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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: Pete Erskine on February 22, 2016, 03:19:26 PM
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I have had several people ask this question off line.
I created this page to explain how I have done it.
http://www.bestaudio.com/s/BTR_antenna_extend.pdf
Info on splitters - there are many others but this is what's in my case
at 500 MHz
Splitters Part no Loss/port Isolation/port Link
1X2 ZAPD-21+ 3.2 22 http://www.minicircuits.com/pdfs/ZAPD-21+.pdf
1X3 ZB3PD1-222+ 5.2 18 http://www.minicircuits.com/pdfs/ZB3PD1-222+.pdf
1X4 ZFSC - 4 -175W+ 6.5 30 http://www.minicircuits.com/pdfs/ZFSC-4-175W+.pdf
1X6 ZB5CS-920-10W-N 7.5 28 https://www.minicircuits.com/pdfs/ZB5CS-920-10W.pdf
DIRECTIONAL TAP
ZGDC10-362HP+ http://www.minicircuits.com/pages/npa/ZGDC10-362HP+_NPA.pdf
<1dB insertion in to out
>12dB loss on tap
used to fet a low level feed off of an rf antenna for the Fiber input of RFOF.
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Thanks Pete,
This is a good diagram on what I was asking on recently. The verification that the passive combining the two antennae is a big help for the two or more areas of reception.
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put the RF amp at the end of the coax
I presume you mean the antenna end of the coax.
Great info! Thanks for posting :)
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I presume you mean the antenna end of the coax.
Great info! Thanks for posting :)
No- I mean the end at the RX. The point I was making was that in a high BG rf noise location such as on 34th street below the Empire State putting the RF amp at the antenna probably will overload it. Moving it to the RX end uses the Coax as protection. Either way it works. There is no inherent advantage to putting it at the antenna unless your coax is so leakly that it picks up noise itself.
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No- I mean the end at the RX. The point I was making was that in a high BG rf noise location such as on 34th street below the Empire State putting the RF amp at the antenna probably will overload it.
-Or get an RF amp with a good bandpass filter and learn to tune it. The old trusty Sennheiser AB 1036 can be tuned very easily with spectrum analyzer with tracking generator and a small screwdriver. The new AB 9000 will also do the trick.
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No- I mean the end at the RX. The point I was making was that in a high BG rf noise location such as on 34th street below the Empire State putting the RF amp at the antenna probably will overload it. Moving it to the RX end uses the Coax as protection. Either way it works. There is no inherent advantage to putting it at the antenna unless your coax is so leakly that it picks up noise itself.
Ahhhhh...got it!
I'm used to putting amps before a long run of coax back to the receiver to make up for cable loss before the fact.
I can see that in a very high noise RF environment, the amp may get overloaded.
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-Or get an RF amp with a good bandpass filter and learn to tune it. The old trusty Sennheiser AB 1036 can be tuned very easily with spectrum analyzer with tracking generator and a small screwdriver. The new AB 9000 will also do the trick.
Can the AB9000 be used stand alone? is there software?
The online info on the AB1036 doesn't say anything about filtering except in specs with the note "adjustable" Is there a more complete manual?
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Can the AB9000 be used stand alone? is there software?
The online info on the AB1036 doesn't say anything about filtering except in specs with the note "adjustable" Is there a more complete manual?
AB 9000 filtering range can be set with a rotary switch when not connected to a 9000 mainframe. The gain is fixed at 17dB, but it would be easy to put in a Mini Circuits NAT-10DC+ to bring it down to 7dB if needed.
The AB 1036 is probably too old for a pdf manual ;-), but it includes a bandpass filter that can be tuned to around 5-40 MHz bandwith. Needs a screwdriver and somebody that knows how a spectrum analyzer works, so not exactly plug and play.
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Thanks for this! What is the benefit of having separate TX1 and TX2 outputs from a BTR base station, only to combine them again? (Especially since this requires a modification to the unit.)
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Thanks for this! What is the benefit of having separate TX1 and TX2 outputs from a BTR base station, only to combine them again? (Especially since this requires a modification to the unit.)
