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Author Topic: wireless frequencies up near the 700mghz range  (Read 8095 times)

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

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com

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

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com

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
Pro Sound Advice, Inc.
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Lee Buckalew
Pro Sound Advice, Inc.

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
Pro Sound Advice, Inc.

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

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com

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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Pete Erskine
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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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