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Author Topic: Cascading Sennheiser AB3 for Multiple Frequency Bands  (Read 347 times)

Christian Ekren

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Cascading Sennheiser AB3 for Multiple Frequency Bands
« on: February 22, 2019, 11:46:58 am »

Hi all,

I have a Sennheiser system of ~32 channels comprised of multiple frequency bands (A & G) with ASA1's and remote-mounted omni A 1031-U antennas (~75' over RG58 w/ A-Band AB3's.) The issue, as expected, is that the G-band systems are hardly getting useable signal level compared to the A-bands. I know you can cascade 2 AB3 units for runs up to ~150ft, but can you cascade 2 AB3's of different frequency bands to create a wide-band booster over a shorter distance?

My initial impressions would be that since the bandpasses of each filter overlap some in the middle, that the increase in noise floor in that area (in this case, centered around ~555MHz) could cause issues, but wanted to ask the experts to be sure..

Alternatively, would just using a Shure UA834WB work better?

Thanks!
« Last Edit: February 22, 2019, 12:13:08 pm by Christian Ekren »
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John Sulek

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #1 on: February 22, 2019, 12:32:46 pm »

Hi all,

I have a Sennheiser system of ~32 channels comprised of multiple frequency bands (A & G) with ASA1's and remote-mounted omni A 1031-U antennas (~75' over RG58 w/ A-Band AB3's.) The issue, as expected, is that the G-band systems are hardly getting useable signal level compared to the A-bands. I know you can cascade 2 AB3 units for runs up to ~150ft, but can you cascade 2 AB3's of different frequency bands to create a wide-band booster over a shorter distance?

My initial impressions would be that since the bandpasses of each filter overlap some in the middle, that the increase in noise floor in that area (in this case, centered around ~555MHz) could cause issues, but wanted to ask the experts to be sure..

Alternatively, would just using a Shure UA834WB work better?

Thanks!

Just one person's opinion but...
I feel your money would be better spent on good quality coax. 75 feet is not that much of a run for LMR400UF or Belden 9913F7.
But I'm not a fan of line amps on the rx antenna unless the distance is a lot longer than 75feet. Usually creates more problems (noise floor) than it solves.
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Mac Kerr

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #2 on: February 22, 2019, 12:40:31 pm »

Hi all,

I have a Sennheiser system of ~32 channels comprised of multiple frequency bands (A & G) with ASA1's and remote-mounted omni A 1031-U antennas (~75' over RG58 w/ A-Band AB3's.) The issue, as expected, is that the G-band systems are hardly getting useable signal level compared to the A-bands. I know you can cascade 2 AB3 units for runs up to ~150ft, but can you cascade 2 AB3's of different frequency bands to create a wide-band booster over a shorter distance?

My initial impressions would be that since the bandpasses of each filter overlap some in the middle, that the increase in noise floor in that area (in this case, centered around ~555MHz) could cause issues, but wanted to ask the experts to be sure..

Alternatively, would just using a Shure UA834WB work better?

Thanks!

The first thing to do is get rid of the RG58. 75' of RG58 has 8.4dB of loss at 550MHz. 75' of LMR400 has 2.3dB of loss at 550MHz. Just by changing to a good cable type you get back 6dB of clean RF and can eliminate the boosters which also add noise. If your layout is such that the mic transmitters are all to one side of your omni antenna you may also get a few dB more by using directional antennas like a LPDA or Helical.

Yes LMR400 costs more than RG58, but works much better. RG58 has no place in RF mic systems except as short jumpers between an antenna distribution and the receivers or less than 25' runs to remote antennas.

Mac
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Christian Ekren

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #3 on: February 22, 2019, 12:45:43 pm »

Just one person's opinion but...
I feel your money would be better spent on good quality coax. 75 feet is not that much of a run for LMR400UF or Belden 9913F7.
But I'm not a fan of line amps on the rx antenna unless the distance is a lot longer than 75feet. Usually creates more problems (noise floor) than it solves.

