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Matt Brodnick

In-Depth Review: Betso Sharkie Active LPDA Antenna

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Over the past couple months, we've had the pleasure of using Betso's Sharkie active LDPA antennae, with its super quiet operation, enhanced construction, and sharp design.

We also tested it against other LPDA sharkfins with integrated amps. 

Please check out our full testing & review here: 

http://www.bamfsound.com/betso-sharkie-review/

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Thanks for the thorough review, Matt. I'm looking forward to trying out the Betsos.

My understanding of the Lectro ALP650 operation, however, is that its active gain setting are +5, +8, and +12 dB; attained by adding jumpers to attenuate by -7 or -5  from the default +12dB gain.

Best,

Ben Lowry

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Evidently the 12 dB (0 attenuator) setting on the Lectro was not engaged by the tester. This would have changed the gain results (??). Further, it appears the reviewer confused the attenuation values (0,-4,-7) with gain settings which are actually a corresponding (+12,+8,+5).  No mention is made of the OIP of the Lectro unit (+41dBm) or of the other competitive units. Finally, output signal level is not the important measurement of an amplified antenna system. Once the output level of the amplified antenna system is above the noise floor of the receiver input (-107 dBm roughly), the SNR of the signal is the important number, i.e., lots of gain is generally useless in an antenna amplifier because the noise is amplified just as much as the signal. The SNR stays constant no matter how much gain you throw at the antenna output. In an audio analogy, a noisy mic in a noisy room is not made quieter by turning up the mixer gain. You only need enough antenna amplifier gain to overcome cable losses (or splitters) plus a couple of dB more. More gain doesn't help but if the amplifier gain minus the cable and splitter losses goes negative, i.e., below 0 dB overall, that will degrade the SNR by that negative amount. The game is rigged since you can't win back SNR but you can always lose.

Also remember, antenna gain is not the same animal as amplifier gain. Since antenna gain implies directionality, signal levels can go up while noise levels stay constant or drop. Amplifiers amplify everything.
Best Regards,
Larry Fisher

 

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Thanks for your notes and clarification, Larry. I'll be updating the review to take this into account. The output signal level was the only numeric value I could reference, with the tools I had available on production. All systems performed realistically depending on location, and these levels typically correlated with how clearly I could hear the transmission. 

At the time of the review, I was unable to find the OIP of the ALP650 or UA874. Wisycom claims +43 dBm of third-order IP on its fin, however if SNR is the real game -- how much difference do these specifications truly make? Is this a case of which product better overcomes signal loss without adding as little unwanted noise to the constant SNR? Is there a point of diminishing returns?

 

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Hi Matt,
Antenna measurements are difficult to make at best and then lab results don't always match up to the real world. An antenna amp should have high IP3, low gain for good input IP3, and low noise. Low noise and low gain conflict as does low noise and high IP3. I am a little bothered that one of the antenna specs lists a current drain of 30 mA. High IP3 amps will pull 300 mA for power reasons.

The ALP650 specs are on the data sheet. See http://www.lectrosonics.com/US/Antennas/product/363-alp650.html
Note we list input IP3 as well as output IP3. The difference is equal to the gain of the device. Input IP3 is the more critical value but rarely given, since data sheets are typically done by the marketing department. Me cynical?

A high IP3 is only meaningful in noisy RF environments, and then maybe one out of ten times. But it is always better to have a high IP3, All else being equal. SNR can be the most critical factor if you are working long distances but if the noise is from interfering RF then the overload performance of the antenna amp becomes more important. That is to say, if the talent is 100 feet from the receiving antenna, the the noise figure of the antenna amp is pretty meaningless since the signal from the transmitter is much higher than the noise floor. The broadcast TV station on the same frequency 50 miles away is going to determine your effective range. Or the video transmitter on the camera 50 feet away. Or the camera link transmitter in your bag 5 inches away.

Back to your questions. Yes, there is a point of diminishing returns. Once you have sufficient amplifier gain to overcome cable and splitter losses, anything more (4 dB more) degrades the overload performance of the antenna amp and maybe the first stage of your receiver. In a perfect world, we would use passive antennas with one for every receiver input and helium cooled super conducting cables. Given cable losses and splitter losses we need a little help from active antennas. Just not a lot of help.
Best Regards,
Larry Fisher

 

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