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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 => LAB: The Classic Live Audio Board => Topic started by: Keith Broughton on February 18, 2014, 03:23:47 PM
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Given that the coax used is of a low loss type (LMR400 or the like) is there more loss over 100' of air or 100' of cable in the 500-700 mhz range?
It's my understanding that cable loss rises with frequency as well.
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Given that the coax used is of a low loss type (LMR400 or the like) is there more loss over 100' of air or 100' of cable in the 500-700 mhz range?
It's my understanding that cable loss rises with frequency as well.
It's not really apples vs apples because each will contribute loss, and each will be part of the overall loss budget. Loss through free air follows the inverse square law, so -3dB for each doubling of distance. Loss through LMR400 at those frequencies is about 3-4dB/100'. If the transmitter to remote antenna distance is short and the remote antenna to receiver distance is long, the cable wins. If the transmitter to remote antenna is long and the remote antenna to receiver is also long, free air may win. You still get the inverse square loss between the transmitter and antenna in both situations, the difference is the ratio between the 2 hops.
Mac
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Thanks Mac :)
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Given that the coax used is of a low loss type (LMR400 or the like) is there more loss over 100' of air or 100' of cable in the 500-700 mhz range?
It's my understanding that cable loss rises with frequency as well.
Hi Keith,
This website has some useful RF calculation tools.
http://www.zytrax.com/tech/wireless/calc.htm
In particular, the free space loss calculator may help to give you some idea of loss through air at various frequencies and distances.
The Times Microwave site has a calculator for most common RF Cable types:
http://www.timesmicrowave.com/calculator/
Thanks must go to Jason Glass for showing me the Times Microwave site. Hope these are of help.
Neil
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It isn't nearly this simple, and I am a few years out of the RF game, but IIRC the characteristic impedance of free air is 377 ohms.
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Given that the coax used is of a low loss type (LMR400 or the like) is there more loss over 100' of air or 100' of cable in the 500-700 mhz range?
It's my understanding that cable loss rises with frequency as well.
Hi Keith,
Here's a simple, quick, and dirty Excel spreadsheet that you can use to compare the losses of two different paths, in which you can manipulate the cable length, antenna gain, and free space loss. It's very handy when trying to decide where to place your antennas.
cleanwirelessaudio.com/Path Loss Comparison Calculator.xls (http://cleanwirelessaudio.com/Path Loss Comparison Calculator.xls)
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Here's a simple, quick, and dirty Excel spreadsheet
Very nice. Thanks for sharing.
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Very nice. Thanks for sharing.
And thanks to you; your planner spreadsheet is the reigning king of free tools!
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cleanwirelessaudio.com/Path Loss Comparison Calculator.xls (http://cleanwirelessaudio.com/Path Loss Comparison Calculator.xls)
Hey Y'all,
I just posted an update to my loss comparison calculator. The previous version contained many egregious errors in the cable attenuation ratings chart. Sorry!
I also added a 585MHz (center of the UHF-TV band) column to make things easier.
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Hey Y'all,
I just posted an update to my loss comparison calculator. The previous version contained many egregious errors in the cable attenuation ratings chart. Sorry!
I also added a 585MHz (center of the UHF-TV band) column to make things easier.
That's some handy Jason!
Thanks
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Hey Y'all,
I just posted an update to my loss comparison calculator. The previous version contained many egregious errors in the cable attenuation ratings chart. Sorry!
I also added a 585MHz (center of the UHF-TV band) column to make things easier.
Jason - You might want to add a field to account for the insertion losses of the number of interconnections between the antenna connector and the next active device (e.g. RX multicoupler input or TX combiner output).
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Jason - You might want to add a field to account for the insertion losses of the number of interconnections between the antenna connector and the next active device (e.g. RX multicoupler input or TX combiner output).
Hi Henry,
The version that I use in-house has fields for other losses; polarization mismatch, actives and passives, interconnects, adapters, etc. ;) I sometimes use passive switching systems that eat up around .3dB per toggle, along with passive combine/splits or active distribution combine/splits, and the more complex the system becomes, the more critical it is that I account for everything inline.
I posted this version in the hope that its simple A/B comparison might be helpful for those who have a predetermined setup, such as a contractor-provided IEM rig with multiple TX + combiner single output or multiple RX + multicoupler diversity inputs, to determine their optimal "rack to stage" antenna+cable combination and placement.
