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Amplified antennas, good or bad?

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I am reading of new products coming on the market that are a concern to me. Amplified antennas should only be used for two reasons. It is to overcome the RF loss of long cable runs to a receiver (50 feet or more) or the RF loss of passive splitters used to drive multiple receivers. Any receiver used for the professional wireless industry has the optimum amount of RF gain as set by the manufacture. When used incorrectly adding gain to the receiver with a powered antenna will only serve to make the receiver overload more easily and will not enhance range any more than a passive directional antenna. Receiving an RF signal is all about signal to noise. Antennas with amplification can only amplify signal and noise at the same time not providing any real advantage. There is a feel good factor in that the signal on a meter may be higher with an amplified antenna but so is the noise so the benefit is hard to see if it is there at all.

The other thing to watch out for is filtered antennas. You must know what the bandpass looks like to know if the filter is doing any good. Zaxcom receiver filters for example have a band pass of +/- 17.5MHz and attenuate about 30 dB  at +/- 50MHz from the center frequency. If an antenna filter in front of the receiver has a +/- 50MHz bandpass and attenuates 20dB at +/- 100MHz away from the center frequency that is good to know as that will not provide good protection from interference. The point is, antenna RF filters are never the same and all manufactures should publish a frequency response graph so the the customer can understand exactly what the product does or does not do. With this information a proper system will be easy to put together.

Glenn Sanders 

President Zaxcom Inc.

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Glenn I hear you. With Zaxcom wireless being digital I thought as long as the RX can read the "0" and "1" of the signal it will detect it properly. So pushing the "1" above a certain threshold will help even with the signal being surrounded by gained noise.

You are saying that a Zaxcom all digital RF signal behaves just like an analog RF signal.

And indeed I just look at my RF meters and notice the RX always switches to the stronger one which is the amplified one. 

I know the theory behind all this yet still my practical experience differs. 

 

 

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i think of amplified antennas in the same way as a microphone. 

whilst turning up the gain increases the signal you do want, you also increase the signal you dont want by the same amount.
then when you get to wherever your compressor is set to, the signal you do want doesnt get any louder, but the signal you dont want does - reducing your signal to noise ratio.

gain in rf also has the potential of adding intermod to your signal as well which cannot be filtered after the amplifier.

 

i worked with a mixer some time ago who had some powered antennas, and when he was complaining that he was getting a load of hits on his radio, he found that turning the amplifier down cured the issue.

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Zaxcom wireless has to see the transmitted signal in its linear range. The receiver is very good at doing this. But overloading it with a powered antenna can lead to dropouts at close range and is not advised. Yes RF systems are a lot like microphones amplifying signal and noise. This is not so much about our system but all RF systems.

 

Glenn

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Audio Wireless has a digitally tuned RF splitter coming soon that is meant to work well with active and passive antennas since it provides further filtering of about 30mhz, if I remember correctly. I was hoping to see how it would perform with the active Betso antennas for my future setup. Maybe someone can chip in on those as its something I also need suggestions on. 

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I attended the sound summit yesterday and Karl Winkler from Lectrosonics said the same thing for not only Lectro but all wireless.

He stressed that the only time to use amplification on antennas is to make up for signal loss from either signal splitting or cable loss. So for example if you loose 4 dB of signal you want to add 4dB or gain. Adding gain will reduce your preformance.

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I know the theory behind all this yet still my practical experience differs. 

That is exactly the thing. The actors suddenly start the take earlier around a corner and now I'm getting dropouts. No time to re-position the antenna so I switch in 10dB of gain and I'm back in businees getting a clean dropout free signal. 

Audio Wireless has a digitally tuned RF splitter coming soon

Don't hold your breath. I have been told about this more than a year ago (started a thread about it, too). Never happened. 

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That is exactly the thing. The actors suddenly start the take earlier around a corner and now I'm getting dropouts. No time to re-position the antenna so I switch in 10dB of gain and I'm back in businees getting a clean dropout free signal.

Maybe there was not much interference on the block that day making your signal to noise ratio very good to begin with.

 

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Maybe there was not much interference on the block that day making your signal to noise ratio very good to begin with.

 

possible, but I'm not talking about a specific occurence, but rather the experience I've had on many days in many locations. 

