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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Justin Goodman on September 09, 2018, 09:09:50 PM
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https://www.rfvenue.com/blog/iem-ultimate-guide-redux (https://www.rfvenue.com/blog/iem-ultimate-guide-redux)
Reading this article, one quote in particular caught my eye. 2nd Question in the response from Brad (bolding mine):
"The second way to minimize loss is through less inline connections. Connecting straight from the combiner to the antenna using one segment of cable is best. Adapting large 'N' type connectors to BNC is not ideal, and 90-degree type adapters are very bad for RF loss."
What about a 90 degree adapter is so bad for RF loss? Is he talking only about an adapter like this: https://www.parts-express.com/parts-express-bnc-right-angle-adapter-male-to-female--090-377 (https://www.parts-express.com/parts-express-bnc-right-angle-adapter-male-to-female--090-377) or also cable which is pre-made with a 90 degree connection like this: http://www.cablesondemand.com/product/CO-058BNCRBNC/URvars/Items/Library/InfoManage/CO-058BNCX200.htm
Either way, what's so bad about them vs a straight connector (like those you'd use in a panel patch bay, to front mount antennas in a rack, etc)?
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https://www.rfvenue.com/blog/iem-ultimate-guide-redux (https://www.rfvenue.com/blog/iem-ultimate-guide-redux)
Reading this article, one quote in particular caught my eye. 2nd Question in the response from Brad (bolding mine):
"The second way to minimize loss is through less inline connections. Connecting straight from the combiner to the antenna using one segment of cable is best. Adapting large 'N' type connectors to BNC is not ideal, and 90-degree type adapters are very bad for RF loss."
What about a 90 degree adapter is so bad for RF loss? Is he talking only about an adapter like this: https://www.parts-express.com/parts-express-bnc-right-angle-adapter-male-to-female--090-377 (https://www.parts-express.com/parts-express-bnc-right-angle-adapter-male-to-female--090-377) or also cable which is pre-made with a 90 degree connection like this: http://www.cablesondemand.com/product/CO-058BNCRBNC/URvars/Items/Library/InfoManage/CO-058BNCX200.htm
Either way, what's so bad about them vs a straight connector (like those you'd use in a panel patch bay, to front mount antennas in a rack, etc)?
Right angle RF adapters are very easy to make incorrectly and thus introduce high insertion losses (>1dB). If it's designed and manufactured properly, it should introduce no more insertion loss than any other good quality adapter, about .5dB. The problem arises from most folk using Amazon or Parts Express as the sole source vendor for all things. There's a reason their stuff is cheap. Properly designed RF components are not inexpensive and the online mass marketers are not the place to be shopping for them. Stay with good quality brand names:
Amphenol (and all their brand variants)
Pasternack
Radiall
RF Industries
Times Microwave
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Right angle RF adapters are very easy to make incorrectly and thus introduce high insertion losses (>1dB). If it's designed and manufactured properly, it should introduce no more insertion loss than any other good quality adapter, about .5dB. The problem arises from most folk using Amazon or Parts Express as the sole source vendor for all things. There's a reason their stuff is cheap. Properly designed RF components are not inexpensive and the online mass marketers are not the place to be shopping for them. Stay with good quality brand names:
Amphenol (and all their brand variants)
Pasternack
Radiall
RF Industries
Times Microwave
You juat brought back memories of terminating right angle Phelps Dodge N-males on RG214. The unbraiding, washers, the little screws that held on the center pin access plan. Field termination was brutal.
The Crimp N connector is Nobel prize worthy.
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The Crimp M connector is Nobel prize worthy.
Even better is the Times Microwave EZ connectors for LMR. The pin is pre assembled in the connector and locks into place on the cable with a nice positive click when the centre conductor is inserted. Then you just have to crimp the outer shield. Their stripping tool does the correct lengths for centre and outer in 2 steps, so no trimming or guess work required. We have had almost 0 failures with this across probably 100+ lengths of LMR400.
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My rule of thumb about RF is that it "goes fast, doesn't like making sharp turns". Good right angle adapters are expensive. Also you don't want to put sharp bends in your coax.
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My rule of thumb about RF is that it "goes fast, doesn't like making sharp turns". Good right angle adapters are expensive. Also you don't want to put sharp bends in your coax.
