Wireless Distributor Problems?

[RadioKnight]

Hello everyone.

So, I have a problem that's leaving me scratching my head. My boss bought me a brand new RF Venue diversity fin antenna to help with the numerous dropouts that occur with our old Shure ULX P4 and ULX S4 systems. (eight receivers each) These receivers are rolled onstage in a travel rack whenever we need more than eighteen lav mics in a show, and are snaked into the booth. (the first 18 mics are Sennheisers, no problems with those)

Most of the time, the Shure's have awful range, less than 30 feet with complete line of sight. The new antenna gives fantastic range, 4-5 bars over 100-150 feet behind two feet of brick wall, but it only seems to work if you hook it up to a single receiver at a time, using a distributor (Shure) kills the effect and reduces the range to 20-30 feet or so.

Is my distro bad? I'm using 75-ohm BNC cable to connect the antenna to the distro, should I be using 50-ohm? Am I missing something important?

Thanks in advance for any help you can give.

 

[BobHealey]

Something definitely seems odd.
Since you've tried the new antenna direct on a reciever, have you also tried the 1/2 or 1/4 wave antennas direct on a receiver too, and seen if the range changed at all?
On the off chance its RF interference, have you used any software for frequency coordination, or just let the receivers scan and auto assign?
I've used SLX and ULX-D on the Shure distributors and have only had range issues when I've had mismatched antennas (SLX G4 antennas cover most, but not all, of ULXD H50).
The one range test I've done, with SLX 1/4 wave antennas on the distribution unit gave me at least 100' through two wooden walls, outdoors, and across a street.

 

[RadioKnight]

Something definitely seems odd.
Since you've tried the new antenna direct on a reciever, have you also tried the 1/2 or 1/4 wave antennas direct on a receiver too, and seen if the range changed at all?
On the off chance its RF interference, have you used any software for frequency coordination, or just let the receivers scan and auto assign?
I've used SLX and ULX-D on the Shure distributors and have only had range issues when I've had mismatched antennas (SLX G4 antennas cover most, but not all, of ULXD H50).
The one range test I've done, with SLX 1/4 wave antennas on the distribution unit gave me at least 100' through two wooden walls, outdoors, and across a street.

I've tried both 1/4 wave and 1/2 wave antennas on the distro before I received the new RF Venue antenna, which covers 470-698 MHz. The results were similar, lots of dropout and terrible range. Unfortunately, I have no RF coordination software, so I let the receivers scan and assign one at a time, but during my tests I only had a single receiver and transmitter active.

 

[themuzicman]

Hello everyone.

So, I have a problem that's leaving me scratching my head. My boss bought me a brand new RF Venue diversity fin antenna to help with the numerous dropouts that occur with our old Shure ULX P4 and ULX S4 systems. (eight receivers each) These receivers are rolled onstage in a travel rack whenever we need more than eighteen lav mics in a show, and are snaked into the booth. (the first 18 mics are Sennheisers, no problems with those)

Most of the time, the Shure's have awful range, less than 30 feet with complete line of sight. The new antenna gives fantastic range, 4-5 bars over 100-150 feet behind two feet of brick wall, but it only seems to work if you hook it up to a single receiver at a time, using a distributor (Shure) kills the effect and reduces the range to 20-30 feet or so.

Is my distro bad? I'm using 75-ohm BNC cable to connect the antenna to the distro, should I be using 50-ohm? Am I missing something important?

Thanks in advance for any help you can give.

You shouldn't be using 75 Ohm cable, problem right there. You should be using 50 Ohm, RG-58 works in a pinch but something lower loss like RG-213 or RG-8 would be better. Your distro is probably fine, though I think you fell into the trap of "buying stuff will clearly fix my issues" which persists a lot in the industry, though without knowing how exactly your rack is wired up it'll be hard to diagnose. Is the Diversity Fin feeding both your Shure and Sennheiser? What does your distribution look like inside the rack? You need to run your system for intermod to make sure you aren't stepping on your own toes, as well as coordinating around DTV broadcasts local to your area. I bet the combination of the two of those is what your issues are.

 

[Jammer]

Since you've tried the new antenna direct on a reciever, have you also tried the 1/2 or 1/4 wave antennas direct on a receiver too, and seen if the range changed at all?

 

[GreyWyvern]

Since you've tried the new antenna direct on a reciever, have you also tried the 1/2 or 1/4 wave antennas direct on a receiver too, and seen if the range changed at all?

Since you've tried the new antenna direct on a reciever, have you also tried the 1/2 or 1/4 wave antennas direct on a receiver too, and seen if the range changed at all?

Ummm....

 

[Jammer]

Should have referenced your post...Didn't see that questions answered, maybe I missed it.

 

[FMEng]

You shouldn't be using 75 Ohm cable, problem right there. You should be using 50 Ohm, RG-58 works in a pinch but something lower loss like RG-213 or RG-8 would be better. Your distro is probably fine, though I think you fell into the trap of "buying stuff will clearly fix my issues" which persists a lot in the industry, though without knowing how exactly your rack is wired up it'll be hard to diagnose. Is the Diversity Fin feeding both your Shure and Sennheiser? What does your distribution look like inside the rack? You need to run your system for intermod to make sure you aren't stepping on your own toes, as well as coordinating around DTV broadcasts local to your area. I bet the combination of the two of those is what your issues are.

I respectfully disagree. The impedance mismatch would make only a dB or two difference, not enough to matter. Impedance matching is a big deal to a transmitter. It matters not at all for a receiver. Low loss cable is important when the lengths are significant, but there just isn't enough attenuation in short jumpers to worry about.

I agree that frequency coordination is essential. You might be camping on a channel with a megawatt TV transmitter on it.

