IEMs - Separate antennas or combine combiners?

Andrew Broughton

Andrew Broughton

Senior
Feb 2, 2011
917
5
18
Back Lounge
www.checkcheckonetwo.com
So let's say you have 12 channels of Sennheiser G3 IEMs and 3 AC-3, all on the same frequency group.

Usually, I see people running them with 3 transmitting antennas (1 on each AC-3 output).
Could the outputs of the 3 AC-3 be fed into another AC-3 and then a single antenna used instead?

Would there be any advantage to combining them again and doing away with 2 of the antennas or would the performance be even worse?
 
Last edited:
Re: IEMs - Separate antennas or combine combiners?

Andrew Broughton said:
So let's say you have 12 channels of Sennheiser G3 IEMs and 3 AC-3, all on the same frequency group.

Usually, I see people running them with be 3 transmitting antennas (1 on each AC-3 output).
Could the outputs of the 3 AC-3 be fed into another AC-3 and then a single antenna used instead?

Would there be any advantage to combining them again and doing away with 2 of the antennas or would the performance be even worse?
Click to expand...

The performance would be worse. You would be amplifying multiple frequencies in the second stage. The amplification should happen before combining to minimize IM.

The Radio Active Designs combiner is a great solution, much nicer than a GX8.

Mac
 
Re: IEMs - Separate antennas or combine combiners?

Andrew Broughton said:
So let's say you have 12 channels of Sennheiser G3 IEMs and 3 AC-3, all on the same frequency group.

Usually, I see people running them with be 3 transmitting antennas (1 on each AC-3 output).
Could the outputs of the 3 AC-3 be fed into another AC-3 and then a single antenna used instead?

Would there be any advantage to combining them again and doing away with 2 of the antennas or would the performance be even worse?
Click to expand...
You could, but it would demand a combiner with bigger input capability, and you would make more intermodulation products.
Think of it this way:
An AC-3 consists of 4 rf amps and a passive combiner. (and some filtering) Each amp has a net gain of 6db, and a maximum input around 100mW.
The Rf amps in the 3 AC-3's will each see 30mW (one carrier) Each carrier is amplified to 120mW (+6 dB) and then put into a combiner with a theoretical loos of -6dB. End result is 4 carriers a 30mW on the output.
If you took the output of the 3 AC-3's into a 4. AC-3 each Rf amp in the last AC-3 would suddenly see 4 carriers of 30mW, which it is not rated for. It would also see the intermodulation products from the previous combiner, and amplify them. Result would be a very "dirty" Rf output.
You could make a workable system by exchanging the 4. Ac-3 with an AC-3200 (it takes 250mW on each input) and use isolators on the outputs of the 3 AC-3'.
 
Re: IEMs - Separate antennas or combine combiners?

Thank you Jens.

Where would one find appropriate isolators and would there be a performance advantage to running the system the way you suggest?

Also, the specs for the AC-3200 II show the inputs rated at 100mW.
 
Last edited:
Re: IEMs - Separate antennas or combine combiners?

Andrew Broughton said:
Thank you Jens.

Where would one find appropriate isolators and would there be a performance advantage to running the system the way you suggest?

Also, the specs for the AC-3200 II show the inputs rated at 100mW.
Click to expand...
www.[URL="http://www.wenteq.com/Ferrite-pdfs/F2332.pdf"]wenteq[/URL].com/Ferrite-pdfs/F2332.pdf as an example. No, but situations exist where you have to amplify a combined output. Think very long antenna cables or multiple room coverage.Sarastech in the UK do some stuff for that: http://www.sarastech.co.uk/iem-distribution-system/2.html
The Ac-3200 will do 250mW, guess they play it safe on the spec sheet. When we did the last Eurovision Danish broadcasting corp. had permission to transmit 250 mW in the full UHF spectrum. We feed 100mW from the SR-2050 into modified AC-3200 which would boost each input to around 250mW. But you need to know what you are doing as the IM products will also be higher...
 
Re: IEMs - Separate antennas or combine combiners?

Andrew Broughton said:
So let's say you have 12 channels of Sennheiser G3 IEMs and 3 AC-3, all on the same frequency group.

Usually, I see people running them with 3 transmitting antennas (1 on each AC-3 output).
Could the outputs of the 3 AC-3 be fed into another AC-3 and then a single antenna used instead?

Would there be any advantage to combining them again and doing away with 2 of the antennas or would the performance be even worse?
Click to expand...

Shure recommends you only use passive combining after the initial active combining stage with IEMs.

http://cdn.shure.com/publication/upload/396/us_pro_antenna_setup_ea.pdf
Page 10, then diagrams starting on page 14.

I've got Sennheiser AC3s with xmitters on the same band passively joined with a shure UA221 then out on RG8 to a transmit paddle. No problems at this time.
 
Re: IEMs - Separate antennas or combine combiners?

Andrew Broughton said:
So let's say you have 12 channels of Sennheiser G3 IEMs and 3 AC-3, all on the same frequency group.

Usually, I see people running them with 3 transmitting antennas (1 on each AC-3 output).
Could the outputs of the 3 AC-3 be fed into another AC-3 and then a single antenna used instead?

Would there be any advantage to combining them again and doing away with 2 of the antennas or would the performance be even worse?
Click to expand...

I asked a few people as Sennheiser a few years ago with this same question, as on occasion, I have over a dozen antennas side stage for a large number of shows (3-5 AC3, 4-8 Receivers, com, etc), and usually 6-8 antennas on a daily basis.

The answer was a lovely Yes/No. They don't recommend more than 8 frequencies off of an antenna, and not to do the combining actively (ie, AC3-AC3). They did mention using passive summing, but you will have loss.

There's not many of these shows where flying a truss or not having enough floor space for antennas is an issue (well, until we discuss the video walls .....), and I haven't had much downtime (me, or equipment) to do some testing to find a decent predictable solution.


BRad
 
Top Bottom