Using a splitter with the Outernet patch antenna and LNA

I didn’t really expect to be able to split the output from the patch antenna/LNA to two RTL-SDR’s so that I could try my hand at monitoring ACARS activity from the satellite on my laptop using JAERO. I had been occasionally moving the antenna/LNA setup back and forth between the two dongles, but that was becoming a nuisance.

However to my surprise, the splitter has not resulted in a significant loss of signal - the Outernet signal is maintaining a SNR of between 5.5 - 6.5 dB. The RSSI has dropped a few points to a level of around -120 but Packet Failure is zero and Signal State is a constant 4.

The RTL-SDR.com v3 dongle continues to power the LNA and I added a DC block to the coax that feeds the second dongle as a safety measure.

The splitter is a good quality BAMF, 5-2300 MHz, 2-way, power passing one I bought on Amazon.

Richard
KE7KRF

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Hi Richard,

Pretty cool! Can you share us a picture of your setup?

Once I was playing with a self made resistive splitter, that eaten up the SNR a bit, around 1…1.5dB loss (my chain was: Antenna -> LNA -> Splitter -> SDR1 + SDR2)

Thanks

Hi Zoltan,

I’ll try to photograph or provide a diagram of my setup in the next day or two. It sounds very similar to what you did. But you created your own splitter - I took the easier way by purchasing one.

The DC block may be unnecessary but I had one anyway so it won’t hurt.

I don’t understand why the RSSI dropped some on the Outernet signal, while the SNR stayed almost constant - I am not logging the levels, but estimate it is less than 1 dB loss. Also packet loss still close to zero – latest figure is zero lost from 15151 packets.

I am also having good reception of aero data on the L-band using JAERO software. Have monitored ADSB for a couple of years but there is some interesting and different information on the L-Band feeds.

Richard
KE7KRF

Hmmm… RSSI drop while same SNR can mean that the SDR input is close to saturation. Unfortunately the gain cannot controlled now to check this…

My splitter made of 3 SMA connectors and 3 resistors:

Zoltan,

Thanks for the photo of your splitter and thoughts on the RSSI issue. Finally made a quick sketch of my setup - too difficult to photograph so sorry if it is not too readable.

Forgot to specify in the drawing that from splitter to other RTL-SDR/ laptop is less than 2 meters of coax.

Sometime could you make a quick diagram with resistor values of your setup. I would like to try this approach. Will your splitter pass DC power?

quick question
would having two patch antennas connected to one SDR via such a device improve reception ?

@Zoltan - Seeing the RSSI drop but the SNR remain about the same is not unexpected and does not imply the SDR input is close to saturation. What it does imply is that the LNA is doing its job by boosting the signal and not adding much noise. Normally the SNR is (more or less) set by the first amplification stage - the noise received by the antenna, in the antenna to LNA cable and the LNA are significant, but after the signal is boosted in the LNA noise added downstream no longer has a significant effect on SNR (unless that downstream noise is large which is unusual in a properly designed system). Splitting the signal after the LNA reduces the signal level (so reduces RSSI) but it also reduces the noise level. Since the reduction in signal and noise is about equal in the splitter the SNR does not change much.

@dans34 - Using two patch antennas could help or hurt. If the cable lengths are matched the signals from the two antennas will be in phase and add to each other. However if the cable lengths are mismatched the signals will be out of phase and subtract from each other. Also, if the signals are in phase the combined antenna will have a different antenna pattern than a single patch, making pointing in the correct direction more important.

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using this start topology, right now 1 input 2 output configuration so 16.6 ohm each 3 resistor.
http://www.changpuak.ch/electronics/resistive_power_split.php

There are better RF splitters out there but this hard to beat when you need to make once quickly at the lab.