BTW, what's your experience with BNC and N inline connector losses? I consistently see specs around .15dB per pair and calculate accordingly. Any thoughts or observations on this?
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BTW, what's your experience with BNC and N inline connector losses? I consistently see specs around .15dB per pair and calculate accordingly. Any thoughts or observations on this?
The IL specs provided by the connector manufacturers are for the theoretical "perfect" connection with a brand new set of connectors. The spec doesn't take into account connector wear & tear after a couple hundred, or even a couple dozen, mating cycles; the imperfect connector installation on the coax, and that degradation over time with handling. After measuring new, purpose built M-C OEM N test cables on our VNA, we calculated a mating IL of about .23dB (IIRC).
Same test done with a couple of short LMR400 cables with RF Industries N connectors we guessed at being about three to four years old yielded a mating IL of just under 1dB (again, IIRC). Used a brand new RFI N barrel for the tests.
My general rule is if the cables and connectors [installations] look good, I allocate .5dB per connection. If the coaxes or hardware connectors look questionable I use 1dB IL per mating.
But in the end we need to keep perspective: When the talent puts their hand over the antenna of the handheld, or the person wearing the bodypack turns so their body is between the pack and the RX antenna, there's a 50ish dB attenuation right there.
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The IL specs provided by the connector manufacturers are for the theoretical "perfect" connection with a brand new set of connectors. The spec doesn't take into account connector wear & tear after a couple hundred, or even a couple dozen, mating cycles; the imperfect connector installation on the coax, and that degradation over time with handling. After measuring new, purpose built M-C OEM N test cables on our VNA, we calculated a mating IL of about .23dB (IIRC).
Same test done with a couple of short LMR400 cables with RF Industries N connectors we guessed at being about three to four years old yielded a mating IL of just under 1dB (again, IIRC). Used a brand new RFI N barrel for the tests.
My general rule is if the cables and connectors [installations] look good, I allocate .5dB per connection. If the coaxes or hardware connectors look questionable I use 1dB IL per mating.
But in the end we need to keep perspective: When the talent puts their hand over the antenna of the handheld, or the person wearing the bodypack turns so their body is between the pack and the RX antenna, there's a 50ish dB attenuation right there.
Henry,
Thanks a million. You have my deep gratitude for sharing, especially considering that you obviously spent significant time and effort on the tests.
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I'm not sure anyone has answered Keith's question. For a given distance (100 feet in OP), the over-the-air propagation loss at a given frequency (say 600 MHz) will be many orders of magnitude higher than the cable loss over that same distance. For example, at 600 MHz a 100' LMR400 cable run has a loss of around 3 dB. At that same frequency the 100' free space path loss is around 58 dB. It's not even close.
Even short transmitter-to-antenna air losses will almost always be higher than any cable run. A 10' free space loss at 600 MHz is about 38 dB. It would take 1200' of LMR400 to get 38 dB of loss at that frequency.
Hope this helps.
Dave
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I'm not sure anyone has answered Keith's question. For a given distance (100 feet in OP), the over-the-air propagation loss at a given frequency (say 600 MHz) will be many orders of magnitude higher than the cable loss over that same distance. For example, at 600 MHz a 100' LMR400 cable run has a loss of around 3 dB. At that same frequency the 100' free space path loss is around 58 dB. It's not even close.
Even short transmitter-to-antenna air losses will almost always be higher than any cable run. A 10' free space loss at 600 MHz is about 38 dB. It would take 1200' of LMR400 to get 38 dB of loss at that frequency.
Hope this helps.
Dave
Umm, yeah, the spreadsheet tool that I offered up calculates the different losses out to several decimals of accuracy:
Hi Keith,
Here's a simple, quick, and dirty Excel spreadsheet that you can use to compare the losses of two different paths, in which you can manipulate the cable length, antenna gain, and free space loss. It's very handy when trying to decide where to place your antennas.
cleanwirelessaudio.com/Path Loss Comparison Calculator.xls (http://cleanwirelessaudio.com/Path Loss Comparison Calculator.xls)
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I'm not sure anyone has answered Keith's question. For a given distance (100 feet in OP), the over-the-air propagation loss at a given frequency (say 600 MHz) will be many orders of magnitude higher than the cable loss over that same distance. For example, at 600 MHz a 100' LMR400 cable run has a loss of around 3 dB. At that same frequency the 100' free space path loss is around 58 dB. It's not even close.
