Connecting antenna to RTL-SDR modem

antennas

#1

Hi @elkos , @pierros

We have attached pictures of two possible layouts of connecting antenna to RTL-SDR.
Which one of this would be a better approach?
Keeping in mind the loss of signal strength and possible RF interference from the modem.

P.S.: We are using Helical Antenna v1

Regards,
Sachin.


#2

~1m of coax should not make a big difference in signal. That said, I prefer to use a USB extension cable for the RTLSDR just to relieve strain on the BBB.

A couple more things:


#3

Hello @cshields ,
Thanks for replying.
1)Regarding the termination issue,we had used this page :
http://www.kerrywong.com/2014/11/16/testing-an-rtl-sdr-spectrum-analyzer/ as a reference for the cabling,without checking in more detail.That apparatus is for frequency spectrum analysis using RTL-SDR and not for antenna connection.So,presumably 50ohm termination is not required for antenna connection ?
2)We have not considered any LNA so far.Your link is not opening.A search for LNA in Aliexpress shows this
http://www.aliexpress.com/wholesale?catId=0&initiative_id=AS_20160710121514&SearchText=lna+amplifier
Some of these LNA are specd for bandwidth of 0.1-2000MHz RF wideband amplifier gain 30dB low-noise amplifier LNA.
Another has been mentioned as:
433MHz Ultra-low noise LNA RF amplifier Moudles Gain 22 dB For Ham Radio receiver amp,which is close to the central frequency of 434MHz for the SatNOGS antenna.
Is it ok to use either of these ?

Thanks again !!
Thomas


#4

There is an impedance mismatch created by this connection and therefor no longer an optimum signal transfer from the antenna to the RTL-SDR and you’ll be losing valuable signal strength.


#5

@PE0SAT so what can be done to rectify this?


#6

For example have a look at the following URL: http://www.qsl.net/dk7zb/Stacking/stacking.htm

If you want to use the T-piece you have to make a impedance transformer. Here you can calculate a transformer that makes it possible to connect two 50 Ohm loads to the 50 Ohm antenna connection from the RTL-SDR. The transformer will replace the T-piece. Also keep in mind that this way of connecting (transformer of t-piece) will lower signals with -3dB.

regards, Jan - PE0SAT


#7

Take the T connection out of the picture. Connect the antenna straight to the RTLSDR or LNA. So it would look something like this:

Antenna - coax - SDR

Antenna - coax - LNA - coax - SDR

or if you have 2 different bands of antennas you could use a diplexer:

VHF Antenna - coax -
.....................- diplexer - coax - LNA or RTLSDR
UHF Antenna - coax -


#8

Hello,
I’ve just installed a LNA based on SPF5189 but I can’t get good results yet. How much gain did you use in “RF-gain” field of satnogs-setup?
Thanks a lot,
Alfredo


#9

Alfredo,

My gain is set to default but I’m using a usrpb200. Gain will vary depending on a lot of factors, but have you tried lowering it?


#10

Have in mind that the values for gains on rtl-sdr gets specific values, you can check them in this wiki page next to the RF_GAIN parameter.

Also you can try a more interactive way to find a good gain by checking with gqrx. Here is a wiki page that describes how you can connect gqrx with your satnogs setup. After connecting you can experiment and find which gain works better, and then pass the closest value in the supported gains into the satnogs-setup (don’t forget to hit apply after the changes :wink: ).