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[Henry Cohen]

Phase shifting of TX combiner inputs has no detrimental affect on transmission quality when combining IEM carriers that operate at different frequencies.

However, it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications . . .

And in high power broadcast applications. There is no such thing as a 500kW amplifier power transistor: Multi 10kW and 100kW power levels are achieved by coherently combining dozens or scores of 100+W transistors to create 2.5kW and 5kW amplifier pallets, which are in turn coherently combined to achieve 100's of kW, all with phase accuracies measured in hundredths of degrees.

[Daniel Levi]

And in high power broadcast applications. There is no such thing as a 500kW amplifier power transistor: Multi 10kW and 100kW power levels are achieved by coherently combining dozens or scores of 100+W transistors to create 2.5kW and 5kW amplifier pallets, which are in turn coherently combined to achieve 100's of kW, all with phase accuracies measured in hundredths of degrees.

Or you use Klystrons/Klystrodes, of course, but that's a bit more "pro gamer  " and not really relevant to the thread, plus if you need that much power for your IEM transmitter then you need a different forum  .

And as you may well know Klystrons/Klystrodes are still very much used in modern broadcast applications, despite the advent of high power transistor based transmitters. The Transmitter at Rowridge on the Isle-of-Wight (http://tx.mb21.co.uk/gallery/gallerypage.php?txid=560 https://web.archive.org/web/20170310145119/http://www.theonlineengineer.org/TheOLEBLOG/rowridge-transmission-site/) uses Thomson transmitters that were new in ~2010 and use Klystrodes for the output stage. 

[Jason Glass]

Or you use Klystrons/Klystrodes, of course, but that's a bit more "pro gamer  " and not really relevant to the thread, plus if you need that much power for your IEM transmitter then you need a different forum  .

Tubes, baby!  Not just for guitar amps and microwave ovens.   

Since we're getting into it, intentional phase shift makes antenna arrays possible, and is critical to beam steering applications such as radar.

https://en.m.wikipedia.org/wiki/Phased_array

[Miguel Dahl]

Phase shifting of TX combiner inputs has no detrimental affect on transmission quality when combining IEM carriers that operate at different frequencies.

However, it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications, but is not something we should ever intentionally do when combining IEM carriers for TX.

How about combining two RX antennas?

[Jason Glass]

How about combining two RX antennas?

That's what DAS is.  Distributed Antenna System.

...it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications...

[Miguel Dahl]

That's what DAS is.  Distributed Antenna System.

Either I don't get something here, or I wasn't clear.

Let's say you have two zones you want to pick up some handhelds or whatever from. Wouldn't a combiner be the correct unit to use?

For instance, real life application here: An outdoor theater, two antennas covering the main "stage" as usual, but one also want to cover an area which is "in the RF-shadows" of the two main antennas. Could one use the ASP-212 to add a third antenna? The ASP-212 is afaik a splitter/combiner. Basically having two antennas going through one coax using a "dumb" combiner, is my point I guess.

I saw RF venue has a unit called 4 Zone. This thing does this as far as I understood. But could one use the ASP 212 to combine "out in the field", and not at WL-land?

[Jason Glass]

Either I don't get something here, or I wasn't clear.

Let's say you have two zones you want to pick up some handhelds or whatever from. Wouldn't a combiner be the correct unit to use?

For instance, real life application here: An outdoor theater, two antennas covering the main "stage" as usual, but one also want to cover an area which is "in the RF-shadows" of the two main antennas. Could one use the ASP-212 to add a third antenna? The ASP-212 is afaik a splitter/combiner. Basically having two antennas going through one coax using a "dumb" combiner, is my point I guess.

I saw RF venue has a unit called 4 Zone. This thing does this as far as I understood. But could one use the ASP 212 to combine "out in the field", and not at WL-land?

Yes, you are describing a distributed antenna system, and correctly understanding how to use a combiner in that application.

Shure shows how to engineer this scenario here https://www.shure.com/en-US/support/find-an-answer/antenna-distribution-2-rooms-4-antennas

Of the many factors to consider when designing such a system, the most important is to avoid conditions where a single carrier, transmitted from a desired performance location, arrives at the receiver through multiple paths that sum to a RSS that is low enough to cause a dropout or audible noise.  Since microphone TX are mobile devices and not rigidly fixed in spatial relation to their RX antennas, each path will impart a unique and dynamic loss or gain and phase shift on the signal.  Combiners and splitters are components that can cause phase shift, as are amplifiers and the electrical length of the entire path.

One of the easiest and most inexpensive ways to mitigate a destructive summing of zones is to insert variable attenuation or amplification into each path, which the user can adjust during or after system deployment.  Variable phase shifting would be wonderful, but I suspect way too expensive.  It's also worth noting that receiver diversity is an enormous factor in DAS, since by design it makes momentary phase cancellations on one side of the diversity pair inaudible.

