We Have Some IEM Dropouts

[Repeater]

Hello all, one of our in-ear RF channels seems to drop out more than the others. We have seven channels, using Shure PSM900 systems. They are combined with a Shure PA821A combiner through 50 ft. of RG59 tothe antenna. The measured loss of the coax (with my service monitor) is 4.5 dB. We are using a Professional Wireless helical transmit antenna 50 ft. max from the talent on stage.

The user of the monitor in the 470 mc. range complains more about dropouts than the other six users which are between 598 and 626 mc. We have coordinated all in-house frequencies using Wireless Workbench, and included any active TV channels and anything else I can think of. I understand that some users may complain quicker than others about problems.

I took a long listen to the offending IEM receiver during rehearsal Tuesday night and found only a couple of random drops. They were hard to replicate a second time as far as trying to find the exact physical location. The next day I again listened to that channel, with no modulation this time, and can carefully find several very small areas where the signal gets noisy, and with careful movement will completely close the squelch.

Such is the nature of multipath, I know. I just would not expect to experience it's effects so close to the transmit antenna. I have checked all seven channels for proper RF output level, along with the output of the combiner to the antenna. All measurements are what I would expect. I have listened carefully to the problematic receiver for interference present while it is at the point of weakest signal, just before the squelch closes, and found little if anything. A spectrum scan with a number of mics and all IEMs operating shows nothing not expected, although I know that will not see products generated within equipment. A spectrum scan with all transmitters off also shows nothing not expected.

The one 470 mc. receiver seems to be the worst, but I can find slightly noisy multipath areas on stage with the other receivers too; it seems to be just a matter of degree.

So what is the 'normal' experience on stages. Is it expected that there may be the occasional IEM dropout with a setup such as ours (not using diversity receivers)? Is perfection within 50 ft. of the boresight of the antenna too much to ask? Is there something I have not checked?

Thanks!
Laryn

 

[DaveySimps]

I would say it is not normal to expect a drop out of with a set up as you describe. What did you use to do your spectrum scan with?

It appears you covered a lot of the typical issues that might contribute to the issue. I can only think of a couple more items. 470 MHz is the lowest your Shure PA821A combiner will deal with, so you might be seeing a potential issue there since you are riding close to that range. Have you sent any of the gear to Shure to be looked at? What model of helical antenna by Professional Wireless are you using? Most go as low as 450 MHz, but the Domed Antenna starts at 470MHz. Should still work on paper, but if you are having issues on the extreme low end of the spectrum you are using with the combiner and antennae, it could prove to be an issue.

Being from the opposite coast of the state, I have an odd issue that might apply depending on how close to Lake Michigan you are (your profile says you are from Western Michigan). My venue is about a half mile from the St.Clair River. We have a lot of freighter traffic up and down the river, as well as concerns with Canada being about a mile away. When we first opened, I had a difficult time with a particular frequency range that was very intermittent. Long story short, the issue proved to an intermod caused by maritime traffic from certain freighters that would go by. It was terribly intermittent, so hard to trouble shoot, and very few people knew to be aware of the issue. I figured it out when speaking to an area harbor master.

 

[Repeater]

I am using an HP Agilent E8285 to do all of the measurements.

We have not sent anything in. The system is relatively new, less than 2 years old, so that helps. We ran the seven transmitters into individual antennas for a time, and 6 months ago added the combiner and antenna. We had problems then, and still have them, although the problems 'then' may have been different ones, I don't know. I have not gotten deep into this problem until this week...

I just was looking at the specs of the PW antenna HA-8089 that we have and it specifies a range of 480 to 900 mc. So that *could* be a part of it, along with the combiner being at the lower end of it's range. WWB gave us some backup frequencies in the 490 mc. range so I will try one of those.

You solved a tough one with the freighters there. We are 2 miles from Lake Macatawa and 4 miles from Lake Michigan. There is some freighter traffic on Macatawa but not a lot, not like you'd have on the river by you there. Plus being 2+ miles away I would not expect any kind of intermod problem from there.

 

[FMEng]

RG-59 is 75 ohm coax, while the other system components are 50 ohms. For a transmitter system, that's a big deal. The mismatch causes extra losses and reduced output power. Change the coax from the combiner to the antenna with RG-8. I prefer Belden 9913F7. The RG-8 will also have much less loss at UHF frequencies. RG-58 would be a good choice for the short jumpers between the transmitters and combiner, because it is small and more flexible.

Also, just a nit pick, but the term is MHz, not Mc. The term Megacycles went out of style about 50 years ago. An mc is a millicycle.

 

[TimmyP1955]

4.5dB loss is less than I would have expected, but perhaps I'm thinking 100' instead of 50'. (Why are the transmitters so far from the performers?)

According to Cohen, the actual impedance of typical devices is so far off spec that in practice the loss from a 75 Ohm/50 Ohm mismatch is unlikely to be more than 1dB.

Place your eye 12" behind the center of the helical's back plate and look through the tube. What you see is what is covered by the antenna according to PWS. I've not seen a spec on how fast it falls off.

Is the axis of the helical perpendicular to the back wall? I wonder if the dropouts could be owing the the signal bouncing off the back wall and cancelling those from the front. You might try a different location for the antenna.

 

[Chris15]

According to Cohen, the actual impedance of typical devices is so far off spec that in practice the loss from a 75 Ohm/50 Ohm mismatch is unlikely to be more than 1dB.

NO NO NO NO NO
I'm sure Henry would concur that rule ONLY applies when talking about receivers.
In transmission systems it is critical to keep everything impedance matched...

