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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Tom Simenauer on August 24, 2017, 06:38:28 am

Title: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 06:38:28 am
Hi.
The venue:

Old church, 40m by 30m with a 30m ceiling. 3 wireless mics, roaming 30 x 35 meters.

Different advice i have had:

 a knowledgeable guy at a well respected audio company here in the UK suggested i place the 2 Sennheiser A1031U omni antenna's close together like around 3 to 5 feet apart.

I have been told by a few people on forums and in retail shops that my plan to have them either side of the stage in the church i do the sound for would be good practice. They would be about 10 meters apart.

I am new to all of this. So i don't know.
Please advice, as this will also determine the length of cable i will need.
Title: Re: Positioning of Omni antenna's
Post by: Keith Broughton on August 24, 2017, 06:52:23 am
Should work fine but remember to buy good quality, low loss coax antenna cable.
Getting all this hardware together is fine but co-ordinating frequencies with the RF environment around you is very important.
The best hardware setup is pointless with poor frequency co-ordination.
Title: Re: Positioning of Omni antenna's
Post by: Jean-Pierre Coetzee on August 24, 2017, 07:11:16 am
If it's much more than 1 wavelength apart it's probably wasted cable. Put it where there is LOS to the mics and separate it by 1 wavelength and you will be set... 10m may be a bit excessive...

Much more importantly keep cable length as low as possible and make sure you co-ordinate, whichever way you decide to do it, using the manufacturers pre-set frequencies should be fine for 3 mics just make sure those frequencies are clear.
Title: Re: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 07:49:17 am
Getting all this hardware together is fine but co-ordinating frequencies with the RF environment around you is very important.
By "co-ordinating frequencies" do you mean scanning with the Sennheiser receiver's and getting a good clear channel for each mic?
Title: Re: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 07:58:21 am
If it's much more than 1 wavelength apart it's probably wasted cable. Put it where there is LOS to the mics and separate it by 1 wavelength and you will be set... 10m may be a bit excessive...

Much more importantly keep cable length as low as possible and make sure you co-ordinate, whichever way you decide to do it, using the manufacturers pre-set frequencies should be fine for 3 mics just make sure those frequencies are clear.
I just googled 606.5mhz wave length, just under 50cm.
So is that the ideal length to separate the antenna's? To get 100%  LOS i have to have them at the front of the stage either side.
What do most conference Q&A people do with mics in the audience?
Title: Re: Positioning of Omni antenna's
Post by: Jean-Pierre Coetzee on August 24, 2017, 08:10:09 am
Go to the sticky section of the wireless forum and read a few of those articles.

Yes honestly 50cm would be enough. 1m would do just fine as well. You will find your 3-5 feet recommendation fits those distances pretty well. You really don't need a ton of space for diversity antennas, the only reason for spacing them further apart would be to increase your coverage range but you would be sacrificing diversity reception.

I have 2 antennae backstage pointed towards the crowd for Q&A in the crowd, but I use directional antennae, with the omni you really won't have a problem.

I would recommend getting them high up and possibly close to the middle of the area they will be used but this would very well be out of your skillset.

Also check your relevant laws in the UK about using wireless mics, from what I remember they are very strict about things there.

For the co-ordination just make sure you use the frequencies from the same bank in the Sennheiser menu and at lease make sure when it is tuned to that frequency there is no dropout when it is used throughout the entire band. I'm not sure if the auto scan keeps wishing the same bank on the units but someone else here will know that and chime in. I generally scan and co-ordinate using WWB...
Title: Re: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 08:35:20 am
Go to the sticky section of the wireless forum and read a few of those articles.

Yes honestly 50cm would be enough. 1m would do just fine as well. You will find your 3-5 feet recommendation fits those distances pretty well. You really don't need a ton of space for diversity antennas, the only reason for spacing them further apart would be to increase your coverage range but you would be sacrificing diversity reception.

I have 2 antennae backstage pointed towards the crowd for Q&A in the crowd, but I use directional antennae, with the omni you really won't have a problem.

I would recommend getting them high up and possibly close to the middle of the area they will be used but this would very well be out of your skillset.

Also check your relevant laws in the UK about using wireless mics, from what I remember they are very strict about things there.