When you are combining more than 1 BTR you want all the transmits separate into the combiner to minimize the intermods created. You never want to amplify an RF signal that has more than 1 carrier. Inside the IEM combiner the signals are amplified first, then combined.
Mac
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Thanks for this info, it could be very helpful on an upcoming gig I have. Would bias power from the DA be enough to power 2 line amps, or would I need a bias-T?
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When you are combining more than 1 BTR you want all the transmits separate into the combiner to minimize the intermods created. You never want to amplify an RF signal that has more than 1 carrier. Inside the IEM combiner the signals are amplified first, then combined.
Mac
I know this is true [amplify a single carrier only] both from experience and listening to wiser men on this forum. but i don't really know why.
Is there a good source to learn a bit about RF amplification/reduction that could explain a little of the issues created when trying to amplify multiple carriers with one amp?
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I know this is true [amplify a single carrier only] both from experience and listening to wiser men on this forum. but i don't really know why.
Is there a good source to learn a bit about RF amplification/reduction that could explain a little of the issues created when trying to amplify multiple carriers with one amp?
You could start here: https://en.wikipedia.org/wiki/Third-order_intercept_point (https://en.wikipedia.org/wiki/Third-order_intercept_point) Beware, this is the mouth of a deep rabbit hole!
Essentially, when you need to amplify multiple carriers with a single amp, the amp must be extremely linear. A measure of an amp's linearity is it's IP3 performance. Amps with high IP3 are expensive, and their IP3 performance is very easily compromised when [the vector sum of the input signal levels]+[the amp's gain] drive the amp into compression or saturation, by the output exceeding the amp's maximum power output rating P1, even by just a little bit.
I often pontificate that amplifying wide bandwidths, and by implication multiple carriers, is generally bad. This is true in most situations where the user has not carefully done the math and specified appropriately precise components for the job at hand. However, it is not strictly forbidden and can work well under certain circumstances. An excellent example is an RF over fiber line feeding an LNA (low noise amplifier) to transmit intercoms to an enclosed hallway that is isolated from the main TX antenna's coverage. I have successfully amplified 6 very low power carriers, post fiber, with a single LNA up to moderately low power levels with minimally transmitted IMD products and other noise. So, it's not impossible but it's not something that is easily hacked together!
-EDIT- Corrected math from product to sum.
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You could start here: https://en.wikipedia.org/wiki/Third-order_intercept_point (https://en.wikipedia.org/wiki/Third-order_intercept_point) Beware, this is the mouth of a deep rabbit hole!
Essentially, when you need to amplify multiple carriers with a single amp, the amp must be extremely linear. A measure of an amp's linearity is it's IP3 performance. Amps with high IP3 are expensive, and their IP3 performance is very easily compromised when [the vector sum of the input signal levels] * [the amp's gain] drive the amp into compression or saturation, by the output exceeding the amp's maximum power output rating P1, even by just a little bit.
I often pontificate that amplifying wide bandwidths, and by implication multiple carriers, is generally bad. This is true in most situations where the user has not carefully done the math and specified appropriately precise components for the job at hand. However, it is not strictly forbidden and can work well under certain circumstances. An excellent example is an RF over fiber line feeding an LNA (low noise amplifier) to transmit intercoms to an enclosed hallway that is isolated from the main TX antenna's coverage. I have successfully amplified 6 very low power carriers, post fiber, with a single LNA up to moderately low power levels with minimally transmitted IMD products and other noise. So, it's not impossible but it's not something that is easily hacked together!
Excellent answer. And I'll dive into the rabbit hole knowing I may well go mad....
I too broke the rules on this once and it worked. Kinda. Wouldn't do it again without a LOT more experience in this area.
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An important takeaway from the TOI/intermod subject is that intermod products drop away twice as fast (in dB) as the original signals that created the problem.
Say your receiver has an apparent problem with an interfering signal. You then put a 10dB attenuator in front of your receiver and find that the interfering signal has dropped by 20dB. The interfering signal never actually existed "on air" - it was created by your receiver!