I have problems with LNA


#1

Hello everyone
I have been experimenting with Satnogs for some time and it is really fun although since my beginning in this adventure I have always had the same problem. I currently have a vertical dual band antenna in a really bad location, but at the moment I can’t change it. The rest of the system consists of a Raspberry and an SDR. Unable to have good signals, I decided to buy an LNA and the results were bad. Only with the antenna and the SDR does everything as good as possible with the location of the antenna work, but when I install the LNA I stop receiving almost completely. I have tried with 2 SDR (rtl-sdr.com and with Nooelec Smartee, also with 2 different LNAs (SPF5189Z and another one that I don’t remember the model) with the same bad result. I have also tried this same configuration in windows and it works fine. can anybody help me?

Thank you in advance


#2

I like wise have been having a great time putting together my fixed ground station.
Regarding the LNA, I have found that the extra gain from the LNA can often cause a blank waterfall. To get things back to ‘working’, you need to go into the Satnogs-setup and turn the RF gain down in the advanced settings.
A good place to start is between 7 and 15.

Of course in your case there might be other issues, like faulty hardware, but finding a good gain setting in the Satnog client is the toughest thing to get sorted for me so far.

EDIT. Are you using a bias T to inject power for the LNA or are you powering the LNA externally? Do you have access to a multi-meter?


#3

Hi thebaldgeek
Thanks for answering so fast, I don’t use Bias T powering the LNA externally from the Raspberry’s USB, I have also tested with a mobile charger. I will test the values from 7 to 15 because I remember that I have not dropped more than 28. I will start testing and inform you.
Thanks again.


#4

I use LNAs with external power mounted at the QFH antennas. rtl-sdrs V3 set with NO rf gain. no If gain No BB gain and NO PPM. This feeds via 2 separate coax into my 568 and 724 ground stations. Look at those results.
I found adding LNA at sdr end of coax only causes problems. However I use a home made VHT UHF combiner (or splitter used backwards) plus home made low gain (10db) preamp to feed a combined VHF/UHF signal to my separate SDRPlay receiver. This has been working successfully for 3 months.
My 2 cents worth.
73 bob vk2byf


#5

Forgot to mention Commercial dual band VHF/UHF have perceived gain only because they have a very low angle of radiation because most amateurs are on the ground. So they are only good for very low satellite passes which puts the about 2000km away.
I too learned this the hard way which is the reason I built several QFH antennas, initially because I wanted to capture Weather maps from NOAA satellites. My ground stations both use home brew QFH antennas. I am particularly surprised how well the UHF one works considering it’s small size.


#6

Looking at your observations, I see that you haven’t got the RF gain set (‘null’). Digging through the satnogs-client and gr-satnogs code, it looks like when the RF gain values are not set, gr-satnogs uses the defaults, which for the rtlsdr are here: https://gitlab.com/librespacefoundation/satnogs/gr-satnogs/blob/master/python/hw_settings.py#L55
You can see where this is used here: https://gitlab.com/librespacefoundation/satnogs/gr-satnogs/blob/master/python/hw_settings.py#L96

So, your station is actually running 32.8 dB of RF gain on the RTLSDR. I suspect if you actually set it to 0 you wouldn’t be seeing much at all.


#7

Yes I know, I used to experiment with different values but then when I got my genuine V3 SDRs I set everything to null because that’s the way everyone else had it and it worked for them so why fix it if it ain’t broken. I’m assuming/hoping this way it’s using hardware AGC. Don’t really care. It works. Next project?

Cheers Bob vk2byf


#8

Well… based on the flow-graphs (i.e. here https://gitlab.com/librespacefoundation/satnogs/gr-satnogs/blob/master/apps/flowgraphs/fsk_ax25.grc#L1031 ), the gain control is set to False = Manual, and will hence be set to that default of 32.8 dB.

From my experience the hardware AGC in the RTLSDRs is not particularly good for our purposes. Better to set a manual gain that results in a slight noise floor rise when your preamplifier is enabled, which indicates you are now externally noise limited.


#9

WOW all of that code just to decode fsk_ax25. In python I suppose?
I have build GNUradio projects before but using available blocks so you will have to be more specific and mention a program line at least.

I have mentioned many times I’m strictly hardware. I’m not a programmer, never will be. I know just enough to be dangerous.

I’m tempted to change RF gain to 32.8 or whatever and change AGC setting but until I know exactly what I’m doing I’m reluctant to fiddle in case I stuff it up. I have roughly 15-20db of gain at the antenna.

Many observations are running on my two humble stations.

When I first started, I ran many observations at various gain setting and never found a better result than what I’m getting now. The main object then, was to figure out the exact local oscillator offset of my 3 fake rtl-sdr V3s.Yes I got stung 3 times but I got my money back from fleebay. I have 3 fair dinkum V3s now.

rtl_test -p got me close but I wanted it spot on.

All good fun. Now I’m looking for my next project. I have a working AZ EL rotator made 2 old TV rotators and an Arduino Leonardo. Might perfect that and bring it on line with 2 crossed VHF UHF Yagies.

Cheers Bob vk2byf