one wireless the only wireless operating in the building?
Is it possible that a second transmitter is on somewhere nearby?
Are there wireless intercoms?
Have you actually scanned the area to see what the interfering frequency signature looks like?  Sometimes you can tell what it is by the shape of the signature on a scope or scanner.

Lee Buckalew
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I let the receiver scan in that bank and I got "0" free frequencies. This happened in two locations, neither of them near the other. At that time I was not aware that when the receiver does a scan, it only scans the selected bank, not all banks. I think my shure wireless scans all the frequencies it covers when it does a scan.
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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #7 on: May 06, 2011, 10:30:32 am »

I let the receiver scan in that bank and I got "0" free frequencies. This happened in two locations, neither of them near the other. At that time I was not aware that when the receiver does a scan, it only scans the selected bank, not all banks. I think my shure wireless scans all the frequencies it covers when it does a scan.

What model is it?
You may be able to connect to your computer using WSM and do a scan.  What Shure wireless model, what frequency range?  Two (or more) wireless systems will create additional broadcast frequencies in the same way that multiple musical notes interact and produce a new harmonic structure.  In wireless RF, these harmonics are intermodulation products.  These do need to be coordinated.  Two mics are not terribly complicated to coordinate but as the quantity increases the difficulty increases exponentially.

So, how many other wireless are you attempting to use together?

Lee Buckalew
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David Parker

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Re: wireless frequencies up near the 700mghz range
« Reply #8 on: May 06, 2011, 10:34:51 am »

What model is it?
You may be able to connect to your computer using WSM and do a scan.  What Shure wireless model, what frequency range?  Two (or more) wireless systems will create additional broadcast frequencies in the same way that multiple musical notes interact and produce a new harmonic structure.  In wireless RF, these harmonics are intermodulation products.  These do need to be coordinated.  Two mics are not terribly complicated to coordinate but as the quantity increases the difficulty increases exponentially.

So, how many other wireless are you attempting to use together?

Lee Buckalew
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yesterday I was only using the one sennheiser. A couple of weeks ago I was using several guitar wireless and wireless inear systems with no problem. The whole problem appears to be the highest bank this unit can use, which is in the 680 and up range. It doesn't seem to matter where I am, I get no free channels in that upper spectrum. That's what made me think that maybe the interference was coming from the new 700mghz usage. I have no problems in the lower bank frequencies.
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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #9 on: May 06, 2011, 10:53:44 am »

yesterday I was only using the one sennheiser. A couple of weeks ago I was using several guitar wireless and wireless inear systems with no problem. The whole problem appears to be the highest bank this unit can use, which is in the 680 and up range. It doesn't seem to matter where I am, I get no free channels in that upper spectrum. That's what made me think that maybe the interference was coming from the new 700mghz usage. I have no problems in the lower bank frequencies.

If you would list out what you are using and in what frequencies (not just ranges) we may be able to offer some help.  As you get higher in the spectrum the points of interference caused by intermod get closer together.  It is quite possible that other wireless are causing the problem although, if it only is happening when the one transmitter is on this would not be the case.
Remember that your in ears are transmitting from the rack mount/tabletop units, not the belt packs.  If those are on it could be the cause of the interference.  This still comes back to laying out ALL of your wireless systems and co-ordinating their frequencies.

Lee Buckalew
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Stuart Pendleton

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Re: wireless frequencies up near the 700mghz range
« Reply #10 on: May 06, 2011, 12:02:19 pm »

David, I don't work for these guys so this is not a commercial plug. I bought Stage Research's RFGuru to help with issues and it has been great.  It has almost made interference during a show a thing of the past.

http://www.stageresearch.com/products/RFGuru/Default.aspx

It is very inexpensive and is updated regularly.  I use it to coordinate 6 wireless guitar/microphones and 5 IEMS and it has done the job.  We work a bar circuit, and ability to save the frequencies on a venue by venue basis has been great.  I walk in, open the file, and pull up the venue if we have been there before and it is ready to go.  I always do a quick test with current data to make sure nothing changed, but it seldom has it required me to modify the file for the next show there.

There are online free resources, but I like having it on my laptop so I can check things at will, even when no wifi access is available to me for online resources.
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Henry Cohen

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Re: wireless frequencies up near the 700mghz range
« Reply #11 on: May 06, 2011, 05:48:20 pm »

my sennheiser wireless has available frequencies right up to the no-mans-land 700mghz range. If I try to use those upper frequencies, I have all kinds of interference. Has anyone else noticed this problem? As I recall, it was low 680's that was having interference.