Even short transmitter-to-antenna air losses will almost always be higher than any cable run. A 10' free space loss at 600 MHz is about 38 dB. It would take 1200' of LMR400 to get 38 dB of loss at that frequency.
Hope this helps.
Dave
However, there is already free air loss before the signal ever gets to the LMR400, and at that point free air is -3dB per doubling of distance, where cable is about 3dB per 100'. It all depends on how long the various paths are. Best practice is get the receivers close to the stage and keep both free air and cable loss to a minimum. Audio over STP has much less loss than RF over LMR400.
Mac
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However, there is already free air loss before the signal ever gets to the LMR400, and at that point free air is -3dB per doubling of distance, where cable is about 3dB per 100'. It all depends on how long the various paths are. Best practice is get the receivers close to the stage and keep both free air and cable loss to a minimum. Audio over STP has much less loss than RF over LMR400.
Mac
Quite true Mac but this particular situation I was in pretty much killed all "best practice" :(
Going into something similar this coming weekend and hope to get a better solution.
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Quite true Mac but this particular situation I was in pretty much killed all "best practice" :(
Going into something similar this coming weekend and hope to get a better solution.
A follow up...
Did another NHL gig and used the same setup with log antennas at the boards and 200' RG8 with amps at the head end.
Worked very well. MUCH less RF congestion in Vancouver as compared to LA!
However, using the calculator that Jason provided, it looks like I would have come out a few db better using helical antennas 150' away from the rink and only 50' of coax, (not including the amplifier gain)
So the question is, would it be better to have a setup where amplifiers are not required?
The amps make up for cable loss but also raise the ambient noise floor and, in my limited understanding, can make the system a bit more sensitive to IM artifacts.
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amps make up for cable loss but also raise the ambient noise floor and, in my limited understanding, can make the system a bit more sensitive to IM artifacts.
Filter for the bandpass..
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A follow up...
Did another NHL gig and used the same setup with log antennas at the boards and 200' RG8 with amps at the head end.
Worked very well. MUCH less RF congestion in Vancouver as compared to LA!
However, using the calculator that Jason provided, it looks like I would have come out a few db better using helical antennas 150' away from the rink and only 50' of coax, (not including the amplifier gain)
So the question is, would it be better to have a setup where amplifiers are not required?
The amps make up for cable loss but also raise the ambient noise floor and, in my limited understanding, can make the system a bit more sensitive to IM artifacts.
Hi Keith,
You're on the right track in that it is generally better to keep the noise floor as low as possible by using antennas with higher axial gain rather than lower gain antennas with amps. That is, if the passive option is sufficient. Most systems can tolerate a few stages of high-quality gain, though, when the ambient conditions aren't too severe and you're not wrangling dozens of channels.
It's wise to be conservative when entering any gain specs for calculations, and to use figures at the low end of spec'd tolerances. I use 9-11dB of gain for PWS helicals in the UHF-TV band. This coincides with my own observations and with NEC models of the antenna. It might bring your comparison results closer together or even tip it the other way.
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Filter for the bandpass..
Good point but, for this application, the Lectro receiver had to work in 21 through 26 blocks.
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Hi Keith,
You're on the right track in that it is generally better to keep the noise floor as low as possible by using antennas with higher axial gain rather than lower gain antennas with amps. That is, if the passive option is sufficient. Most systems can tolerate a few stages of high-quality gain, though, when the ambient conditions aren't too severe and you're not wrangling dozens of channels.
It's wise to be conservative when entering any gain specs for calculations, and to use figures at the low end of spec'd tolerances. I use 9-11dB of gain for PWS helicals in the UHF-TV band. This coincides with my own observations and with NEC models of the antenna. It might bring your comparison results closer together or even tip it the other way.
That's kind of what I thought but wanted some opinions.
The LA gig would have been better without the amps as the RF environment was very hostile.
The Vancouver gig was much cleaner so the amps worked out.
The big issue with this particular gig was the fact that I couldn't get antennas anywhere close the "best practice" locations.
Thanks for everyones input :)