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As end users of this gear in the field, we become somewhat a kind of engineer ourselves, not in the classic sense, but at the front lines trying to make this stuff work.  Spec is great and stories of hundreds of yards of range is impressive, but sometimes shit hits the fan and you can't get 50 feet (sometimes can't get 5 feet, but that's not a signal power / reception issue).  I appreciate having the power and tools at my disposal to make the scene work without holding up production.  If having powered antennas helps me do the job, I appreciate the flexibility in having them at my side.  The warning about overload - duely noted.  What I'd really like to see from wireless manufacturers is some more informative RF metering capabilities.  The single axis "RF bar" or "Christmas tree" could be replaced with some sort of 2-axis meter perhaps?  An axis for RF modulation level and a side band that would illustrate signal to noise, I'd imagine it could look much like half of a stereo phase correlation scope.  Perhaps powered antenna manufacturers could also provide some sort of built in signal geneator.  If we can observe the RF modulation level with a pig tail, then we can more accurately calculate the dB drop from a 50' length of RG-11.  More informative metering could then tell us if we are realizing a gain in performance, or just introducing too much noise to amplify too.

I think the future will be more wireless packed into a tiny bandwidth shared with other devices.  2.4Ghz?  5Ghz? 900Mhz?  With digital modulation could have ultra narrow independently bandpassed antennas, one for each actor, with c-stand mounted multi-receiver, bristling with these antennas on PTZ heads and helical antennas tracking cast as they move about.  At that point, my coax would be feeding a MADI signal that could run hundreds of feet back to me.  The utility position could evolve into some sort of AWACS-like EW/RF/radar operations tech scanning the RF horizon for bogies.

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Constantin - how do you switch on 10dB of gain? With me using the Sennheiser AB the signal won't pass the amp if that is not powered.

no, I have not the same experience. I use the Sennheiser AB3700, which is separate from the antenna. And it definetely does pass the signal. I know this, because I used to use them with my Octopack where I could switch on power as needed. Now on my Venue the power is more difficult to switch on or off so it's always on. Standard setting on the AB is 5dB. This makes sense as it accounts somewhat for cable length and the patchbay pass-through. 

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That is exactly the thing. The actors suddenly start the take earlier around a corner and now I'm getting dropouts. No time to re-position the antenna so I switch in 10dB of gain and I'm back in businees getting a clean dropout free signal. Don't hold your breath. I have been told about this more than a year ago (started a thread about it, too). Never happened. 

Well like many manufacturers they too take their time after the news is out but I was at their office last week and after a long chat about the units I am pretty confident we will see them soon.

Sent from my iPhone using Tapatalk

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Sennheiser support confirmed that the AB3 attenuates the signal when no power is supplied.

AB3 or AB3700? Anyway, it's possible that it attenuates. Still, I would only switch it in as needed

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its worth noting that just attaching antenna cables to your receivers - without an antenna attached - will give you signal at the receivers.

when you switch off your antennas, you may just be seeing that signal. 

 

some of the powered antennas i have seen have a 0dB option. 

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I am curious to know if anyone has used the Betso's and what they think of them and wether if its possible to use them as passive antennas without power when the extra gain is not needed?

 

 

Edited by Erkal Taskin
bad english

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I am curious to know if anyone has used the Betso's and what they think of them and wether if its possible to use them as passive antennas without power when the extra gain is not needed?

 

 

I am using the Betso Sharkie antenna with the Broady as122 DA antenna in a hiden camera show and i still can not believe how I managed to make such distance as far as 300 feets with  out any concern about dropout I used it active way but sometime I give + or - DB depending my need

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I am using the Betso Sharkie antenna with the Broady as122 DA antenna in a hiden camera show and i still can not believe how I managed to make such distance as far as 300 feets with  out any concern about dropout I used it active way but sometime I give + or - DB depending my need

Hey, thats good to hear. Are you also able to use it without power as passive or does it only work with power? 

 

 

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These reasons are why we are a big fan of the Shure 874 antenna a lot of the times.  Feed it power all the time and set appropriately.  Even has attenuation if things start getting real close and you actually need to bring signal levels down.  Normally we set for 0dB, unless we are using longer runs or have to use light weight cable.  Wish it had a 3dB setting though.

http://www.shure.com/americas/products/accessories/wireless-systems/wireless-systems-antennas/ua874-active-directional-antenna

I do like the idea of a Signal to Noise meter a lot though, that would be great.

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i am presently at work and I did the test (turn off the DC feed on the DA) making the antenna passive. I do not get a good result I should try directecly in the receveiver but I don't have the sma to Bnc adapter

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