To be honest, that would be my first preference as well. Wrapped a show last year with 200' runs of 9913 and no boosters for 50+ channels and it worked fantastic.. unfortunately I am not in a position in this venue to make those types of calls, as this is their brand new installed system and any big changes need to go through the integrator since its still under warranty. I was not very pleased to see RG58 in-use, either.

I could most likely get away with adding/swapping a couple rented boosters, but pulling new wire through the conduits, etc. opens a can of worms.

May be able to convince them to look at cable alternatives in the future, but we're also in a big time crunch as the production opens next week.

The first thing to do is get rid of the RG58. 75' of RG58 has 8.4dB of loss at 550MHz. 75' of LMR400 has 2.3dB of loss at 550MHz. Just by changing to a good cable type you get back 6dB of clean RF and can eliminate the boosters which also add noise. If your layout is such that the mic transmitters are all to one side of your omni antenna you may also get a few dB more by using directional antennas like a LPDA or Helical.

Yes LMR400 costs more than RG58, but works much better. RG58 has no place in RF mic systems except as short jumpers between an antenna distribution and the receivers or less than 25' runs to remote antennas.

Mac

Thanks, Mac! This is a brand new educational space that I am coming into as a guest Sound Designer for their inaugural musical production. I wish I had the opportunity to be involved back when the system was being designed by the integrators - higher quality cable runs and LPDA antennas would have been one of my first requests, along with solutions for a few other system quirks that are quickly becoming apparent (and thorns in my side...)

« Last Edit: February 22, 2019, 12:51:24 pm by Christian Ekren »
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Henry Cohen

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #4 on: February 22, 2019, 02:26:24 pm »

unfortunately I am not in a position in this venue to make those types of calls, as this is their brand new installed system and any big changes need to go through the integrator since its still under warranty. I was not very pleased to see RG58 in-use, either.

So it sounds like the designer of the RF system (the integrator?) did not provide a proper design or deployment, and thus it falls back on those folk to do so. What performance testing did they do to commission the RF system?
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Henry Cohen

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Russell Ault

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #5 on: February 22, 2019, 03:01:38 pm »

[...] can you cascade 2 AB3's of different frequency bands to create a wide-band booster over a shorter distance?

My initial impressions would be that since the bandpasses of each filter overlap some in the middle, that the increase in noise floor in that area (in this case, centered around ~555MHz) could cause issues, but wanted to ask the experts to be sure..

Alternatively, would just using a Shure UA834WB work better?

Cascading two band-limited amplifiers would have the effect of giving you more amplification than you'd like in the overlapping area (not just noise, but signal, too, which can cause overload and desens) with rapidly diminishing signal outside of the overlap area. It wouldn't likely make your G band mics work much better (especially at the top end) and would almost certainly reduce the signal strength for your A band mics (especially at the bottom end).

You can use a combination of AB3 bands, but not cascaded. If you passively split the antenna feeds just before the distro, you can send one half through A band AB3s and then on to the A band mics, and the other half through G band AB3s and then on to the G band mics.

If you need to us a single point of amplification then you'll need to use an amp with a pass band that covers everything you're distributing to; the UA834WB would do this.

-Russ
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Christian Ekren

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Re: Cascading Sennheiser AB3 for Multiple Frequency Bands
« Reply #6 on: February 27, 2019, 12:19:53 pm »

Update: the integrator has some RF Venue wide-band boosters on-order.

In the meantime, I temporarily swapped the main antenna lines from RG58 to some 100' RG6 lines I already had on-hand for video monitors and took all the boosters out.

Even with the impedance mismatch (and length), we've got full signal strength on all 32 channels with the omni paddles (surprisingly!)

Another challenge in this space - it has x28 A-band and x4 G-band.... Good thing we're in the middle of nowhere (midwest) Wireless Workbench was able to come up with enough compatible freqs and a couple backups to boot.

Show is up and running better than expected. Thanks again for the advice everyone!

Sent from my LG-H931 using Tapatalk
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