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Author Topic: IEM distribution antenna / combiner  (Read 1321 times)

jasonfinnigan

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Re: IEM distribution antenna / combiner
« Reply #10 on: June 16, 2014, 09:30:52 pm »

One of the guys from a Shure has given a great presentation a number of times on intermod. He has a spectrum analyzer and proceeds to move a couple of belt packs closer together, then with three. Looking at the spectrum analyzer you can see the intermod products.

He shows that if you pick channels at random, you may get problems. Also, if you use the tools provided by the manufacturer (wireless workbench in this case), you will get channels that are not going to put intermod products on top of other active frequencies.


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I think I've seen that demo before.

But from experience I can tell you that intermod is a big issue from when I worked on cell towers dealing with both Point 2 point Wifi (site to site Air Fiber connections) and emergency services signals. There's a reason they make multiplexers.
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Lyle Williams

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Re: IEM distribution antenna / combiner
« Reply #11 on: June 17, 2014, 06:17:32 am »

Ok, I did some calculations and the intermod performance of a properly combined single antenna IS better than separate antennas, given the way sound people are likely to deploy those antennas.  Ie, close together and without crossed polarisation.

Rough calculations here on intermod product strength from two 30mW signals for B-band gear I have here seem to be about 1uW with the ZAPD-21 as a passive combiner.  Separate antennas yield worse performance until you get about 5m antenna separation.  A BNC-T is simply dreadful, maybe 100x worse than the passive combiner.

A circulator will be better than the ZAPD-21, but not hugely better.  Maybe 6-10dB?

How much do we care about a millionth of a watt interference as long as it is out-of-band relative to our gear?

(But I stand corrected.   :-)   )
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Steve.Oldridge

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Re: IEM distribution antenna / combiner
« Reply #12 on: June 17, 2014, 11:53:44 am »

Just wondering about these .....
I have 2 of the Sennheiser EW300 units each with 2 body backs which I run in mono.
I have them next to each other in a rack with the antennas to the front and I have had zero problems and love them.
 I have the chance to purchase one more with one pack used but new condition and was wondering whether if I decide to add one more system if I need to think about getting one of those antenna combiner / distributer thingies. Is it really necessary at this point and in fact why would it be necessary at all? ... Benefits etc....

Deb.. this thread got real technical, real quick.
FWIW - We have 4 PSM units and a Carvin EM900 in the rack with no intermod (so far) - all with front mount antennas, located on stage as band does not have FOH person.

We did look at the extra expense of antenna combination options, talked to a few folks who [basically] concluded that over the range in question, being stage located, not being mixed/xmitted from FOH, that we should NOT need the extra equipment expense. So far that has been proven to be correct.
YMMV.
« Last Edit: June 18, 2014, 09:58:38 pm by Steve.Oldridge »
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Debbie Dunkley

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Re: IEM distribution antenna / combiner
« Reply #13 on: June 17, 2014, 11:58:29 am »

Wow…thanks everyone - "be careful what you wish for"..
I understood only a small part of what has been discussed here - you peeps are all way too clever with this stuff!!!
Anyway, am I right in saying that adding one more unit to the 2 I already have wouldn't bring any problems for the most part?
The guy didn't get back to me on the Sennheiser EW300 I was looking at so I won't be adding another system just yet but when one comes up at the right price I'll probably give it a go without the antenna distributor and if I get problems, I'll just have to add one !!!.
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Debbie Dunkley

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Re: IEM distribution antenna / combiner
« Reply #14 on: June 17, 2014, 12:01:50 pm »

Deb.. this thread got real technical, real quick.
FWIW - We have 4 PSM units and a Carvin EM900 in the rack with no intermod (so far) - all with front mount antennas, located on stage as band does not have FOH person.

We did look at the extra expense of antenna combination options, talked to a few folks who [basically] concluded that over the range in question, being stage located, not being mixed/xmitted from FOH, that we should need the extra equipment expense. So far that has been proven to be correct.
YMMV.

