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Author Topic: IEM antenna spacing?  (Read 624 times)

Kevin Maxwell

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IEM antenna spacing?
« on: May 26, 2019, 06:25:56 pm »

IEM antenna spacing?

I am helping with an install in a church and we will be adding a total of 7 Shure IEM systems. We will be using 2 Shure PA411 Four-Port Antenna Combiners and 2 Shure PA805SWB Passive Directional Antennas. Because the 8 port antenna combiners are A LOT more money. How far apart is the recommended distance these 2 antennas should be mounted apart from each other?

These will both be on the stage right wall as close to the ceiling as possible and behind where the people using the IEMs will be. So it should be a relatively clean line of site to their receivers. From there to center stage is about 40feet. The transmitters themselves will be on the other side of that wall, about 10 feet from the antennas in a closet. We will be using Shure UA825 25' UHF Remote Antenna Cable, RG8X/U. BTW the wireless mic receivers are already in that closet and are going to be in the same rack as the IEM transmitters. The antennas for the receivers are on a distribution system, the antennas are just above and slightly behind the users of the mostly handheld wireless.

Thank you for any help you can be with this. 
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Dave Garoutte

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Re: IEM antenna spacing?
« Reply #1 on: May 26, 2019, 09:05:58 pm »

You might be able to run the full 4-port combiner out into one of the the not full 4-port ins and use a single antenna.
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Erik Jerde

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Re: IEM antenna spacing?
« Reply #2 on: May 26, 2019, 09:35:48 pm »

You might be able to run the full 4-port combiner out into one of the the not full 4-port ins and use a single antenna.

Itís usually not recommended to run an active combined into another active combiner.  I donít recall the reasoning but itís a no-no.  You can use a passive combiner though.  Iíve effectively used a Shure passive Y to combine a couple sennheiser antenna combiners without problems.  Thatís why the larger Shure combiners have passive combiner inputs.

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Russell Ault

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Re: IEM antenna spacing?
« Reply #3 on: May 27, 2019, 01:50:18 am »

Itís usually not recommended to run an active combined into another active combiner.  I donít recall the reasoning but itís a no-no.  You can use a passive combiner though.  Iíve effectively used a Shure passive Y to combine a couple sennheiser antenna combiners without problems.  Thatís why the larger Shure combiners have passive combiner inputs.

I'm pretty sure it's because you'll overload the input amplifier for the cascaded input on the second combiner in the chain. Active combiners often have an amplifier on each input to compensate for the loss caused by the passive part of the combiner (~7 dB for a 4-way combiner), and this amplifier is only rated to take in so much signal.

The AC 41 is rated for a maximum input of 30 mW, with the understanding that 30 mW in should give you 30 mW out to the antenna, but that's per channel. If my math is right, multiplied by 4 channels, the output of an AC 41 can be throwing almost an eighth of a watt of power, which is way higher than what the next AC 41 can handle.

-Russ
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Keith Broughton

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Re: IEM antenna spacing?
« Reply #4 on: May 27, 2019, 06:02:48 am »

While the passive splitter would be effective in this application, I  would like to know minimum spacing requirements if 2 or more TX antennas are in play.
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Miguel Dahl

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Re: IEM antenna spacing?
« Reply #5 on: May 27, 2019, 11:10:02 am »

While the passive splitter would be effective in this application, I  would like to know minimum spacing requirements if 2 or more TX antennas are in play.

My guess so far apart and/or at such a different angle to the Rx that when the Rx is obstructed from one angle, it can still get RF from the second antenna.

Some years ago I did a major event where Sennheiser had their own guys doing the RF, and he/they mounted both IEM and Wireless mic antennas on both sides of the stage.
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Pete Erskine

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Re: IEM antenna spacing?
« Reply #6 on: May 27, 2019, 02:12:14 pm »

While the passive splitter would be effective in this application, I  would like to know minimum spacing requirements if 2 or more TX antennas are in play.

My practice is TX antennas are 4' apart and 8' from any RX antenna.  This spacing includes any wifi and cellular antennas which are always 10'- 20' away from any of my antennas.  Digital transceivers, like Freespeak, Bolero or CrewCom, count as TX antennas.
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Kevin Maxwell

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Re: IEM antenna spacing?
« Reply #7 on: May 27, 2019, 02:14:24 pm »

My guess so far apart and/or at such a different angle to the Rx that when the Rx is obstructed from one angle, it can still get RF from the second antenna.

Some years ago I did a major event where Sennheiser had their own guys doing the RF, and he/they mounted both IEM and Wireless mic antennas on both sides of the stage.

In My case each antenna is for different IEM transmitters (4 on one and 3 on the other) so there is no diversity coverage coming into play at all.
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Kevin Maxwell

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Re: IEM antenna spacing?
« Reply #8 on: May 27, 2019, 02:17:48 pm »

My practice is TX antennas are 4' apart and 8' from any RX antenna.  This spacing includes any wifi and cellular antennas which are always 10'- 20' away from any of my antennas.  Digital transceivers, like Freespeak, Bolero or CrewCom, count as TX antennas.

