Building a No Rotator station


#1

Hi to all!

I’m trying to build a no rotator ground station but I can’t find a few components. For example the LNA with FM-Notch module is not available anywere.

Can for example use this one: https://www.amazon.de/gp/product/B076D354LW/ref=ox_sc_act_title_1?smid=A1TIGMJ4AZ1K1A&psc=1
in series with this one:
https://www.ebay.com/itm/Wideband-0-01-4GHZ-21DB-for-HAM-Radio-RTL-SDR-LNA-Low-Noise-Amplifier-Module-zg/332507060123?epid=15006590541&hash=item4d6af7179b:g:SM0AAOSwI7tZ1x2G
in order to build the LNA amplifier and the filer as well?

Secondly, where can I find an already made turnstile antenna at around 137MHz?


#2

This LNA is broadband. It offers no low-pass to protect against nearby celltowers 1.8 GHz and up. It also does not notch out FM. Ideally you really want to power the LNA at the antenna with Bias Tees. And, ideally, you would have a LNA/SAW filter@137MHZ/LNA combination.

–Konrad


#3

Thanks for your answer!

It seems that this LNA here: https://www.ebay.com/itm/Bias-Tee-Wideband-1MHz-3GHz-for-HAM-radio-RTL-SDR-LNA-Low-Noise-Amplifier-50VDC/172460255780?epid=2118865505&hash=item28276eaa24:g:m0YAAOSwAURap-XX
is closer to what you are saying but still without the FM suspention. But if I use it series with this one:
https://www.amazon.de/gp/product/B076D354LW/ref=ox_sc_act_title_1?smid=A1TIGMJ4AZ1K1A&psc=1
probably I will do the job! Is that correct?

If no… where can I find the suggested LNA+FM Filter from this guide: https://wiki.satnogs.org/No_rotator?

Basically I will use the base station with a turnstile antenna at around 137MHz… so, the LNA together with a SAW filter that you said will do the job I want, but I couldn’t find anything available. Any suggestions here?


#4

The NOAA satellites are very strong. You may not need an LNA at all. But if you were to need one, the bias tee you selected will do just the trick to supply DC power into the coax feeder up to the antenna. The bias tee also blocks DC from going into your RTL-SDR.

The FM bandstop filter is only necessary if you are near an adjacent FM transmitter that is overloading your front end. You could also be near a a cell tower, etc. that causes similar front-end overloading, etc. This bandstop filter passes DC through so that you can run an LNA in front of it.

The HAM Radio RTL SDR LNA does not have a bias tee on-board, so you would need a second one to tap DC from the feeder coax at the antenna and block DC from going to the antenna for it could actually have a DC short (Eg. a gamma match or loop antenna, etc. The LNA can be very useful for low-signal reception.

If you find an LNA with a SAW filter, the FM bandstop won’t be necessary. The SAW is a very narrow bandpass filter that blocks everything on both sides.

I really like Adam, 9A4QV’s videos. This is a passive filter, which won’t need power from the bias tee, but your LNA will.

Remember, these are all remedies to fix things. Try it without. Then solve the situation as needed.

–Konrad


#5

So, if NOAA (plus Meteor satellites) are very strong is it possible to totally eliminate the LNA (at least for now) and use just the FM bandstop filter that I’ve posted above (the link is from amazon.de) because I do have FM transmitters close to me?

And if yes, the connection will be SDR module -> FM Filter -> Turnstile Antenna?


#6

This looks very reasonable as a starting point:
Turnstile / QFH Antenna --> Coax --> FM Filter --> RTL-SDR

If there’s additional interference, we’ll deal with it.

–Konrad


#7

Perfect! Now I’m trying to find a good FM stop filter.

Its seems that this one:


is the best out there and the second one is the one that I’ve posted above.

The problem is that the one from RTL-SDR probably creates something like 1.5dB attenuation… is this ok?


#8

John,
There’s an old saying: TANSTAAFL (There ain’t no such thing as a free lunch). All filters, all connectors, mixers, everything introduces losses. 1.5 dB insertion loss is quite reasonable for a band-stop filter. You pay the 1.5 dB loss in order to rid yourself of front-end overload from the FM transmitter nearby. If you don’t have this problem, then you don’t need the filter and you don’t incur the 1.5dB of insertion loss. It all comes out of your link budget.

Receiving satellites is in large part a big series of trade-offs in order to get the best possible signal to noise ratio. In the end, it’s about how much energy you receive from the bits that are sent to the amount of energy you receive from noise. At the end of lengthy satellite calculations you will see Eb/No … do you have enough energy left over to decode the message?

For powerful and strong satellites, it seems like no big deal. With the weak ones, every dB will count. So unless you really need that filter, don’t use it. Most people in suburbia live have problems with FM broadcasters. But do try it without. You may not need it.

–Konrad


#9

This is what I use and have had GREAT success with it!


#10

I just press the Bye Now button on eBay! Thanks for your help!!!


#11

And what about the SDR dongle? It seems that the suggested one is the NooElec NESDR Smart. Is it possible to plug a better one for better performance?

Any suggestion about this topic?


#12

I personally recommend the rtl-sdr.com “v3” dongle… its cheap and it works very well.

(for full disclosure, I have a USRP B200 on my station at the moment, and my decode results are very similar to what I got with the rtl-sdr.com v3)


#13

I have been using a RTLSDR from adafruit. (The small ones) with out an LNA so far. :stuck_out_tongue:

I need to invest in some better stuff.


#14

I wouldn’t go replacing that rtlsdr anytime soon - it should be good enough! For your station I would recommend an antenna upgrade - a turnstile or lindenblad would be a great upgrade.


#15

Perfect! rtl-sdr v3 dongle it is!!!

Do I have to change anything or to do any other configuration on the satnogs code in order for this dongle to work properly or is’t “plug and play” like the suggested NooElec Smart?


#16

Pretty much plug and play. I don’t have eith an LNA or filter on mine and its all loft mounted so very much ‘less than ideal’ and I still get reasonable results.

You can try it out and tweak as per the wiki if you need to


#17

Ok, what about the micro SD card size? I can see that SatNogs image can fit easily on a 16GB micro SD card size.

Is this a good option or it’s better to use a bigger one (like 32GB) to be future proof?


#18

The SD card size is not important, as n a Raspberry Pi the SATNOGS code uses a RAM disk (a virtual disk on the RAM), to ensure it can get the read/write speeds. 16GB should be more than plenty.

Make sure it is a Class 10 SD card though, to minimize chances of it developing faults.


#19

Perfect! I will get a 16GB Class 10 Sandisk micro SD card!

And my final question… what about the 50Ohm RF cable from the antenna to the rest of the system?

I am going to use a waterproof enclosure but this one will be on the lowest side of a 4 meter antenna mast. Is the RG213 a good choice or I have to go to an LRM400 cable? Or something even cheaper?


#20

Start with what you have available. You can always upgrade later. Your RTL SDR and LNA should give you good sensitivity, and if the observations turn out to be too low signal-to-noise, the antenna may be next up for upgrading.

For my station I went through several upgrades; from a cheap RTL SDR to a v3 one, from not using an LNA to using one with an FM notch, and from using ground plane antennas to turnstiles. With each upgrade the signal-to-noise increased significantly.