Is it my old age?

[ZL2BMH]

Hello All.

Could someone tell me please, does rtl-sdr.com sell 137.5 MHZ LNA's

I went to the store but came away more confused than when I went in. Okay, I'm coming up 80yrs of age very quickly but I'm darned if I could find one (for NOAA/Meteor Sats).

Any help would be really appreciated.

Regards.

[tim_rtd]

Probably not for a narrow band of frequencies. They are usually broad range LNA's. Something like 25-2000Mhz is generally what's available.... They do work...The next question is do you use the silver V3 dongle? This unit has Tbias capability whereby the dongle can power the LNA. Otherwise you need to externally power the LNA itself with a power supply or battery.

[rtlsdrblog]

We don't have specific LNAs for 137.5 MHz. Generally you will find that an LNA for 137 MHz is not needed, and may make things worse.

Why do you think you need an LNA?

[ZL2BMH]

Thank you both for your replies, much appreciated.

Jim.

[KC7BYY]

An FM Broadcast filter “might” help. Typically they are strong signals between 88mhz to 108mhz. It’s a notch filter.

[aurgathor]

While there are some band specific LNAs, I don't think there is one for that frequency.

If you really want one, you could put a bandpass filter in front of a wideband LNA.
Here is a 116-152Mhz Band Pass Rf Filter: https://www.ebay.com/itm/116-152Mhz-Ban ... SwCGVYB2RT

You can probably find better and cheaper ones, or you could just make your own.

[snn47]

The BP filter diagram that aurgathor attached looks like a low pass filter to me, even though the text suggests otherwise.

Using the same website I get a different diagram

_#.PNG (8.76 KiB) Viewed 678 times

I personally would use a helical, or better a cavity bandpath filter. Just look for a 2m BP filter.

[aurgathor]

Yes, a cavity filter would be much better than most any LC filter, but I think they are more expensive, too.

As for the discrepancies in the circuits, rf-tools have quite a few options (i.e.: topology)now that weren't there last year. I generally trust the filters generated there, although I've seen a few cases where the circuit contained values that didn't make any sense such as fF capacitors for relatively low frequency filters that disappeared after a minor change in the starting parameters.

BTW, 100 nH is the default value as per:

[snn47]

I remember a few publications for preamps receiving Wx-satellite e.g. in UKW-Berichte/VHF Communication Magazine, which employed simple LC filter with air inductor and FET. More recent design employ integrated LNA for RTL-SDR, e.g. DG8GB http://www.gunthard-kraus.de/Vortrag_We ... m_2014.pdf
But unfortunately UKW-Berichte have stopped producing PCB and kits, so you might look for used ones.

But there are many other publications, incl. 2m preamp design where you can retune the filters.

Or you can buy tunable band pass-Filter that works that includes the 137 MHz range (e.g. Surplus from Signal Corps) and use a standard preamp/LNA.

It all depends on how much gain your preamp has to provide to complement your antenna, while how much attenuation the filter has to provide, depends on your RF-environment at your QTH. No one can advise you how much filtering you might need, if a simple LC filter is sufficient or if need a combination of bandpass and notchfilters.

PS:aurgathor
while it is nice to have webpages calculate filter for you, building a filter that really comes close to the calculated filter performance is another.

[snn47]

Maybee you could change the topic to something like "137 MHz NOAA preamp" to make this thread easier to find

I just checked, there are a number of 137 MHz preamps with bandpath filter available, e.g.
https://www.wraase.de/de/shop/#aa-137
http://www.hf-berg.de/HFC/vorverstaerker_weissblech.htm look for NOAA
https://stecker-shop.net/epages/27edac8 ... ucts/03852
MVV 137 http://www.oh1sa.net/data/mirrors/www.u ... orver.html