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Author Topic: Can receive antennas interfere?  (Read 5420 times)

Pete Jones

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Can receive antennas interfere?
« on: November 12, 2018, 09:28:45 AM »

Hello, just wondering if anyone can explain what's really going on here? I can't really find a useful answer anywhere else.

A number of manufacturers recommend not putting multiple whip antennas near to each other, e.g. in a rack (let's assume these antennas have good line of sight and not too much metal around them). Why is this? Can something which is receiving interfere, intermodulate with or otherwise affect another receiver? Seems counterintuitive. Or is it that they block signal from one another?

Also Shure recommend to only cascade to one additional ULX-D unit, given the choice do you:
A- use three sets of whips for six units or
B- cascade all six units together and put one set of whips on the top unit
C- it'll work fine either way

Any help greatly appreciated!

Pete

Sent from my EVA-L09 using Tapatalk

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Don Boomer

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Re: Can receive antennas interfere?
« Reply #1 on: November 12, 2018, 11:43:32 AM »


Antenna farms are never a good idea.

When a RF signal reaches your antennas the electrons in the antenna are energized by the electro-magnetic signal thereby absorbing some signal. At the same time the moving charge is creating an oscillating dipole that will emit it’s own signal. When you have an “antenna farm” these signals now absorbed can create arrays and/or change the polar characteristics of all the antennas.

All of this might not be a big factor, but in these days of shrinking bandwidth and increasing interference best practice would suggest you get yourself an antenna distro.
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Russell Ault

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Re: Can receive antennas interfere?
« Reply #2 on: November 12, 2018, 01:58:24 PM »

Also Shure recommend to only cascade to one additional ULX-D unit [..]

The bold text at the bottom of the FAQ is Shure's explanation of the ULXD's cascade limit.

Here's my understanding: to make cascading work, you have to setup unity-gain signal splits, and the only way to do that in RF is to use an amplifier of some kind to offset the ~3.5dB loss of an impedance-matched RF split. Amplifiers are inherently non-linear, and so they distort the signal, which tends to be in the form of increased intermod strength and a higher noise floor. For digital wireless microphones, where intermod products tend to show up as an increase in the noise floor anyway, this effect is compounded. Making things worse is that digital wireless microphones (except for Lectro's digital hybrid) have to operate a lot closer to the noise floor than their analogue counterparts to fit their signal within a spectral mask that was designed with FM transmission in mind. I'm assuming this is why Shure only recommends cascading a single receiver for ULXD (or even Axient Digital, if I'm reading the documentation correctly), but the UR4+ was officially up for cascading 10 receivers.

-Russ
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Lyle Williams

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Re: Can receive antennas interfere?
« Reply #3 on: November 13, 2018, 02:11:39 PM »

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Pete Jones

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Re: Can receive antennas interfere?
« Reply #4 on: November 14, 2018, 11:11:08 PM »

Thanks Russell and Don. I might have to do a few experiments to see which has a worse effect, more antennas or too many cascades. We do usually just use antenna distro, but it was a discussion on a job where one wasn't available.

Sent from my EVA-L09 using Tapatalk

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Russell Ault

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Re: Can receive antennas interfere?
« Reply #5 on: November 15, 2018, 01:34:12 AM »

Thanks Russell and Don. I might have to do a few experiments to see which has a worse effect, more antennas or too many cascades. We do usually just use antenna distro, but it was a discussion on a job where one wasn't available.

For my money I'd take a small antenna farm over cascading 3x the number of units you're supposed to. You won't break anything, but I suspect that by the time you get to the end of the chain of six your CNR is going to be pretty hopeless...

-Russ
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Keith Broughton

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Re: Can receive antennas interfere?
« Reply #6 on: November 15, 2018, 07:42:10 AM »

You "might" get acceptable results cascading 3 units, but no more!
A reasonable compromise might be 2 sets of  3 cascaded receivers.
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Nathan Salt

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Re: Can receive antennas interfere?
« Reply #7 on: November 17, 2018, 01:36:28 AM »

I have cascaded 3 ulx-d's and it worked. Didn't take any measurements on the 3rd unit though. Might do that in the shop sometime to see how bad it actually is
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Nathan Salt

Ike Zimbel

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Re: Can receive antennas interfere?
« Reply #8 on: November 17, 2018, 03:22:42 AM »

I have cascaded 3 ulx-d's and it worked. Didn't take any measurements on the 3rd unit though. Might do that in the shop sometime to see how bad it actually is
With no TX's turned on, if all three sets of RF displays are dark, you're fine. If you see lights on the 3rd unit that you aren't seeing on the first two, then your noise floor is higher on the 3rd one.
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~Ike Zimbel~
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Keith Broughton

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Re: Can receive antennas interfere?
« Reply #9 on: November 17, 2018, 09:00:06 AM »

With no TX's turned on, if all three sets of RF displays are dark, you're fine. If you see lights on the 3rd unit that you aren't seeing on the first two, then your noise floor is higher on the 3rd one.
Good tip Ike👍
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ProSoundWeb Community

Re: Can receive antennas interfere?
« Reply #9 on: November 17, 2018, 09:00:06 AM »


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