I would like to have at least two different set-ups share the same antenna for testing purposes. How best is this accomplished? I presume there is some sort of splitter/combiner required. Has anyone built a home-made unit? If so, can you provide details?
Much thanks in advance, for any insight that can be provided, Tom.
(1) signal loss increases with the number of ports; it has to as you’re splitting up the signal. Therefore, use a splitter with as few ports as possible. For a high-performance splitter/combiner, cost also increases with number of ports.
(2) terminate unused ports properly (with a 50 or 75-ohm termination)
(3) splitters/combiners have an operating frequency range. Be sure what you pick works at your frequency. ADS-B is 1090 MHz, or 1.09 GHz.
(4) splitters/combiners (and other similar RF goodies) are designed for a characteristic impedance, 50 Ohms or 75 Ohms. 50 Ohms is commercial/R&D/industrial. 75 Ohms is cable TV and home satellite.
With those simple rules–
What you’re going to use depends on what kind of cabling you’re using. Good, low-loss 50 Ohm coaxial cable is expensive and the connectors have to be done correctly or you lose signal. On the other hand, a good 75 Ohm RG-6 cable used for satellite dishes is cheap and low loss.
The Mini-Circuits stuff is great. It’s also expensive. I use a ZX-10-2-20+ splitter to drive two SDRs from one antenna-amplifier chain. They’re good for 200 to 2000 MHz. I have a few of them. New, they’re $30 each. (the 4-port beastie mentioned above is around $100 new). eBay is a good source for used Mini Circuits gear.
You might find some splitter/combiners at larger home improvement centers, as well as places that deal with satellite gear. You shouldn’t be paying more than $10 for a 2 port splitter/combiner. Be sure it will work for ADS-B – I’d look for gear that says it will work to at least 2000 MHz (2 GHz).
Please post links and ask questions – we’re full of opinions around here!
Not sure, it might. You can find out, though, by using rtl-power to do spectrum sweeps with and without the splitter in the circuit (terminate the unused port, either with a proper 50 Ohm terminator, or connected to another SDR to provide the termination.
I think it’s a good idea to do a spectrum sweep using your chosen antenna and feedline to give you an idea of what you’re up against in the RF environment.
I agree on benchmarking with rtl-power, but on what platform? Is this utility something that I should install onto a Windows Notebook and plug the dongle into to do the testing? Or, can it be run from the Raspberry Pi?
OK, it does seem to be installed even on the PiAware SD card image from FA that I am using. I did a ‘sudo rtl_power -?’ and was presented with a help file showing the command line options. I think I’ll mount a flash drive for it to write its output file and give it a try.
I’ll let you know how it goes.
Thank you, Tom.
I presume that I must stop dump1090 from running while I perform the rtl-power tests? As my 1st attempt failed due to ‘usb_claim_interface error -6’.
OK, after killing dump1090 I did a 10 min. scan. Now I have a .CSV file, how do I make sense of the data? I tried to follow the heatmap.py method, but am lost. Is there a MSWindows tool to analyse the .CSV file?
OK, so I’m now testing two systems on the same antenna. One is running the FlightAware SD PiAware image and the other is running the PiAware addon to a Dump1090-Mutability install.
I plan on letting them run for a week to compare results. Preliminary stats are favouring the Dump1090-Mutability install. But after the initial week long test I’ll swap the dongles and/or splitter outputs to see if they affect the comparison.
Do not forget that introducing the splitter, the reception will be poorer than just with the one dongle. You will spoil the noise figure of the each dongle by the same number in dB that you loose in the splitter. Roughly for the same value the S/N will degrade comparing to the single dongle.
The spec sheet, see link below, on the splitter lists a 3db loss through the splitter and my testing with rtl_power proved this was the case. However there was no loss of coverage or reduction in positions reported from several weeks of testing before and after the splitter was added to the circuit. I contribute this to the filter affect of the splitter on frequencies below 800 MHz. Here’s my set-up…
I am researching and probably will add a filter between the antenna and amp at some point in the near future, to further reduce interference from cellular signals in the 900 MHz range near-by my location.