Multiple Antennae on IEM transmitter

[Adam Robinson]

I've seen it done before, but I'm in a situation where I'd like to try it: I'd like to take the output from one IEM transmitter (Sennheiser G3) and send it to a paddle and into my GX8 (this is for my cue mix). I've been just using the paddle (or a small whip antenna) because I sit behind the helical that's connected to my GX8 combiner, but am having range issues on the paddle when I leave monitor world, so I was hoping to be able to do both.

Is doing this as simple as using something like a PWS passive 2-way splitter? Surely there are placement considerations to be had so intermod doesn't get out of hand.

 

[Jason Glass]

Re: Multiple Antennae on IEM transmitter

Hi Adam,

Absolutely do not ever transmit one RF source from more than one antenna. The only time that is ever acceptable is when the TX antennas are located in different RF environments that are completely isolated from each other. The resulting problems from multiple TX antennas in the same space is multipath interference on steroids.

You might try swapping out your local paddle or whip with an omni dipole like the Sennheiser 1031-U, mounted high on a mic stand between monitor world and the stage. It should give you decent coverage throughout monitor world and onstage.

 

[Adam Robinson]

Re: Multiple Antennae on IEM transmitter

That's why I've never done it - I figured the interference from not being able to isolate the two antennae would be murder. That being said, I watched a friend do it on his tour where he had to shoot a helical at the stage for the artist's IEM and then another at FOH for a part of the show where there was a B stage where they'd perform a couple songs. They had no issues with it (using two helicals). In my case, clearly the patterns of a paddle and a helical antenna are not at all complimentary. I'm still left wondering whether what they were doing was an exception to the rule or if they just skirted RF disaster on a regular basis.

 

[Jens Bacher]

Re: Multiple Antennae on IEM transmitter

Maybe this is an application for the RF Spotlight placed right at monitorworld instead of a whip? The field strenght from that would probably be low enought on stage to make it work.
Another alterntive would be to take your output directly to this baby: Professional Wireless Systems-antennas-log-periodic-dipole-array-lpda-antenna-bi-directional

 

[Henry Cohen]

Re: Multiple Antennae on IEM transmitter

Absolutely do not ever transmit one RF source from more than one antenna. The only time that is ever acceptable is when the TX antennas are located in different RF environments that are completely isolated from each other.

To expand on the concept, it is possible to create a "different RF environment" between the primary IEM TX antenna (for the performers on stage) and a secondary antenna by the monitor mixer, and essentially Jens has the right idea. (I do recommend, and have used the Spotlight for this application. Do note I am a consultant to RFVenue.) Given that we're currently dealing with FM systems, and its characteristic capture ratio, all that's required is a consistent difference of >12dB between the two TX signals at the receiver's antenna to eliminate phase distortion. Thus if the primary antenna's signal is weak enough at the monitor position to actually drop out, this would mean the average signal strength is likely hovering around -80 to -85dBm. Using a 2-way way splitter (an unequal directional coupler is even better) and placing a small whip antenna down low amongst the electronics racks right by the monitor mixer's knees or ankles, or the Spotlight on the floor, will provide more than enough signal strength to reliably surpass the 12dB difference needed to mitigate phase distortion at the monitor mixer's IEM pack while providing sufficient signal to avoid drop outs, yet will completely fall into the noise floor by the time it hits the near side of the stage.

 

[Tim Perry]

Re: Multiple Antennae on IEM transmitter

Are you sure you want to sacrifice coverage? Sending half the power elsewhere wont cut the RF field in half... but it will reduce it.

 

[Henry Cohen]

Re: Multiple Antennae on IEM transmitter

Are you sure you want to sacrifice coverage? Sending half the power elsewhere wont cut the RF field in half... but it will reduce it.

You seem to be presuming the ERP (without the intended splitting) is just enough to cover the performance area. If ERP and field strength at the furthest required coverage point has sufficient link budget (headroom), the 3dB loss from the split will not be harmful. That said however, I typically use directional couplers, generally 10dB (meaning the coupled port is 10dB down from the input with the mainline loss at about .5dB).

 

[kristianjohnsen]

Re: Multiple Antennae on IEM transmitter

Absolutely do not ever transmit one RF source from more than one antenna.

