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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 14, 2014, 09:21:59 AM
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I understand the principal of calculating long run cable loss in RF mic systems then using the appropriate front end amp gain to compensate.
If I am using the Shure or the PWS helical , do I consider the "10-14 db gain" in that calculation to determine amp gain?
So, if I have 8 db of cable loss and the antenna has 10 db of "gain" do I not use an amplifier?
Is the type of antenna not considered in cable loss compensation calculations?
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"Gain" is a misleading term of art, misused in this instance (and others). It should be referenced/charted as "Forward Tuned Losses v. Omni Losses". In this manner antenna types display their advantages (mislabeled as "gain"). No passive device produces gain.
Modern, off-the-shelf RF devices respond well to "best practices":
Use high quality, frequency-tuned antennas. (Helical is better than Yagi paddle. Yagi paddle is better than omni stick).
Minimize transmitter-antenna distances.
Use high quality antenna cables. (LMS400 or equiv/better. Use RG58 to tie up a drooping auto tailpipe - temporarily).
Keep antenna-receiver distances short.
Only if transmitter-receiver distances MUST exceed 100ft, should an an inline RF amp be considered. They are not panaceas for ignoring other upstream criteria, and often unnecessary. JMO....
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I understand the principal of calculating long run cable loss in RF mic systems then using the appropriate front end amp gain to compensate.
If I am using the Shure or the PWS helical , do I consider the "10-14 db gain" in that calculation to determine amp gain?
So, if I have 8 db of cable loss and the antenna has 10 db of "gain" do I not use an amplifier?
Is the type of antenna not considered in cable loss compensation calculations?
Hi Keith,
Fortunately, path loss through cable and antenna is calculated with simple arithmetic. It only gets more complicated when you introduce free space loss for a total path loss or "link budget" calculation. You generally start with the antenna's conservative gain spec (lowest value, because gain varies across any wideband antenna's frequency range), then subtract your cable loss from that number, then subtract free space loss if you need to take it that far. You can leave out free space loss if you're simply comparing different combinations of coax lengths and antenna types, in situations where the TX and RX antenna locations will always be the same.
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"Gain" is a misleading term of art, misused in this instance (and others). It should be referenced/charted as "Forward Tuned Losses v. Omni Losses". In this manner antenna types display their advantages (mislabeled as "gain"). No passive device produces gain.
Not true. There are many passive devices that can produce gain. Antennas are just one of them.
Antenna gain is typically measured in either dBi (referenced to an ideal isotropic antenna) or dBd (referenced to a dipole). This gain should be used in your overall wireless link budget calculation, and yes it is real gain.
Another item to consider--if you introduce any active gain (i.e. an amplifier) into your system try to keep that component as close to your receive antenna as you can. This will keep your system noise figure as low as possible. So for long coax runs place any gain at the antenna end of the cable, not the receiver end.
My .02
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Not true. There are many passive devices that can produce gain.
The Second Law of Thermodynamics seems to refute this.
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The antenna type, dipole, yagi, quad, log, etc. are all factors in determining gain, as is the orientation of that entenna type relevant to frequency and distance. Antennas are passive devices whose gain can be measured with gain referring to a gain in power.
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The Second Law of Thermodynamics seems to refute this.
I don't understand how the second law (The entropy of any isolated system not in thermal equilibrium almost always increases) refutes this. I hope you will explain.
A passive boost in antenna gain is the result of directivity. For any boost of gain in one direction, gain is lost in other directions. Energy is neither created nor destroyed, conforming to the law of conservation of energy.
The peak gain of an antenna is not mislabeled in the context of RF engineering terminology, but I agree that it is often misunderstood.
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Informally, the Second Law posits that you cannot get more out than you put in, no free lunch, etc. That should suffice for this forum.
My reply suggested that the RF-world inner circle term of art was misleading to the casual user. "Less lossy" does not equal "gain" for most of us. To wit: If the stock market fell by 50%, but your portfolio only sank by 30%, would you be rewarding your broker for a 20% gain? I think not..
The OP was also asking guidance regarding inline RF amplifiers for prosound usage. My reply (which I believe helpful and supported by experience) promotes the notion that for most applications inline RF amplifiers are unnecessary.
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I understand the principal of calculating long run cable loss in RF mic systems then using the appropriate front end amp gain to compensate.
If I am using the Shure or the PWS helical , do I consider the "10-14 db gain" in that calculation to determine amp gain?
So, if I have 8 db of cable loss and the antenna has 10 db of "gain" do I not use an amplifier?
Is the type of antenna not considered in cable loss compensation calculations?
It depends... If you want the normal range/performance of a PWS helical, but with 75 metres of coax, you should calculate the loss in the cable and make up that loss with an amplifier just after the antenna.
If you only need the range of a standard omni antenne, but actually have a directional antenna, you can include the "gain" in the antenna in the calculation.
Another thing to remember is to watch out with wideband rf amps on directional antennas. It is quite easy to overload it with stronf RF signals like UHF Motorolas etc. So keep the antennas some metres away from any RF transmitter/ TX antenna. Whenever possible use bandpass filtering before amplifiers. Sennheiser AD-12, Sennheiser AB-1036, the new Sennheiser antennas for the 9000 series, Lectro UFM50 etc.
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Informally, the Second Law posits that you cannot get more out than you put in, no free lunch, etc. That should suffice for this forum.
Your general interpretation of the second law seems to be confused with the first law (conservation of energy), but neither applies to the issue antenna axial gain. An antenna is not a "perpetual motion machine" but is rather a transducer. It is understood that energy is being supplied to a TX antenna by a transmitter or to an RX antenna by electromagnetic waves traveling through space.
My reply suggested that the RF-world inner circle term of art was misleading to the casual user. "Less lossy" does not equal "gain" for most of us. To wit: If the stock market fell by 50%, but your portfolio only sank by 30%, would you be rewarding your broker for a 20% gain? I think not..
When defining transducer gain, values given in dB are always a comparison to some reference value, whether you're an Audio Engineer or an RF Engineer. Dave has already noted that in the case of antennas, it is compared to an ideal isotropic's (dBi), or an ideal dipole's (dBd), inherent gain.
The OP was also asking guidance regarding inline RF amplifiers for prosound usage. My reply (which I believe helpful and supported by experience) promotes the notion that for most applications inline RF amplifiers are unnecessary.
I agree with your advice on amplifers, but you may have misunderstood the OP. He stated that he understands an amplifier's proper application, but was wondering if the same methods of path loss calculation apply to passive high-gain antennas like PWS helicals and LPDAs. The answer is yes.
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It depends... If you want the normal range/performance of a PWS helical, but with 75 metres of coax, you should calculate the loss in the cable and make up that loss with an amplifier just after the antenna.
If you only need the range of a standard omni antenne, but actually have a directional antenna, you can include the "gain" in the antenna in the calculation.
Another thing to remember is to watch out with wideband rf amps on directional antennas. It is quite easy to overload it with stronf RF signals like UHF Motorolas etc. So keep the antennas some metres away from any RF transmitter/ TX antenna. Whenever possible use bandpass filtering before amplifiers. Sennheiser AD-12, Sennheiser AB-1036, the new Sennheiser antennas for the 9000 series, Lectro UFM50 etc.
Thank you for answering the question.
I figured that was the way it works but wanted to confirm.