Wireless mics, coms, IEMs, IFBs are permitted to operate 470-608MHz and 614-698MHz in the UHF-TV spectrum. It sounds like you're experiencing inteference from either an outside source (TV station/translator/booster, other wireless mics/coms/IEMs/IFBs, or video wireless assist) or IMD due to mixing.
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Henry Cohen

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Henry Cohen

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Re: wireless frequencies up near the 700mghz range
« Reply #12 on: May 06, 2011, 05:59:07 pm »

Two (or more) wireless systems will create additional broadcast frequencies in the same way that multiple musical notes interact and produce a new harmonic structure.  In wireless RF, these harmonics are intermodulation products.  These do need to be coordinated.

Not all harmonics create IMD, and a not all IMD is a result of harmonics. Harmonics generally only occur when an RF component goes into saturation (denoted as third order intercept point or IP3) caused by too much RF energy and emits harmonics of the frequenc(ies) comprising the overpowering energy: fx2, fx3, fx4, etc. This is not the IMD we typically experience when frequencies mix in a non-linear (amplified) stage in the mathematical pattern of 2A-B, 2B-A, A+B-C, A-B+C, etc.


Quote
Two mics are not terribly complicated to coordinate but as the quantity increases the difficulty increases exponentially.

Two mics require no coordination (beyond appropriate selection of clear spectrum and channel bandwidth). Only with the introduction of a third transmitter does the result of mixing produce a potential inteferer.
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Henry Cohen

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David Parker

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Re: wireless frequencies up near the 700mghz range
« Reply #13 on: May 07, 2011, 10:46:09 am »

my sennheiser wireless has available frequencies right up to the no-mans-land 700mghz range. If I try to use those upper frequencies, I have all kinds of interference. Has anyone else noticed this problem? As I recall, it was low 680's that was having interference.

This conversation went way over my head in a hurry! Whew! Used the mic in question last night with one of my regular bands. 6 total wireless running within the band. No problems whatsoever. Seems there is something "in the air" in Houston in the upper 600 mghz. I dropped it down to the 627 area and had no problems at all.
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Henry Cohen

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Re: wireless frequencies up near the 700mghz range
« Reply #14 on: May 07, 2011, 11:04:56 am »

I checked the sennheiser zip code chart and 692-704 shows to be vacant in the houston area. My problem was in this range. Hmm?

Yet again proving the manufacturers' website FCC look ups are not comprehensive. If one checks directly with the FCC Media Bureau database, we find two construction permits issued for channels 46 and 49 in the Houston area:
KPBX (ch 46) is a low power DTV station probably in testing.
KEHO (ch 49) is another LP DTV station in testing.


Expect more low power stations to come online in the next few years as the FCC just recently ended a moratorium on LPTV applications.
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Henry Cohen

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Lee Buckalew

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Re: wireless frequencies up near the 700mghz range
« Reply #15 on: May 07, 2011, 11:06:58 am »

Not all harmonics create IMD, and a not all IMD is a result of harmonics. Harmonics generally only occur when an RF component goes into saturation (denoted as third order intercept point or IP3) caused by too much RF energy and emits harmonics of the frequenc(ies) comprising the overpowering energy: fx2, fx3, fx4, etc. This is not the IMD we typically experience when frequencies mix in a non-linear (amplified) stage in the mathematical pattern of 2A-B, 2B-A, A+B-C, A-B+C, etc.


Two mics require no coordination (beyond appropriate selection of clear spectrum and channel bandwidth). Only with the introduction of a third transmitter does the result of mixing produce a potential inteferer.

Thanks for the corrections Henry,
I was talking about two mics being able to be too close together and cause problems. 

Lee Buckalew
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Henry Cohen

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Re: wireless frequencies up near the 700mghz range
« Reply #16 on: May 07, 2011, 11:32:15 am »

Thanks for the corrections Henry,
I was talking about two mics being able to be too close together and cause problems. 

Lee Buckalew
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Misunderstood you. You are correct, though even mid-tier products today have to be quite close to exhibit LO and/or IF mixing with the tuned frequencies.
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Henry Cohen

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Pete Erskine

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Re: wireless frequencies up near the 700mghz range
« Reply #17 on: September 06, 2011, 05:24:41 pm »

Here is a cheat sheet for HOUSTON TV current as of April this year.  The red TV channels are Digital TV and you cannot find clear channels within them.  As you can see there isn't much open space in Houston.  If you are underground or indoors you may be able to use weak channels.


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Re: wireless frequencies up near the 700mghz range
« Reply #17 on: September 06, 2011, 05:24:41 pm »


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