Wireless dropouts mystery...

D

Daniel Postilnik

Junior
Jan 14, 2011
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San Francisco, CA
Imagine the following situation (NYC): A room about 60' x 40'x9' (ceiling height) full of fitness equipment and sweaty bodies, pretty hot and humid inside. Fitness instructor is using a headset mic into a ULX1 transmitter. The receiver is at the rack in another room, but there are 2 x 1/2 wave antennas on the ceiling, so the instructor is never more than 12' or so from either one.

Instructor reports that there are dropouts at three positions in the room, and she allegedly experiences those dropouts consistently in the same place. Group/channel for the M1 band of transmitters is within Shure's recommended list. When the transmitter is off and I'm watching the receiver, there is a very faint flicker in the first light of the "RF" column of lights (presumably from a stray source somewhere nearby). When the transmitter is on and I'm watching the receiver during some other instructor's class, the "RF" column of lights is completely full almost all of the time.

One thing to note is that the instructor with the complaint walks the room during class, while almost every other instructor stays in one place at the front of the room, roughly equidistant from the two antennas.

Who's in the mood to play detective? And in any case, what course of action is available to me other than changing the wireless channel?
 
Re: Wireless dropouts mystery...

My first guess would be bad cable from one of the antennas to the receiver. Sounds like a typical multipath dropout situation that occurs on non-diversity receivers (or when only one antenna works on a diversity system).

Yes, I got a bad batch of cables from the (major wireless system) manufacturer once, so it does happen.
 
Re: Wireless dropouts mystery...

Milt Hathaway said:
My first guess would be bad cable from one of the antennas to the receiver. Sounds like a typical multipath dropout situation that occurs on non-diversity receivers (or when only one antenna works on a diversity system).

Yes, I got a bad batch of cables from the (major wireless system) manufacturer once, so it does happen.
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Would the receiver know that it wasn't successfully switching between two antennas? The A/B lights are definitely switching occasionally to indicate that both are working fine.
 
Re: Wireless dropouts mystery...

Milt Hathaway said:
Hmm, if it's switching antennas then they both should be working equally well.

Next guess...
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If it's a consistent location, something in the building at that location? HVAC unit or electrical equipment that is giving of spurious emissions that interferes with the signal? Something electronic in a piece of exercise equipment?
 
Re: Wireless dropouts mystery...

Matt Tudor said:
If it's a consistent location, something in the building at that location? HVAC unit or electrical equipment that is giving of spurious emissions that interferes with the signal? Something electronic in a piece of exercise equipment?
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The HVAC/electrical could be the cause, but none of the equipment has any electronics on it.

If it were HVAC electrical - would there be any solution that doesn't involve ripping it all out?

Unfortunately it's just one instructor who has this issue.
 
Re: Wireless dropouts mystery...

Just because the reciever switches betwen a and b doesn't exclude a bad antenna cable.

Connect the reciever with some short cables and verify.

There is a slight risk that both antennas are in a blind spot. Try moving one antenna a copule of inches...
 
Re: Wireless dropouts mystery...

Robert Lofgren said:
Just because the reciever switches betwen a and b doesn't exclude a bad antenna cable.

Connect the reciever with some short cables and verify.

There is a slight risk that both antennas are in a blind spot. Try moving one antenna a copule of inches...
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Robert, this is an install scenario. Things can't really moved around easily.

Can you tell me more about these "blind spots"?
 
Re: Wireless dropouts mystery...

Robert Lofgren said:
There is a slight risk that both antennas are in a blind spot. Try moving one antenna a copule of inches...
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+1 This is the most likely fix for the trouble you describe, although you may end up moving an antenna a few feet or more.

Also make sure that the polar radiation patterns of both antennas are appropriate for each antenna to cover the entire performance area, and aim them accordingly. If you're using LPDA antennas (paddles), they have around 70 degrees of coverage, which would necessitate their locations to be near the corners of the room, pointed toward the diagonal opposite corner. PWS Helicals are around 60 degrees. Sennheiser A5000CP are around 80 degrees. You might consider omnidirectional antennas if corner locations would be difficult to accomplish.