Just attended a Sennheiser RF class where the rep explained about a Coldplay tour where the band moved over to a second stage at the back of the venues mid-show. They had an IEM transmitter for each performer at either end set to the same frequency, and the transmitters either side were all programmed to be able to be muted with the press of only one button so that the performers' IEM packs would ever only "hear" one transmitter.

Just thought I'd share as I thought it was a very cool solution. Perhaps Chris (Johnson?) can share, he's one of their techs and I think I've seen his name around here.

 

[Jason Glass]

Re: Multiple Antennae on IEM transmitter

Just attended a Sennheiser RF class where the rep explained about a Coldplay tour where the band moved over to a second stage at the back of the venues mid-show. They had an IEM transmitter for each performer at either end set to the same frequency, and the transmitters either side were all programmed to be able to be muted with the press of only one button so that the performers' IEM packs would ever only "hear" one transmitter.

Just thought I'd share as I thought it was a very cool solution. Perhaps Chris (Johnson?) can share, he's one of their techs and I think I've seen his name around here.

Wow, that is a good idea. It seems like it would be easy enough to implement with WSM and network control.

 

[Tim Perry]

Re: Multiple Antennae on IEM transmitter

You seem to be presuming the ERP (without the intended splitting) is just enough to cover the performance area. If ERP and field strength at the furthest required coverage point has sufficient link budget (headroom), the 3dB loss from the split will not be harmful. That said however, I typically use directional couplers, generally 10dB (meaning the coupled port is 10dB down from the input with the mainline loss at about .5dB).

No so henry, but i do presume that reception to the artists/actors to be paramount and if an expensive RF combiner/booster is needed plus expensive directional antennas are employed that anything done to degrade the signal would be counterproductive.

After re-reading the post a couple of times I have come to realize that the poster wants to utilize the power gain of the GX8 but in a different coverage area. I guess a better plan would be to get a small booster amp to bring the TPO (total power out) up to 250 mW and keep on using a seperate antenna.

 

[Henry Cohen]

Re: Multiple Antennae on IEM transmitter

No so henry, but i do presume that reception to the artists/actors to be paramount and if an expensive RF combiner/booster is needed plus expensive directional antennas are employed that anything done to degrade the signal would be counterproductive.

Diverting RF energy does not necessarily equate to "degrading the signal", if the RF gain structure is sufficient.

After re-reading the post a couple of times I have come to realize that the poster wants to utilize the power gain of the GX8 but in a different coverage area. I guess a better plan would be to get a small booster amp to bring the TPO (total power out) up to 250 mW and keep on using a seperate antenna.

Using an amplifier after the transmitter combiner is not a better plan: Amplifying combined signals will result in a tremendous number of additional intermod products. Any amplification must take place prior to combining.

 

[Tim Perry]

Re: Multiple Antennae on IEM transmitter

Diverting RF energy does not necessarily equate to "degrading the signal", if the RF gain structure is sufficient.




Using an amplifier after the transmitter combiner is not a better plan: Amplifying combined signals will result in a tremendous number of additional intermod products. Any amplification must take place prior to combining.

let me clarify: use the amplifier instead of the combiner.

 

[Chris Johnson]

Re: Multiple Antennae on IEM transmitter

Just attended a Sennheiser RF class where the rep explained about a Coldplay tour where the band moved over to a second stage at the back of the venues mid-show. They had an IEM transmitter for each performer at either end set to the same frequency, and the transmitters either side were all programmed to be able to be muted with the press of only one button so that the performers' IEM packs would ever only "hear" one transmitter.

Just thought I'd share as I thought it was a very cool solution. Perhaps Chris (Johnson?) can share, he's one of their techs and I think I've seen his name around here.

Hello!

Yes, Indeed, thats how it works.

That doesn't mean that the packs wouldn't have worked at that distance (150m or so from main stage), but this way you have guarantees. Sennheiser WSM allows you to RF mute and unmute TXs, but this is tedious, and in the set there was only 30 seconds or so to get this done accross 10 transmitters. So Sennheiser wrote a custom app that meant it could be done much quicker.

This is ultimately 1 of 2 possible solutions. The other solution would be to use an RF over Fiber modulator which would allow you to use 1 set of transmitters. You'd just need to switch the RF output between the onstage antennae and the RF over fiber link. This could probably be a mechanical switch though, something easy to have built.