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Author Topic: Extending BTR antennas to get more coverage  (Read 4835 times)

Andrew Outlaw

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Re: Extending BTR antennas to get more coverage
« Reply #10 on: March 03, 2016, 12:09:43 pm »

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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brian maddox

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Re: Extending BTR antennas to get more coverage
« Reply #11 on: March 03, 2016, 01:17:17 pm »

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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brian maddox
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Jason Glass

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Re: Extending BTR antennas to get more coverage
« Reply #12 on: March 03, 2016, 09:24:14 pm »

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  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.
« Last Edit: March 05, 2016, 09:50:42 am by Jason Glass »
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brian maddox

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Re: Extending BTR antennas to get more coverage
« Reply #13 on: March 03, 2016, 10:44:39 pm »

You could start here: 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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brian maddox
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Lyle Williams

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Re: Extending BTR antennas to get more coverage
« Reply #14 on: March 05, 2016, 02:07:48 am »

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!
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Re: Extending BTR antennas to get more coverage
« Reply #14 on: March 05, 2016, 02:07:48 am »


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