 

[FMEng]

NO NO NO NO NO
I'm sure Henry would concur that rule ONLY applies when talking about receivers.
In transmission systems it is critical to keep everything impedance matched...

That's exactly right. You can get away with impedance mismatch on a receiver with no effect. In fact, 75 ohm coax might be a benefit on a long run, even with the mismatch, because it has a bit less loss per foot than a similar diameter 50 ohm cable.

Transmitters do not like running into a mismatch. It will reduce output power and it can also cause the final stage to generate spurs (garbage) in the signal. I've worked on transmitters of ranging in size from <1 Watt to 25,000 Watts, and frequencies from 500 kHz to 6 GHz, and I've never seen one that didn't get unhappy with a load that didn't match well.

One issue with the impedance mismatch is that a good portion of the energy produced by the transmitter gets reflected back into output stage where it is dissipated as heat. That reflected power never gets coupled to the antenna. In order for all of the power to be radiated by the antenna, the impedance of the entire antenna system has to match the transmitter.

 

[FMEng]

Is the axis of the helical perpendicular to the back wall? I wonder if the dropouts could be owing the the signal bouncing off the back wall and cancelling those from the front. You might try a different location for the antenna.

Anytime an antenna is placed in a real world environment with metal around, there are reflections which cause nulls in the signal pattern. Experimentation by moving it would be a good idea. Getting the antenna away from metal studs, rebar, light grids, and anything metal can only help.

 

[Repeater]

After much study and many more checks and measurements, I've found at least 7 changes I will / have made, thanks to each of the previous posts---

A. Replace the coax to the antenna with lower loss and shorter 50 ohm cable.
B. I found a short/wrong antenna on the G6 split receiver. Swapped with one of the K1 receivers, which had a long/wrong antenna, to correct.
C. I lowered the squelch settings on several receivers.
D. Orient the natural bend of the antenna away from the body, but this is not always practical because of clothing which can push the antenna against the body anyway.
E. Moved the system on 470 MHz to 497 MHz to within the published range of the HA8089 antenna. (Meaning no disrespect to Herr Hertz in the earlier post, I occasionally revert to the real meaning of the term...).
F. Trying to keep the helical antenna as close as possible to the talent, within set restrictions. Distance makes a big difference.
G. One of our people (the one with more dropouts) often wears an electric guitar transmitter on his belt too. Advising him to always keep it opposite his IEM receiver.

The use of 75 ohm cable from transmitter to antenna is not condemned by Cohen, but an allowed 2nd choice. Impedances of the antenna or combiner output are not exactly 50 ohms in any (IEM) system. They vary over a fairly broad range, so the 75 ohm cable can often be just as good a match to the system at any particular frequency in the system as 50 ohm cable. That said, 50 ohm is his first choice, as it is mine.

What is a good choice for the antenna cable? LMR400 is way too stiff to move around on stage. Is there a more flexible low loss cable that is used by anyone here?

Is it possible to get the helical antenna too close to wireless mics on stage, with the possibility of generating intermod products within the mic transmitters? (I guess any intermod generated *should* not be a problem if all stage frequencies have been coordinated properly, yes?) I don't want to find out the answer on a Sunday morning...

How about the Shure P10R. Diversity reception should help with this I think... Any experience?

Thanks for the help!

 

[Stookeybrd]

G. One of our people (the one with more dropouts) often wears an electric guitar transmitter on his belt too. Advising him to always keep it opposite his IEM receiver.

Ding Ding Ding! If the transmitter in operating anywhere in the frequency range of the IEM on the same person, there will be huge issues. The IEM is getting desensitized by having a transmitter so close to it. Even if it is on the other side of the waist. The best solution is to have the frequencies as separated as possible, and ideally, have zero overlap of the two band splits.

Is it possible to get the helical antenna too close to wireless mics on stage, with the possibility of generating intermod products within the mic transmitters?

For the gear that we use in professional audio, intermod products do not exist within the transmitters. All the pro level gear is very linear and intermod products occur in the front end filters of receivers.

How about the Shure P10R. Diversity reception should help with this I think... Any experience?

The PSM1000 series is my current favorite IEM system. WWB6 compatibility, diversity receive antennas, Cue Mode(that doesn't suck), and silent muting audio. Shure added a fantastic squelch-mute that keeps a lot of unwanted noise out of the ears when the pack has week signal strength.

 

[Repeater]

Well the guitar pack is on 680 MHz and the IEM pack is (now) on 497 (was on 470). Yeah I'm definitely aware of the possibility of desense, having worked with repeaters for many years. I did some subjective testing while the IEM was getting a slightly noisy signal, and then moving the guitar pack near and touching the IEM. There was some desense when the antennas were within an inch or so of each other. Any more than that there was no desense. Seems like putting them on opposite sides should be sufficient in this case.

As far as intermod "within the transmitters", it can actually be generated in any active stage if the signal(s) is strong enough. In two way equipment it is common to experience intermod being generated in the final stage of a transmitter from other close by transmitters if there is not enough filtering on the transmitter to keep external signals at a low enough level. I haven't had enough experience with wireless mics to know if it is possible on a typical stage.

Thanks for the positive endorsement of the PSM1000 system. Maybe someday...

 

[JDH]

This may be a horribly obvious question, but have you tried different frequencies with the "problem" unit? Plug all your current freqs into WWB (don't change any other units), then just go for like 5 or 10 backup frequencies, and try as many as it will spit out.