[Miguel Dahl]

Yes, you are describing a distributed antenna system, and correctly understanding how to use a combiner in that application.

Shure shows how to engineer this scenario here https://www.shure.com/en-US/support/find-an-answer/antenna-distribution-2-rooms-4-antennas

Of the many factors to consider when designing such a system, the most important is to avoid conditions where a single carrier, transmitted from a desired performance location, arrives at the receiver through multiple paths that sum to a RSS that is low enough to cause a dropout or audible noise.  Since microphone TX are mobile devices and not rigidly fixed in spatial relation to their RX antennas, each path will impart a unique and dynamic loss or gain and phase shift on the signal.  Combiners and splitters are components that can cause phase shift, as are amplifiers and the electrical length of the entire path.

One of the easiest and most inexpensive ways to mitigate a destructive summing of zones is to insert variable attenuation or amplification into each path, which the user can adjust during or after system deployment.  Variable phase shifting would be wonderful, but I suspect way too expensive.  It's also worth noting that receiver diversity is an enormous factor in DAS, since by design it makes momentary phase cancellations on one side of the diversity pair inaudible.

So, sorry for beating the question to death here.. The way I described the application with the Sennheiser 212 unit is a valid approach. BUT, (this was my original question): phase issues etc can be an issue. (I asked originally if it's possible one can see a 180deg phase shift from one antenna comparing to the other by using different lengths and gauge coax cables) so it would null out at the receiver end.

EDIT: I read the link you provided, and that's also been my understanding of it the whole time. But in my example antennas would overlap each other, so now I understand that overlapping antennas can deal more damage, obviously. Thumbs up, thanks.

But. Can I ask. Let's say you have two whips in free field, spaced just a few meters apart. Will it be a very noticable reduction in performance comparing to just using one whip (or paddle for example)? Can you end up with detrimental performance? I know how it works with wavelengths comparing to spacing between antennas. But in a "random" situation, will the performance of the WL straight up suck, or is it more that some planets needs to align in a line first, before one one notices it once in a while?

[Jason Glass]

Let's say you have two whips in free field, spaced just a few meters apart. Will it be a very noticable reduction in performance comparing to just using one whip (or paddle for example)? Can you end up with detrimental performance? I know how it works with wavelengths comparing to spacing between antennas. But in a "random" situation, will the performance of the WL straight up suck, or is it more that some planets needs to align in a line first, before one one notices it once in a while?

It will range from being less than ideal to straight up sucking.

For simplicity, we'll assume free space, theoretical lossless cables of equal length, and antennas of exactly equal gain.  We'll also assume a theoretical combiner that has perfect phase coherence from input ports to output port and excellent port isolation.

Every passive two-way combiner imparts a mandatory -3 dB loss, at best, and additional insertion losses from real-world imperfections.

At all positions where the TX antenna is located an equal distance to both RX antennas, the signal will travel through two paths and arrive at the combiner in-phase and constructively interfere, and sum to +3 dB gain, merely making up for the -3 dB combiner loss.  FWIW, all of these positions are located within a plane passing halfway between the antennas and orthogonal to the axis formed by a line between the two antennas.

At every other position where the difference in distance is a multiple of 1 wavelength, constructive interference will occur but not enough to make up for the combiner loss.  The determining factor here is the difference in signal strength between the two paths.  The closer together their signal strengths are, in perfectly aligned phase, the less the net loss will be.

At all positions where the difference in distance is 1/2 wavelength, the signal will arrive 180° out of phase and destructively interfere and sum to -∞ dB loss.

At every remaining position where the difference in distance is a multiple of 1/2 wavelength, destructive interference will occur in addition to combiner loss.  The determining factor here is the difference in signal strength between the two paths.  These interference losses will be severe because the antennas aren't located very far apart, assuring that the difference in signal strength between the two paths is small.  The closer together their signal strengths are, at 180° out of phase, the greater the net loss will be.

Every position in between all of those described above also results in a phase shift and net loss that is worse than a single antenna without a combiner.

This scenario is an intentional creation of multiple signal paths, resulting in textbook multipath interference and net losses for all but a relatively small volume of TX locations.

Here is a fabulous simulator to visualize what we're discussing with animated graphics.  https://phet.colorado.edu/sims/html/wave-interference/latest/wave-interference_en.html Here's a quick video primer on using it to show multipath interference.  http://cleanwirelessaudio.com/PHET_Multipath_Simulation.mp4

It's a really good thing that our gigs aren't in theoretical free space!  Reflections are enormously helpful to real-life RF work.  But it's also good that we can use free space as a modeling tool to optimize our systems because reflections are staggerlingly complex to calculate in 3D space.  Murphy's law applies.  Our best chance at success is to remove as many of his possibilities as we are allowed and free space lets us at least anticipate the basics.

[ProSoundWeb Community]