LNA Safety Question

[Scott Elliott]

Looking at LNAs like the Nooelec SAWbirds, typical OP1dB output of 18 dBm far exceeds the safe input of a AD9361 based SDR such as a Pluto.   The 9361 datasheet gives ABSOLUTE MAXIMUM RATINGS: RF Inputs (Peak Power) 2.5 dBm.  Sure, H1 is supposed to be quiet and yes some LNAs have narrow bandpass filters, but some nearby cell tower with long tails or some intermod finding a side lobe could could do some real harm.  I've never heard anyone complain, so what is the real risk?  

[Marcus D. Leech]

On 2025-09-24 20:57, Scott Elliott wrote:

Looking at LNAs like the Nooelec SAWbirds, typical OP1dB output of 18 dBm far exceeds the safe input of a AD9361 based SDR such as a Pluto.   The 9361 datasheet gives ABSOLUTE MAXIMUM RATINGS: RF Inputs (Peak Power) 2.5 dBm.  Sure, H1 is supposed to be quiet and yes some LNAs have narrow bandpass filters, but some nearby cell tower with long tails or some intermod finding a side lobe could could do some real harm.  I've never heard anyone complain, so what is the real risk?   --

The SawBird is filtered between the two stages.  Should be fine.  Yeah, you might occasionally experience intermod in the downstream receiver, but hugely unlikely to
  experience dangerous levels of RF, unless you're deployed AT a cell-site.  Which, well, you'd never do for radio astronomy.

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[Scott Elliott]

Thanks Marcus.  I'll try to not plant my dish under a cell tower or TV station.  Does anybody ever add a second stage amp in the Rx chain? -scott

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[Marcus D. Leech]

On 2025-09-24 21:07, Scott Elliott wrote:

Thanks Marcus.  I'll try to not plant my dish under a cell tower or TV station.  Does anybody ever add a second stage amp in the Rx chain? -scott

The SawBird has about 40dB of gain, with a SAW filter between the two stages.  I've never really needed an interstitial amplifier except for very-long (100m) runs.

At the observatory, our feed-line lengths are fairly short as these things go--down the feed legs and into the receiver cabin.  Maybe 10m.

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