Well... AC power and analog audio also "go fast," but right angle adapters are pretty normal in those contexts. Really the issue/question was is there something specific to RF and 90 degree turns as the quote from the article seemed to indicate.
Henry's answer makes a lot of sense that manufacturing a quality one repeatably is more difficult than a $2 part allows for, and that a working professional who may show up to work on a tour but not necessarily own all the gear might have experience in the wild suggesting 90 degree adapters having a high susceptibility to failure/poor performance.
I actually do need some 90 degree adapters (shallow rack) so it caught my eye. I've ordered the pre-built Amphenol cables at the appropriate lengths.
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Well... AC power and analog audio also "go fast," but right angle adapters are pretty normal in those contexts. Really the issue/question was is there something specific to RF and 90 degree turns as the quote from the article seemed to indicate.
RF moves quite a lot faster than analog audio, though. It's where you have to start thinking in terms of it being a transmission line that can carry a train of waves, rather than something that's effectively always the same point in the wave over the whole length of the wire.
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Technically, I think they both move at the speed of light through copper.
The RF just wiggles a lot faster.
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Technically, I think they both move at the speed of light through copper.
The RF just wiggles a lot faster.
It took me a while to really understand the difference between audio and RF. RF moves *really* fast, basically like old cartoons where you'd see lumps of water moving through a hose. It's possible for there to be multiple waves moving through a cable of practical length. This is why you get reflections if the other end isn't properly terminated, because the waves have to go somewhere.
Audio on a wire, on the other hand, is like a solid rod inside a tube. If you wiggle one end of the rod, the other end wiggles right in time. You could feel it if somebody held the other end steady instead of allowing it to wiggle. This is why you don't have to worry about termination and reflections on audio lines, unless they're really seriously long (miles).
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Technically, I think they both move at the speed of light through copper.
Technically at about 70% the speed of light ;)
That would be the propagation velocity.
Lee
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Technically at about 70% the speed of light ;)
That would be the propagation velocity.
Lee
I did say through copper.
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I did say through copper.
Dave, I don't think that I am following you here. Electricity does not travel at the speed of light through copper, it travels at approximately 70% of the speed of light.
Lee
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Dave, I don't think that I am following you here. Electricity does not travel at the speed of light through copper, it travels at approximately 70% of the speed of light.
Lee
The typical value for the speed of light (C) refers to its speed in a vacuum.
The medium through which it travels effects the actual speed.
The speed of light through glass is different than the speed of light through air, which is why lenses work.
If the medium is copper, it is .7C.
That is the speed of light through copper.
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The typical value for the speed of light (C) refers to its speed in a vacuum.
The medium through which it travels effects the actual speed.
The speed of light through glass is different than the speed of light through air, which is why lenses work.
If the medium is copper, it is .7C.
That is the speed of light through copper.
But what if the copper is blocking the light?
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But what if the copper is blocking the light?
There is a lot more to the electromagnetic spectrum than visible light.
Copper only blocks light longer than 115 nm wavelegnth. Above that it is mostly transparent to light. X-rays mostly pass right through.
spectrum (https://www.google.ca/imgres?imgurl=https://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Electromagnetic-Spectrum.svg/300px-Electromagnetic-Spectrum.svg.png&imgrefurl=https://en.wikipedia.org/wiki/Electromagnetic_spectrum&h=456&w=300&tbnid=vr3K2hAoqmrPJM:&q=electromagnetic+spectrum&tbnh=160&tbnw=105&usg=AFrqEzebRB_xlsj0NuKr5aTEI6ZryTGmtw&vet=12ahUKEwi795XaxbbdAhX9HDQIHZh-C_gQ9QEwAHoECAgQBg..i&docid=5I6i8EJ0ysnPwM&sa=X&ved=2ahUKEwi795XaxbbdAhX9HDQIHZh-C_gQ9QEwAHoECAgQBg#h=456&imgdii=DGlX3Q6Rql7-eM:&tbnh=160&tbnw=105&vet=12ahUKEwi795XaxbbdAhX9HDQIHZh-C_gQ9QEwAHoECAgQBg..i&w=300)
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The typical value for the speed of light (C) refers to its speed in a vacuum.
The medium through which it travels effects the actual speed.
The speed of light through glass is different than the speed of light through air, which is why lenses work.
If the medium is copper, it is .7C.