Thank you. So the 2 TX antennas should be 4 feet apart from each other and in this case the RX antennas for the wireless mics are at least 25 feet away from the TX antennas. 
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Keith Broughton

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Re: IEM antenna spacing?
« Reply #9 on: May 28, 2019, 06:40:15 am »

My practice is TX antennas are 4' apart and 8' from any RX antenna.  This spacing includes any wifi and cellular antennas which are always 10'- 20' away from any of my antennas.  Digital transceivers, like Freespeak, Bolero or CrewCom, count as TX antennas.
About what I guesstimated.
Thanks Pete :)
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Henry Cohen

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Re: IEM antenna spacing?
« Reply #10 on: May 28, 2019, 08:02:29 pm »

I'm pretty sure it's because you'll overload the input amplifier for the cascaded input on the second combiner in the chain. Active combiners often have an amplifier on each input to compensate for the loss caused by the passive part of the combiner (~7 dB for a 4-way combiner), and this amplifier is only rated to take in so much signal.

This is part of the reason. Even if the composite power level is below that of the amplifier's 1dB compression point (P1), the amplifiers in the combiners we typically use are not linear enough to amplify multiple carriers; IM products, as well as harmonics, will result, generally strong enough to be detrimental to the coordination.
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Henry Cohen

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Henry Cohen

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Re: IEM antenna spacing?
« Reply #11 on: May 28, 2019, 08:06:19 pm »

My practice is TX antennas are 4' apart and 8' from any RX antenna.  This spacing includes any wifi and cellular antennas which are always 10'- 20' away from any of my antennas.  Digital transceivers, like Freespeak, Bolero or CrewCom, count as TX antennas.

Whereas this is a very good rule of thumb for the RF power levels we typically use, horizontal antenna spacing actually depends on composite power levels, any filtering in use, frequency separation and vertical spacing separation. So higher RF power levels means for physical separation, and/or more filtering, and or greater frequency separation.
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Russell Ault

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Re: IEM antenna spacing?
« Reply #12 on: May 28, 2019, 08:34:46 pm »

This is part of the reason. Even if the composite power level is below that of the amplifier's 1dB compression point (P1), the amplifiers in the combiners we typically use are not linear enough to amplify multiple carriers; IM products, as well as harmonics, will result, generally strong enough to be detrimental to the coordination.

Right, that makes sense. Is this a linearity/power tradeoff as compared to the amplifiers in antenna distros?

Whereas this is a very good rule of thumb for the RF power levels we typically use, horizontal antenna spacing actually depends on composite power levels, any filtering in use, frequency separation and vertical spacing separation. So higher RF power levels means for physical separation, and/or more filtering, and or greater frequency separation.

Is there a more generic rule of thumb for this sort of thing based on power at the antenna instead of distance? Something along the lines of "an IEM signal should show up in another combiner's output at less than X dBm"?

When dealing with uncombined IEM transmitters, is frequency separation as helpful without additional filtering? It seems like, under these circumstances, separation is only going to cause 3rd-order IMD products to be spewed further afield, but without reducing their magnitude?

-Russ
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Henry Cohen

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Re: IEM antenna spacing?
« Reply #13 on: May 28, 2019, 10:24:06 pm »

Right, that makes sense. Is this a linearity/power tradeoff as compared to the amplifiers in antenna distros?

Not really. It's due more to the signal levels. The average carrier level reaching the input stage of a multi-coupler is less than -40dBm, as compared to +15dBm to +20dBm for outbound (IEM, coms, IFB) signals, or about 1,000,000 times stronger. With an RX multi-coupler the composite power from all carriers and general RF noise rarely exceeds a few dBm (milliwatts), unless you're using a preamp, in which case the multi-coupler can be placed into saturation.


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Is there a more generic rule of thumb for this sort of thing based on power at the antenna instead of distance? Something along the lines of "an IEM signal should show up in another combiner's output at less than X dBm"?

No real math rule of thumb due to the number of variables, and a lot due to the characteristics of any given gain stage in multi-couplers, combiners, receivers, and even reflected energy back into combiner output if isolators aren't used. The best rule of thumb as it concerns low power wireless mic/coms/IEMs/ IFBs is as Pete said, ". . . TX antennas are 4' apart and 8' from any RX antenna", and the more the better. That said, anytime you can reduce the unwanted energy by at least 20dB, is a 99% reduction.


Quote
When dealing with uncombined IEM transmitters, is frequency separation as helpful without additional filtering? It seems like, under these circumstances, separation is only going to cause 3rd-order IMD products to be spewed further afield, but without reducing their magnitude?

Yes and no. Those IM products can only be generated in a non-linear stage that has decent performance at the two IEM frequencies in question. If one of those frequencies is outside the range of the device of concern, then the resulting IM products will be very weak. That said, filtering and isolators are always beneficial to performance.
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Henry Cohen

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Gian Luca Cavalliini

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Re: IEM antenna spacing?
« Reply #14 on: June 02, 2019, 02:41:43 pm »




No real math rule of thumb due to the number of variables, and a lot due to the characteristics of any given gain stage in multi-couplers, combiners, receivers, and even reflected energy back into combiner output if isolators aren't used. The best rule of thumb as it concerns low power wireless mic/coms/IEMs/ IFBs is as Pete said, ". . . TX antennas are 4' apart and 8' from any RX antenna", and the more the better. That said, anytime you can reduce the unwanted energy by at least 20dB, is a 99% reduction.


Also, IMHO, minimum distance depends from antenna type and relative polarization. Not recommended, but I've used sometimes one receiving horizontal LPDA a few inch ahead of a vertical transmitting LPDA without any problem (guess what, coax too short from rental company...). Just put both vertical and problem arise. If receiving is omni, in the same range of a IEM transmission by Helix, you need to stay as far as possible, also taking care of the directivity of the Helix (Cardioid? Hyper?).
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