For the co-ordination just make sure you use the frequencies from the same bank in the Sennheiser menu and at lease make sure when it is tuned to that frequency there is no dropout when it is used throughout the entire band. I'm not sure if the auto scan keeps wishing the same bank on the units but someone else here will know that and chime in. I generally scan and co-ordinate using WWB...
thanks.
And if i buy 10meter low loss cables so i can have flexibility where i place the antenna's will that be ok?
Title: Re: Positioning of Omni antenna's
Post by: Jerome Malsack on August 24, 2017, 11:01:23 am
I like to place them with one out front in the audience and one in the back of the stage.  This config is sometimes hard to reach. But it allows the person to move around and turn around and the antenna will have line of sight.  Hard for the person to turn and move and not have one of the antenna not getting a signal.   The one in the back corner is subject to multi path problems so position could be questioned.  Out from the walls with different distance will help  8 inch from wall and 20 inch from back wall.

circles are antenna  cube is the stage and the rest is open to audience.
Title: Re: Positioning of Omni antenna's
Post by: Jean-Pierre Coetzee on August 24, 2017, 12:56:51 pm
General rule of thumb you want as little excess cable as possible so experiment a bit but make sure you make the cable as short as possible. BNC connectors are not hard to terminate so I would say  cut the excess the moment you can an put another(good quality) connector on the shorter cable.
Title: Re: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 01:31:19 pm
General rule of thumb you want as little excess cable as possible so experiment a bit but make sure you make the cable as short as possible. BNC connectors are not hard to terminate so I would say  cut the excess the moment you can an put another(good quality) connector on the shorter cable.
Its not a permanent install, i will pack up after every gig.
AND the system will be used in other halls in London.
Title: Re: Positioning of Omni antenna's
Post by: Tom Simenauer on August 24, 2017, 01:39:14 pm
The cables i have been recommended are RG213, expensive, but probably worth it.

YET ANOTHER thing to consider i have been told, (wow this antenna stuff is WAY more complicated than audio!!) is its best NOT to have different cable lengths for the antenna's?
But i have a cunning plan if this is so. Nothing better than a hunch.
Use RG213 for the 20 meter cable and a lower grade cable for there 10 meter, so it might even out any important difference?

I am sorry for filling this forum up with newbie questions, but i like to be really on top of things, especially as i am about to do the sound for a very high profile guest, and i want a painless gig.

Thanks for all the help guys
Title: Re: Positioning of Omni antenna's
Post by: Jean-Pierre Coetzee on August 24, 2017, 01:50:15 pm
Its not a permanent install, i will pack up after every gig.
AND the system will be used in other halls in London.

In that case sure go for it, would maybe look at getting something that you can use to scan RF with though, if your event is at all high profile a wireless dropping out is a big issue. Likewise maybe some directional antennae are not the worst idea, I can imagine London is quite active and some null points on your antennae might actually be useful.

I've also never had issues with big bags of water(humans) blocking RF with a handheld mic and since most belt packs tend to be placed behind people I find the rear positioning to be better but YMMV. Did have dropout on one mic that followed whatever frequency I selected, I happened to be rewiring the rack and reseating the BNC connectors solved it immediately... Should generally be step 1 for troubleshooting the more I work in this industry the more the problem is quite stupid, like a connector being partially disconnected or just not connecting correctly due to corrosion/oxidation.

Maybe the issue is that your guys have the mics down by their stomaches the whole time and not near their head where it should be.

Also I would strongly recommend power conditioning/UPS for the wireless mics, power issues cause massive headaches with wireless.

Regarding different cable lengths I would likely hope that Henry can pop by and give is some actual science on that.
Title: Re: Positioning of Omni antenna's
Post by: Mac Kerr on August 24, 2017, 01:53:13 pm
General rule of thumb you want as little excess cable as possible so experiment a bit but make sure you make the cable as short as possible. BNC connectors are not hard to terminate so I would say  cut the excess the moment you can an put another(good quality) connector on the shorter cable.

Not necessarily. RF follows inverse square law through the air, it has a defined loss through cable. Getting the antennas closer to the transmitters by running longer cable often has less loss than antennas on shorter cable that are farther from the transmitters. Longer cables may also allow you to position antennas on both sides of the stage to have better coverage of the area of concern.

Mac
Title: Re: Positioning of Omni antenna's
Post by: Jean-Pierre Coetzee on August 24, 2017, 01:55:13 pm
Not necessarily. RF follows inverse square law through the air, it has a defined loss through cable. Getting the antennas closer to the transmitters by running longer cable often has less loss than antennas on shorter cable that are farther from the transmitters. Longer cables may also allow you to position antennas on both sides of the stage to have better coverage of the area of concern.

Mac

Why I said as short as possible, what I meant was don't have a 5m coil laying next to the antennae for no reason.