Is it my old age?

Talk about anything else here.
Post Reply
ZL2BMH
Posts: 23
Joined: Mon Jul 16, 2018 8:49 am

Is it my old age?

Post by ZL2BMH » Sun Sep 06, 2020 2:44 am

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
Posts: 162
Joined: Fri Jun 30, 2017 11:34 am

Re: Is it my old age?

Post by tim_rtd » Sun Sep 06, 2020 4:58 pm

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
Site Admin
Posts: 2816
Joined: Mon Nov 19, 2012 11:54 pm

Re: Is it my old age?

Post by rtlsdrblog » Tue Sep 08, 2020 12:51 am

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
Posts: 23
Joined: Mon Jul 16, 2018 8:49 am

Re: Is it my old age?

Post by ZL2BMH » Sun Sep 13, 2020 12:57 am

Thank you both for your replies, much appreciated.

Jim.

KC7BYY
Posts: 7
Joined: Fri Oct 09, 2020 12:35 am
Location: Bellevue, Washington USA

Re: Is it my old age?

Post by KC7BYY » Sun Nov 29, 2020 10:30 pm

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

aurgathor
Posts: 106
Joined: Wed Apr 25, 2018 8:14 am

Re: Is it my old age?

Post by aurgathor » Sun Dec 13, 2020 12:17 pm

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.
Attachments
1375-filter.png
1375-filter.png (8 KiB) Viewed 704 times

snn47
Posts: 239
Joined: Tue Dec 27, 2016 11:00 pm

Re: Is it my old age?

Post by snn47 » Sun Dec 13, 2020 6:47 pm

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
_#.PNG (8.76 KiB) Viewed 679 times
I personally would use a helical, or better a cavity bandpath filter. Just look for a 2m BP filter.

aurgathor
Posts: 106
Joined: Wed Apr 25, 2018 8:14 am

Re: Is it my old age?

Post by aurgathor » Mon Dec 14, 2020 1:04 am

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:
Attachments
lc-filter-setting.png
lc-filter-setting.png (10.85 KiB) Viewed 658 times

snn47
Posts: 239
Joined: Tue Dec 27, 2016 11:00 pm

Re: Is it my old age?

Post by snn47 » Tue Dec 15, 2020 10:38 am

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.
Last edited by snn47 on Tue Dec 15, 2020 6:51 pm, edited 1 time in total.

snn47
Posts: 239
Joined: Tue Dec 27, 2016 11:00 pm

Re: Is it my old age?

Post by snn47 » Tue Dec 15, 2020 6:33 pm

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

Post Reply