You haven't told us what you are using for distribution. Is it passive? A passive, 16 way splitter loses 14 dB of signal, which is huge. An active splitter has little to no loss, but intermod becomes a critical factor with one, making frequency coordination even more important.

If the distribution has a wall-wart supply, don't overlook the basics. Check it with a voltmeter. If the wall wart has a DC output, it should have very little AC ripple,

 

[RadioKnight]

Right, the distributor is an active 4-way with a 5th output for cascading. I bought 75-ohm on my boss's instruction, he told me that 75-ohm cable is for low loss applications, and that 50-ohm is more suitable for power-handling situations.

 

[RadioKnight]

The 9-2 receiver on the left is using two 1/2 wave antennas, as you can see, it picks up the lav pack across the stage with no problems. The 9-2 on the right is getting signal from the distro being fed by the new antenna just out of frame. If I bypass the distro, then I get strong signal all around.

 

[rsmentele]

Channel coordination software is free from Shure!

http://www.shure.com/americas/products/software/wireless-workbench/wireless-workbench-6

 

[BobHealey]

Channel coordination software is free from Shure!

http://www.shure.com/americas/products/software/wireless-workbench/wireless-workbench-6

And that version even knows what to do with Sennheiser units, to avoid frequency clashes.

 

[themuzicman]

I respectfully disagree. The impedance mismatch would make only a dB or two difference, not enough to matter. Impedance matching is a big deal to a transmitter. It matters not at all for a receiver. Low loss cable is important when the lengths are significant, but there just isn't enough attenuation in short jumpers to worry about.

Yes, I agree that in short jumpers it own't make a difference, but I am assuming worst case that the poster was putting the antenna some length from the rack. It's not ideal to use 75 Ohm jumpers in your racks, but it doesn't make a crazy amount of difference which you are correct about.

Right, the distributor is an active 4-way with a 5th output for cascading. I bought 75-ohm on my boss's instruction, he told me that 75-ohm cable is for low loss applications, and that 50-ohm is more suitable for power-handling situations.

Your boss should go get a refresher on impedance, I think they are confused. 50 Ohm is actually a compromise between a low-loss cable and a high-power cable, with 75 Ohm cables handling even less power with and generally having better low-loss characteristics than the 50 Ohm.

 

[FMEng]

Both of you are sort of right. When you are connecting a transmitter to an antenna, you want all of the available power to reach the antenna to be radiated. To do that, the transmitter output stage, the coaxial cable, and the antenna's impedance all have to match. Any mismatch results in some of the power going toward the antenna being reflected back to the output stage and being lost as dissipated heat. Generally speaking, most transmitters, whether it be a milliWatt IEM system, a 100 Watt HAM radio, or a 35,000 Watt FM broadcast transmitter, are standardized on 50 ohms impedance. A transmitter and antenna could just as easily be designed for 75 ohms, but they seldom are. It's just easier to design for what all of the test instruments, connectors, antennas, test loads, etc. use.

For a transmitter of any significant size, keeping reflected power to a minimum is important. The term Voltage Standing Wave Ratio (VSWR) is a mathematical expression of how well matched the load (antenna and coax) is to the output stage. We broadcast RF guys live by that number. A VSWR of 1:1 is perfect. A VSWR of more than 1.5:1 means something is wrong and it may be getting ready to let the smoke out.

75 ohm cables generally have less loss for a similar physical size. For example, lets compare RG-58 and RG-59 at 440 Mhz. RG-58 (50 ohms) -10.4 dB per 100 feet. RG-59 (75 ohms) -7.6 dB per 100 feet. Both can handle more than 100 Watts of power, and above that we'd use bigger connectors and fatter cable, so that becomes irrelevant.

Receivers tend to have input impedances that are all over the map. Some are 75 ohms, some are 50 ohms, and some could be almost any value, because it simply doesn't really affect the performance. Sometimes, the input impedance is chosen to match the native impedance of the connector type used. For example, BNC and F connectors lean toward being 75 ohms. Type UHF and N connectors are 50 ohms. Basically, unless you put your receiver on the bench and measure it, you don't know what the input impedance is.

Deep trivia: the UHF connector was designed when "ultra high frequency" meant 50 MHz. Today UHF is more like 500 MHz, and UHF connector isn't very good at those frequencies.

 

[RadioKnight]

Update.

Through process of elimination, I've determined that the Shure distro is indeed malfunctioning. Using different antennas and both 75-ohm and 50-ohm cables all lead to poor signal performance and dropouts. Bypassing the distro fixes it every time. I guess I'll have to stick to the old whip antennas until I get some cash for new distributors.

 

[JD]

Update.

Through process of elimination, I've determined that the Shure distro is indeed malfunctioning. Using different antennas and both 75-ohm and 50-ohm cables all lead to poor signal performance and dropouts. Bypassing the distro fixes it every time. I guess I'll have to stick to the old whip antennas until I get some cash for new distributors.

Coming to this thread late, but it's makes sense that the distro is bad. As for 75/50 ohm cable. Yea, you do inject a bit of a loss, but what you were experiencing was a ton of loss. Recently, I put a Sennheiser rack together with 75 because the vendor had the 50's on backorder. Did some A/B comparisons with another rack I had wired wit the 50's and could not tell the difference. I am sure there is some loss but it was not big enough to show up on the displays.

 

[BobHealey]

Update.

Through process of elimination, I've determined that the Shure distro is indeed malfunctioning. Using different antennas and both 75-ohm and 50-ohm cables all lead to poor signal performance and dropouts. Bypassing the distro fixes it every time. I guess I'll have to stick to the old whip antennas until I get some cash for new distributors.

Give Shure a call, might not be too much to get it repaired vs replacing it. There's not much to them.