Wireless Mounting a large antenna on the roof!?

[Standby]

I'm a high school theater manager and yesterday the head of facillities and maintenance for my school district was heading to the grid. After asking him what he was up to, he said they were looking to install an antenna on top of the roof for "district communication." I asked, "like for the school bus radios etc?" and he replied,"yes."

Now I'm guessing that an antenna to reach over the entire school district will have to be pretty powerfull. I asked him to wait to install anything so that we can research the potential interferance problems.

So CB sound guru's -- do you think that an antenna used for communicating between school bus radios etc, mounted outside on top of the roof of the fly loft would interferre with our wireless system. We have 16 shure systems of which some are ULX1-J1 on frequencies of 554 - 590 MHz and some are ULX-M1 with freq's of 662-698 MHz.

Second question - the company that installed the system in our space put the antenna distribution system in the booth which is 100 feet from the plaster line under the balcony. One antenna is on the back wall of the sound booth side and one is on the back wall of the lightbooth side which has unopenable glass windows.

We're having some dropping out issues and I have a company tech coming out to analyze our set up and see about picking up better antennas and getting them out of the booth and out from under the balcony overhang.

Any thoughts ?

 

[Derrick]

I think it is a good idea you have to want to analyze this further. However questions do arise.

What frequency will the new system be on?
Will it be a simplex or repeater system?
What will the transmitter power output be?
What is the gain of the selected antenna?
What losses will be in the antenna feedline ?
What is the distance above the roof line the antenna will be mounted?

You have supplied your current range of wireless frequencies in use, which is good. Although for a complete interference study (intermod calculations) the actual frequencies in use are needed to check for problems.

Note, if this is a repeater system, the "antenna" may not have to be "all that powerful".

Something else to be considered is the Maximum Permissable Exposure (MPE) of the RF field.

The communications shop providing the system "should" be able to help you with the above information and studies. Let the board know if this results in a deadend. There are several qualified folks on here that can give valuable input.

More information is also needed regarding your current drop out problems. Could you describe in more detail the current setup?

 

[TimmyP1955]

How far are your wireless antennas from the transmitters?

How long are the cables from the antennas to the splitters?

What type of cable is being used?

 

[mbenonis]

Here's the short answer: no, it won't cause a problem, and they should go ahead and do it. The type of radio system they're installing (a land mobile radio system) will likely operate at 150 MHz, 460 MHz, or (less likely) 800 MHz. It's most likely a repeater system, and that means it'll probably run somewhere around 25-100W max. That said--this is way outside the front end window of your receivers, and so even if the system is transmitting (which it probably would not be during a show anyway), the received signal inside your receivers (where intermod is most likely to occur) will be small enough to neglect (and this is for 460 MHz...if it's at 150 MHz, then you're golden for sure).

Also, it's worth noting that the (presumably metal) roof will help to attenuate the signal significantly, which will help you even more.

As far as your mic antenna question goes: what kind of antennas do you have now? If you are not using *passive* LPDA (shark fin) antennas, you should buy a pair and use them. Under the balcony overhang isn't necessarily bad as long as the antennas are pointed at the stage. Make sure they are separated by at least a few feet (preferably as far apart as is practical).

 

[Footer]

In college we had the campus radio tower on top of our mainstage theatre. It has campus radios and our campus radio station antenna's on it. We never had an issue with any wireless. However, you could pick up the station rather clearly on non-shielded cables. That is an issue easily resolved.

As others have said, you should be fine.

 

[fx120]

If it's for district-wide communication it's probably operating on a pretty low frequency, I'd guess down in the low VHF range. Some smaller districts still use commercially available CB radios which I believe operate somewhere down in the 10Mhz range, but I'd bet that they're using something like the business band which (if I recall correctly) is in the 150Mhz range, and some in the 450Mhz range.

Your mics should be fine.

 

[mbenonis]

In college we had the campus radio tower on top of our mainstage theatre. It has campus radios and our campus radio station antenna's on it. We never had an issue with any wireless. However, you could pick up the station rather clearly on non-shielded cables. That is an issue easily resolved.

As others have said, you should be fine.

If I had to guess, an AM station?

 

[Footer]

If I had to guess, an AM station?

Nope, 89.5 WJMU. In high school we had a high power AM station's antenna about a 1/2 mile from our theatre. You could cleary hear it from just about any speaker if you put your ear up to it.

 

[Chris15]

As Mike and others have noted, you ought to be fine RF wise. Down here you'd be looking at either 5 or 50W with 5 having a notional 2km coverage and 50 a notional 20km coverage.

This doesn't however mean the curious minds on here wouldn't like to know the answers to some of the questions posed earlier...

We have a 10kW FM transmitter sitting on top of our university and we don't have issues.

Perhaps an issue that may arise is that typically a radio mast has equipment needs to feed it, where were they planning to install that? If it's a single repeater then it may well only be a single rack unit box, but it's still going to have to go somewhere and they may be thinking of using your grid space... It will also need power...