The performers' short distance (12') to the antennas is relevant, but how long are the coax cables and what type are they? Significant losses occur with long cable runs.

Daniel Postilnik said:
Can you tell me more about these "blind spots"?
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As a professional audio engineer who must use wireless, you'll benefit greatly from studying and understanding Antenna diversity - Wikipedia, the free encyclopedia and Multipath propagation - Wikipedia, the free encyclopedia.
 
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Re: Wireless dropouts mystery...

Daniel Postilnik said:
The HVAC/electrical could be the cause, but none of the equipment has any electronics on it.

If it were HVAC electrical - would there be any solution that doesn't involve ripping it all out?

Unfortunately it's just one instructor who has this issue.
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With out removing the suspect equipment, the solution would be extra shielding around the offending piece of equipment, tied to a convenient ground.

Robert's suggestion also has merit, and might be easier to test for. Just add a length of coax between the antenna and whatever plate it's mounted to. Move the antenna to a new location and temporarily attach it to something with tape, wireties, or chewing gum for the purposes of testing. Try a couple spots. If the move fixes the problem, then you have to figure out how to make it permanent. If moving the antenna doesn't fix the problem, put it back on the original mount. If you find a location in the process that causes a marked increase in signal at the transmitter, you may have found a possible source of interference.
 
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maybe you did answer it yourself when you named "non-diversity".
The antennas should be placed closed together at lamda/4 wavelengt distance, keep in mind to calculate with speed of light not sound. @800MHz it is about 4".

...the most common misusage I see is one antenna upper left corner and the B antenna lower right corner of a room. And people saying that gives the best coverage.
yep ...but it is non diversity, in a good diversity installation the reciever is heavily switching between two antenas. ...need more coverage? install more diversity pairs

Tilt one antenna some degrees maybe 30 degrees to cover polarisation losts. ...up to 6dB

And of course find out the damping of the antenna cable and maybe boost ...but avoid clipping

pimp the beltpack antenna to avoid it is direct connected to the sweat skin ...cost up to 20dB!

...just some ideas from far away :)
 
Re: Wireless dropouts mystery...

Bodo Felusch said:
maybe you did answer it yourself when you named "non-diversity".
The antennas should be placed closed together at lamda/4 wavelengt distance, keep in mind to calculate with speed of light not sound. @800MHz it is about 4".
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I don't think so.

The antennas should be placed FAR APART compared to the wavelength. The whole point of a diversity receiver is to have the two antennas in different environments so that both antennas can't (or at least most likely won't) both be sitting in nulls due to RF mulitpath interference at the same time.
 
Jeff Carter said:
I don't think so.
so that both antennas can't (or at least most likely won't) both be sitting in nulls due to RF mulitpath interference at the same time.
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this is correct, and the reason why diversity technology does exist. I don't want to discuss about Lamda 1/4 or 1/2 or 1 this depends on the antena type and so. BUT we discuss the minimum space at that point. If you place the antennas far away then one has allways a higher level and you do not have a diversity setup anymore, if then the drop out because of reflection summing occours ...you have a drop out. Place the antennas near to each other about 20-40cm and tilt one about 45 degrees is a perfect starting rule!
 
Re: Wireless dropouts mystery...

Bodo Felusch said:
maybe you did answer it yourself when you named "non-diversity".
The antennas should be placed closed together at lamda/4 wavelengt distance, keep in mind to calculate with speed of light not sound. @800MHz it is about 4".

...the most common misusage I see is one antenna upper left corner and the B antenna lower right corner of a room. And people saying that gives the best coverage.
yep ...but it is non diversity, in a good diversity installation the reciever is heavily switching between two antenas. ...need more coverage? install more diversity pairs

Tilt one antenna some degrees maybe 30 degrees to cover polarisation losts. ...up to 6dB

And of course find out the damping of the antenna cable and maybe boost ...but avoid clipping
Click to expand...

