Re: Best Gain Settings for 137 mhz reception


At those freqencies why not build yourself a common gate J-310 preamp.  Nominally 12 dB of gain, 1.5 dB NF, narrow band design.  IF and ONLY IF you MUST have a preamp, this is a good choice as the gain is not too great, NF is good, and the dynamic range is to die for.  Google:  N6CA preamps or something like that.  They are very simple to build and really difficult to "get it right". 

Dave - W0LEV

PS:  I'd still recommend an FM SBF in front of it all.  DON'T, DO NOT, use a wideband unfiltered "DC-to-Light" preamp!

On Sat, Sep 9, 2017 at 9:54 AM, jdow <jdow@...> wrote:
LNA4All has a lot of gain. In most situations 5 dB gain is enough to set system noise figure well enough. 10 dB might be needed if you have a long coax run. With 20 dB you almost certainly have too much gain involved. That causes IMD from digital saturation effects without improving reception particularly. For beset results you need to play with the individual gain settings on the R820T2 chip. It will still be a little problematic. So a specialized 162.500 MHz filter might be your way out of trouble. A gain setting around 10 would be WAY WAY too high, regardless. I have a 20 dB preamp here and have the expected problems with it. I need to use some subtle filtering to just make up the cable loss and about 6 or 7 dB of extra gain over the amplifier's useful range.

If GB is anything like the US pager transmitters are your likely source of problems. "Good luck" filtering them They are witnin band for anything other than SAW filters or very selective filters based on helical resonators. (The latter are fun to play with. But, the mechanical design for such filters is awkward.)

If you can find the major source of RF energy you can combine a selecting 162.5 MHz filter with a tuned coaxial stub to null the interference source. And for God's sake, reduce the AirSpy gain - a lot. (I use around minimum settings on R820T2 based RTLSDR dongles, which have even worse problems with overload.)

In the final analysis, the number of digitized bits is going to set your usable instantaneous dynamic range. If you improve noise figure 4 dB that means interfering sources that just barely cause overload before will be on the order of 7 dB into overload. (4 dB for NF improvement and 3 dB or more for the extra gain needed to "set" the noise figure below the R820T2 noise figure.)

Mumble mumble - where can I get a 10 dB attenuator that passes DC with no attenuation? That's a start....

{^_-}   Joanne

On 2017-09-09 07:28, David J Taylor via Groups.Io wrote:
From: Ruben Navarro Huedo (EA5BZ)

Hello friends:
I am trying my Airspy R2 receiving NOAA and Meteor M2 sats.
My setup is:
Wimo TA-1 Turnstile Antenna ( --> LNA4ALL --> Around 10 meters Aircell 7 coax --> Airspy
I have been trying with Linearity and using a Gain value around 10.
If i use a higher gain value i start to have some intermittent BCFM interferences.
My question is:
Could i obtain better SNR values using Free Gain mode? What adjustment is the best?
How do you think i could obtain better SNR values? Using another LNA? Using a BCFM filter?

Thank's a lot for your help.


Are you sure you need any extra gain?  I'm testing with Linearity, Gain: 14, Sample rate: 3 Msps, Decimation: 64, using an Airspy Mini.  Are you sure what's causing the interference?  I get pager transmissions on both 138 and 153 MHz which show up as bands - some with a near 1 minute repetition interval.

Ideally a filtered pre-amp, such as:

but unfortunately this is out of stock.  Therefore I would go for a broadcast FM reject filter.  I see that Amazon offer one from a Greek amateur - SV1AFN:

All being well the forthcoming Airspy HF+ will perform better.

David GM8ARV

Dave - WØLEV
Just Let Darwin Work

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