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Author Topic: relocating wireless mic antenna  (Read 2294 times)

Gary Fitzpatrick

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relocating wireless mic antenna
« on: January 14, 2016, 04:37:35 pm »

Hi,

I have an installation job coming up where I'm going to be installing a Sennheiser EW135 into a church. The audio rack is located in the back rom which was an addition to the original church building. The walls between the "back room" and the main church are fairly thick...maybe 25 inches in places and solid stone walls.

Would it work if I ran a good quality co-ax cable from the audio rack into the church body, and put the 1/4 wave antenna on the ends of the co-ax?
Was thinking maybe of placing one antenna at the top of the church, and one maybe half way down? the first cable would be maybe 20 feet long, and the second maybe 40 feet long. What grade of co-ax would you recommend for this?

thanks
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Keith Broughton

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Re: relocating wireless mic antenna
« Reply #1 on: January 14, 2016, 05:18:18 pm »

Hi,

I have an installation job coming up where I'm going to be installing a Sennheiser EW135 into a church. The audio rack is located in the back rom which was an addition to the original church building. The walls between the "back room" and the main church are fairly thick...maybe 25 inches in places and solid stone walls.

Would it work if I ran a good quality co-ax cable from the audio rack into the church body, and put the 1/4 wave antenna on the ends of the co-ax?
Was thinking maybe of placing one antenna at the top of the church, and one maybe half way down? the first cable would be maybe 20 feet long, and the second maybe 40 feet long. What grade of co-ax would you recommend for this?

thanks
A 20' separation is really not required to get the diversity to work. A good line of sight to the transmitters is important.
If possible, keep the antenna cables reasonably close to the same length.
LMR 240 would be a good choice of cable.
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Gary-Fitzpatrick

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Re: relocating wireless mic antenna
« Reply #2 on: January 14, 2016, 05:19:25 pm »

What separation would you recommend?


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Jeff Carter

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Re: relocating wireless mic antenna
« Reply #3 on: January 14, 2016, 07:54:48 pm »

On the end of an external coax like that, I believe you will need 1/2 wave antennas rather than 1/4. But 1/2-wave dipole antennas are still very inexpensive.

The idea behind separating the antennas is to make it unlikely that both of them will suffer dropouts from multipath interference at the same time, which in practice means ensuring they're separated by at least a couple of wavelengths. At UHF I'd say 3-4 feet should be plenty.

Keeping cable lengths similar is all about keeping the levels from both antennas roughly similar; for the same reason it's best if the antennas are roughly the same distance from the transmitter.
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Lyle Williams

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Re: relocating wireless mic antenna
« Reply #4 on: January 15, 2016, 04:24:56 am »

For rx antennas, a nice impedance match isn't so important.

Use the existing antennas, or any others of a similar size.  Use any coax you like, but the LMR stuff is good.  Separation doesn't matter that much, but the antennas should be at 90 degrees to each other.
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Keith Broughton

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Re: relocating wireless mic antenna
« Reply #5 on: January 15, 2016, 06:21:51 am »

On the end of an external coax like that, I believe you will need 1/2 wave antennas rather than 1/4. But 1/2-wave dipole antennas are still very inexpensive.

The idea behind separating the antennas is to make it unlikely that both of them will suffer dropouts from multipath interference at the same time, which in practice means ensuring they're separated by at least a couple of wavelengths. At UHF I'd say 3-4 feet should be plenty.

Keeping cable lengths similar is all about keeping the levels from both antennas roughly similar; for the same reason it's best if the antennas are roughly the same distance from the transmitter.
Good catch on the 1/4 wave antenna. You can make a round plate to create a plane and use a bulkhead coax connector but it seems easier to just grab some Shure antennas.
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Henry Cohen

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Re: relocating wireless mic antenna
« Reply #6 on: January 15, 2016, 01:23:51 pm »

If possible, keep the antenna cables reasonably close to the same length.
and
Keeping cable lengths similar is all about keeping the levels from both antennas roughly similar; for the same reason it's best if the antennas are roughly the same distance from the transmitter.

Absolutely unnecessary to keep the coax cables the same length. The concept is to get the best signal strength AND the best CNR at each receiver input as possible. This is primarily achieved by placing antennas in the best locations to cover the desired performance area while having significantly diverse perspectives in order to mitigate signal loss from multipath, performer movement and set piece movement. The shortest, and lowest loss, coax should be used for each antenna. Let the comparator (or phase correlation) circuit in the receiver do its job and choose the optimum antenna. The only thing using a longer than necessary coax gets you is less RF signal at the receiver, and that's not usually the goal.
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Lyle Williams

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Re: relocating wireless mic antenna
« Reply #7 on: January 15, 2016, 04:25:18 pm »

The limiting factor of receivers is normally RF signal-to-noise ratio.  Receivers are rarely operating anywhere near their RF sensitivity limits.  A bit of coax loss will reduce the WL mic signal, but also reduce the accompanying RF noise.

Regaining unobstructed line-of-sight between the mic and the antenna is the goal.

(Don't put your antennas right next to a source of electrical noise like a flouro or dimmer light either!)
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Gary-Fitzpatrick

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Re: relocating wireless mic antenna
« Reply #8 on: January 15, 2016, 07:55:48 pm »

Many thanks for all your help and suggestions :)


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Re: relocating wireless mic antenna
« Reply #8 on: January 15, 2016, 07:55:48 pm »


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