Wireless RF Dropouts

[Rhett]

I'm having RF dropouts with Shure SLX bodypacks. The handhelds work fine. I run 8 SLX4 receiver units in the H5 (518-542 MHz) bandwidth. The antennas are run through a passive splitter to feed both distribution amps. The distance from the pulpit to the booth is approximately 150 feet.

Upon examination of the antenna system, I noticed two things.

First, the antenna is partially obstructed. My common sense tells me this isn't good, but then maybe its not a big deal. Check out the pictures below, and gimme your thoughts on why a reputable installer would do this.



I decided to try a different mount position with the other antenna. I took some spare rack components and came up with this. Would this be beneficial or is this a wasted effort? See below.



Second, I noticed that the antenna is model UA820A (774-865 MHz). Were the wrong antennas installed? My common sense tells me that an antenna would pick up more gain if its tuned closer to the frequency its receiving. I did some poking around and found that Shure makes a UA820H4 (518-574 MHz) antenna. Would it make much of a difference if I installed these instead?

Thank you for your much needed RF wisdom.

Rhett

 

[Chris15]

I think you've hit the nail on the head with your second question...
You'll get much better performance from using antennas within their bandwidth, so you can either order the Shures you've identified or build your own half waves on a centre frequency of 530 MHz.

Once you have the right antennas, then you can look at repositioning them if the problems persist.

Have you checked the beltpack antennas aren't damaged?
Are your frequencies coordinated?

 

[avare]

Chris, like always, immediately identified the main cause. AFTER the right frequency range antenna issue, is the distance being transmitted.

Wireless mic and IEM systems are intended for short range sending of signals. First option is locate the antennas closer to the stage, using coax from the antenna to the splitter. If that is not likely to occur, investigate using directional antennas. Either log-periodic, AKA shark fins, or circularly polarized.

Good luck!

Andre

 

[Rhett]

Thanks for that. I will order two frequency-tuned omni antennas for around $60. If this isn't enough, I'll return and look at Shure's directional amplified antennas. Regarding reclocating the antennas closer to the stage, the cable run would have to be greater than 200 feet. How much dB loss is that with RG-58?

Would you go ahead and remount Antenna A like I did for Antenna B, or is the obstruction not that big of a deal?

I'm assuming the frequencies were software balanced by the installer, but have no easy way of finding out. Do I need to purchase this software? Or is it basically a set of formulas I can calculate myself with pencil and paper? I'm interested in improving the RF as much as I can.

 

[WooferHound]

A no cost and almost instant solution would be to relocate the receivers to the stage. The antenna wires will be short to the receivers and then the audio is sent down the sound snake to the House Mix position. The disadvantage to this is you can't see the display on the receiver, but the signal strength will be huge.

 

[Rhett]

I thought about that WooferHound. I've rented an additional 16 SLX units for productions, placed them on stage near the snake, and never had any drop outs on the rentals. But I do like knowing if the units are on, have a low battery, seeing the audio level, stuff like that.

When I need that many wireless units, I would have to run some wireless from the booth anyway, as my digital snake has a 32 channel limit. So for all of those reasons, I'd like to try and keep them in the booth if possible. But that is a possibility that I've wrestled with.

 

[avare]

Regarding reclocating the antennas closer to the stage, the cable run would have to be greater than 200 feet. How much dB loss is that with RG-58?

With RG-58 ~ 22 dB. With RG-59 INCLUDING THE MISMATCH ~ 14 dB. Jim Brown has an excelent article Which Coax for Wireless Mics that covers the cable selection physics vs cost very well.

Good luck and keep us informed on your iminent success!

Andre

 

[Rhett]

Hey, that's intersting. I've always been told 50 ohm for antennas. I've got tons of RG-6 cable that really wouldn't be a cost factor, that project is long gone. Would you recommend that?

Should I use a 10 dB line amp to make up the difference?

And for my third question, the software RF coordination that was menioned, how do I look into that?

Rhett

 

[mbenonis]

Lots to reply to here.

1) You're definitely right on having the wrong band for your antennas. That said, at a distance of 150', I would seriously be looking at directional antennas. Avoid AMPLIFIED/ACTIVE antennas at all costs and go for a PASSIVE directional antenna, like an LPDA (shark fin). You don't need a CP/Helical antenna.

2) No need for a line amplifier. This is the for the exact same reason you don't need an active antenna. The problem is that these amplifiers often distort and it's very difficult to know this is going on other than when you find that you get random dropouts or other distortion that can't be traced.

3) RG-6 is a good idea, and I'd say you could run it up to about 50' without too much loss. Above that, look at LMR400 or something. On the topic of 50 Ohm cable and impedance matching, the reality is that a dipole antenna is matched to about 72 ohms at the center frequency, and other antennas also vary from type to type. You aren't going to lose that much in the mismatch, and whatever gets reflected will come back and get partially absorbed anyway, so it's not a big deal. Impedance matching for transmitters (like IEM transmitters) is more critical to avoid burning up the final amplifier stage.

4) Moving the receivers closer to the stage is always a good idea.

5) The obstructions probably aren't the major problem here...I'd say the wrong antennas plus distance is the critical factor.

Mike

 

[Rhett]

Avoid AMPLIFIED/ACTIVE antennas at all costs and go for a PASSIVE directional antenna, like an LPDA (shark fin).

Is it reasonable to assume that I'm losing far more RF through 150' of air than I would through 200' of cable?

I've already ordered the frequency-specific omnidirectional antennas. If this does not fix the problem, I will relocate them near the stage with RG-6 cable.

 

[mbenonis]

Without doing a link budget, probably...but that, of course, assumes good coax cable. I would go as far as you practically can with the RG-6, but if you have to make a detour through some space that costs you 50+ feet then you might want to reconsider the antenna location.

 

[Rhett]

What if I ran the cables straight from the booth to the drop ceiling over the stage mounted them upside down? Are we looking at any polarity issues?

 

[Chris15]

Antennas won't care if they are mounted upside down.
Are you sure on that 50m distance? The size of the grand piano in comparison with the antenna in the original photos would tend to indicate a much shorter distance...

I'm going to suggest that once you have the right antennas if there's still problems, then you look at remoting antennas etc.

 

[Rhett]

Thanks Chris. Its not 150' to the piano, good eyes. Its more like 80'. It is a ways to the pulpit though, and part of my thought is calculating when a presenter walks all the way to the other side of the stage, which would be 150' to the booth. The RF has to work all over the room at times, and a 150' radius from the booth covers most of it.

If I were to run a cable to the spot over the piano, I would be using about 120 feet of cable, knowing the route above the ceiling. An antenna to the other side of the stage would be quite a bit more. I agree that I won't relocate the antennas unless the new ones don't solve the problem.

 

[2mojo2]

I run 12 channels of Shure SLX gear in our high school theater.
I placed the antennas and receivers at the stage and carry the signal back on the snake.
I had initial problems with dropouts.
A rep at Shure ran the software for me and sent me a list of the frquencies I should be using.
It worked like magic.

 

[Rhett]

After three weeks, I'm happy to report no drop-outs. I installed the proper antennas and programmed the frequencies that Chris15 worked out for me. This forum is priceless. Stay tuned, more problems to come!

 

[Chris15]

Glad to know that things are working nicely...