That is the speed of light through copper.
Isn't that what the velocity of propagation value indicates?
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Isn't that what the velocity of propagation value indicates?
Yes, that is why I was not understanding.
Also, the standard use of (c) as the constant for the speed of light is understood as the speed of light in a vacuum.
It can sometimes be used as the speed of light in any medium while (c), with a subtext 0, is used to reference the speed of light in a vacuum.
Lee
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Yes, that is why I was not understanding.
Also, the standard use of (c) as the constant for the speed of light is understood as the speed of light in a vacuum.
It can sometimes be used as the speed of light in any medium while (c), with a subtext 0, is used to reference the speed of light in a vacuum.
Lee
Dave's original response did not refer to c, it referred to the speed of light in copper, which I clearly understood to be the speed of the wave in the medium.
The speed of the wave is simply the ratio of the wavelength and period, and in my mind does not really factor into the cause of reflections.
What happens when you put a sub playing a note with a 40 hz fundamental in a 25 ft room?
What happens when you put an offer signal with a wavelength of .7m to 7m on a cable whos length is similar to that length?
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What happens when you put a sub playing a note with a 40 hz fundamental in a 25 ft room?
What happens when a wave hits the wall of the pool?
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What happens when a wave hits the wall of the pool?
What I am getting at is this is NOT a case of simple reflection.
The oscillation carrying the RF signal is actually a longitudinal wave that is modulated on to the transverse waves of the electric and magnetic fields. The signal oscillation is fundamentally one dimensional. That is why is is perhaps easier to picture the signal as an oscillation in the voltage or current values rather than by a distribution of charge.
Now the key point, the superposition of the primary signal with it's reflection can mean the summation of a frequency with a slightly different frequency as the signal is modulated. This sets up another layer of modulation (called beats) that oscillates much slower than the original signal. This is actually the same thing you listen for when you tune one guitar strings against another.
In other words, weird things happen with reflections and superposition when the wavelength of oscillation is similar in size to the space in which the oscillation occurs.
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Dave's original response did not refer to c, it referred to the speed of light in copper, which I clearly understood to be the speed of the wave in the medium.
I can see how it could be read that way although it could also be read as referencing both RF and analogue audio transmission through copper to "the speed of light" which is commonly used to refer to the speed of light in a vacuum.
Technically, I think they both move at the speed of light through copper.
The RF just wiggles a lot faster.
I was not trying to argue here, just trying to clarify so that those reading understood the difference between "the speed of light" as commonly used and "the speed of light transmitted through copper".
Hopefully this thread has been able to clarify what certainly appeared to be confusion at points.
So, yes, the basic understanding is that electricity travels through copper at the same velocity that light travels through copper.
Neither electricity nor light travel through copper at the commonly understood reference of "the speed of light" which would be the speed of light in a vacuum or "c".
Lee
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As if the physics of a signal passing through a cable aren't complicated enough, here's yet another concept to consider.
At audio frequencies, an alternating current passes through almost the entire diameter of the conductor and radiates very little at mic and line voltages. At radio frequencies, signals pass only through the outermost portion of the center conductor diameter due to skin effect and radiate, reflecting back and forth between the outside diameter of the center conductor and the inside diameter of the shield, passing through the dielectric material of the coaxial cable, down the cable's entire length. The velocity factor of said cable is the percentage of the speed of light through copper that the dielectric material imparts on the signal.
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As if the physics of a signal passing through a cable aren't complicated enough, here's yet another concept to consider.
At audio frequencies, an alternating current passes through almost the entire diameter of the conductor and radiates very little at mic and line voltages. At radio frequencies, signals pass only through the outermost portion of the center conductor diameter due to skin effect and radiate, reflecting back and forth between the outside diameter of the center conductor and the inside diameter of the shield, passing through the dielectric material of the coaxial cable, down the cable's entire length. The velocity factor of said cable is the percentage of the speed of light through copper that the dielectric material imparts on the signal.
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And, to bring all of this back to the OP. RF cable is designed to have the distance between the conductor, dielectric, and shield be as consistent as possible. With a 90 degree connector, this means that the conductor inside would have to be be bent very, very precisely to maintain a consistent distance to the outer walls of the connector. I think it's a safe bet that that level of precision manufacturing is not going to be present in a cheap connector from Amazon.