If two antennas are within one wavelength of the carrier frequency they are receiving identical signals. The ideal diversity system has each antenna receiving a unique path from the transmitter, and rapidly switching between both antennas. If you are limiting both antennas to a single wavelength than you can theoretically have full cancellation from multi-path interference. If you have two antennas widely spaced in a diversity system it is statistically improbable to ever have signal loss from multi-path cancellation. You are correct that a good diversity system is switching heavily between the two antennas, but it should really be switching heavily between two different signal paths. If it is simply switching between antennas within the same signal there isn't a difference between the signals.

If you have antennas with a single polarization, positioning them 90 degrees from each other and +/-45 degrees from a common axis will give you the best set-up to avoid signal loss due to polarization mismatch. Ideally, use a circularly polarized antenna for situations in need of full polarization coverage.

The main reason to boost a receive signal is to overcome cable loss. If you have cable runs under 50ft(>100ft for low loss cable), move your antennas before adding an amplifier to the signal chain. RF amplifiers can cause more trouble than they're worth.
 
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Re: Wireless dropouts mystery...

Bodo Felusch said:
this is correct, and the reason why diversity technology does exist. I don't want to discuss about Lamda 1/4 or 1/2 or 1 this depends on the antena type and so. BUT we discuss the minimum space at that point. If you place the antennas far away then one has allways a higher level and you do not have a diversity setup anymore, if then the drop out because of reflection summing occours ...you have a drop out. Place the antennas near to each other about 20-40cm and tilt one about 45 degrees is a perfect starting rule!
Click to expand...

Bodo, you are correct about polarization diversity but completely incorrect about colocation of the antennas. Diversity antennas are most effective when separated by multiple wavelengths. Their locations and aiming are indeed dependent on antenna type, and both antennas' radiation patterns should be arranged to overlap and cover the entire performance area. Relying only on polarization diversity is far less effective than multi-wavelength spacing. The best setup is a combination of both.

Cameron's reply to your post is 100% correct.
 
Re: Wireless dropouts mystery...

Bodo Felusch said:
this is correct, and the reason why diversity technology does exist. I don't want to discuss about Lamda 1/4 or 1/2 or 1 this depends on the antena type and so. BUT we discuss the minimum space at that point. If you place the antennas far away then one has allways a higher level and you do not have a diversity setup anymore, if then the drop out because of reflection summing occours ...you have a drop out. Place the antennas near to each other about 20-40cm and tilt one about 45 degrees is a perfect starting rule!
Click to expand...

If one always has a higher level then we're good to go!
when it ends up in a null though we want to maximize the chances that the other antenna is NOT in a null. in which case it is important to have them spaced as far apart as possible (within reason and coverage patterns etc)

Jason
 
ok guys ...i give up ...but don't ask for mystery dropouts again when antennas are placed far away. Now we've learned that this is impossible ;)

but again ...check yout beltpack antenna is not direct connected to the sweat body and skin

over and out
 
Re: Wireless dropouts mystery...

Bodo Felusch said:
ok guys ...i give up ...but don't ask for mystery dropouts again when antennas are placed far away. Now we've learned that this is impossible ;)

but again ...check yout beltpack antenna is not direct connected to the sweat body and skin

over and out
Click to expand...

Of course it's possible. In fact, I see it happen often and must move an antenna to shift a dropout location out of the performance area. But it is statistically far less likely than when the antennas are located within 1 wavelength of each other.

+1 on your good advice to keep antennas disconnected from the performer's skin. Medical bandage tape is an excellent barrier when packs are worn under clothing.
 
Re: Wireless dropouts mystery...

Robert Lofgren said:
Just because the reciever switches betwen a and b doesn't exclude a bad antenna cable.

Connect the reciever with some short cables and verify.

There is a slight risk that both antennas are in a blind spot. Try moving one antenna a copule of inches...
Click to expand...

Try unplugging one coax from the receiver at a time, see if coverage changes.
 
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