Thanks Steve…ironically I just posted my comment as yours was posting too. I did get rather overwhelmed with the info being provided but I respect everyones knowledge and experience here  so I never want to sound unappreciative.

Good to know you are not having issues going this route...
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A young child says to his mother, "Mom, when I grow up I'm going to be a musician." She replies, "Well honey, you know you can't do both."

Keith Broughton

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Re: IEM distribution antenna / combiner
« Reply #15 on: June 17, 2014, 02:11:38 pm »

To clarify, and simplify...
 "antenna combiner" takes multiple outputs from IEM transmitters and combines them to transmit on one antenna.
"Antenna distribution" takes one receive antenna and distributes it to multiple receivers.
You can use a "powered" (receive) antenna on a distribution system but only a "passive" (transmitter) antenna can be used on a combiner
Hope this helps :)
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Steve.Oldridge

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Re: IEM distribution antenna / combiner
« Reply #16 on: June 17, 2014, 04:07:44 pm »

Thanks Steve…ironically I just posted my comment as yours was posting too. I did get rather overwhelmed with the info being provided but I respect everyones knowledge and experience here  so I never want to sound unappreciative.

Good to know you are not having issues going this route...

Yeah.. the guys on this forum are great! !!  Very knowledgeable and very helpful.. and some of them have more than helped me at times... I don't do the SR thing much anymore and have gone to the other side of the stage, but still try to stay "up" on things and try to apply what I've learned to current band.

Much as I actively prefer someone at FOH, reality is a number of the venues we play (35%?) have house systems and of the rest, many require that you mix from side of stage as they don't want console/wires out front.
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Rob Spence

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Re: IEM distribution antenna / combiner
« Reply #17 on: June 17, 2014, 11:42:26 pm »

Ok, I did some calculations and the intermod performance of a properly combined single antenna IS better than separate antennas, given the way sound people are likely to deploy those antennas.  Ie, close together and without crossed polarisation.

Rough calculations here on intermod product strength from two 30mW signals for B-band gear I have here seem to be about 1uW with the ZAPD-21 as a passive combiner.  Separate antennas yield worse performance until you get about 5m antenna separation.  A BNC-T is simply dreadful, maybe 100x worse than the passive combiner.

A circulator will be better than the ZAPD-21, but not hugely better.  Maybe 6-10dB?

How much do we care about a millionth of a watt interference as long as it is out-of-band relative to our gear?

(But I stand corrected.   :-)   )

Last I checked, 6-10dB is a big deal.

Debby, are you using a tool to determine your frequencies? If so, be sure to include wireless mics as well.

Sent from my iPad using Tapatalk HD
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Lyle Williams

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Re: IEM distribution antenna / combiner
« Reply #18 on: June 18, 2014, 02:39:33 am »

Last I checked, 6-10dB is a big deal.

To clarify, the offending intermod signal (which should fall on a frequency that you are not using) would be about 50dB down in strength from your main signal if using the passive combiner.  Using a circulator/duplexor might mean that the offending signal might be 56-60dB down in strength from your main signal.

Even if it was zero dB down, it would cause you no problems unless it fell on another frequency you are using!  If you stick to vendor recommended intermod-free channel allocations you should largely be immune from self-generated intermod problems.

This is nothing to do with the quality of the audio.  This is just to do with how much RF garbage you leave around for others who may use the RF spectrum.  As wireless mics are a secondary user of spectrum, you should worry (or not worry!) about the interference caused by your main signal before you worry (or not) about your intermod products.  :-)

(RF-wise, this issue is about as big a deal as two people with mobile phones sitting next to each other on a bus....)
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jasonfinnigan

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Re: IEM distribution antenna / combiner
« Reply #19 on: June 18, 2014, 03:12:24 pm »


(RF-wise, this issue is about as big a deal as two people with mobile phones sitting next to each other on a bus....)

I don't care to go into all the details now but, this isn't the same thing at all.
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