FlightAware Discussions

How do I allow two set-ups to properly share an antenna?

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.

We have several of these here at FlightAware to test multiple feeders at once on the same signal.


Make sure you also get terminators to put on the unused ports.

Some rules for splitters/combiners:

(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.

Looking at satellite gear, Amazon has the http://www.amazon.com/Skywalker-Splitter-Off-air-Satellite-SKY23302D/dp/B008UD4LRM splitter for $7.19 with free shipping – it covers 5 to 2300 MHz, which is just fine.

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!

bob k6rtm

Thank you for the information.

I found one of these kickin’ around work and I think I’ll give it a try…

Good solid construction with “N” connectors – should work a treat! Well balanced, good isolation, and it can handle 10 watts if you’re using it to combine signal sources (spec sheet at http://fxr.com/products/power-splitters/~/media/Microlab/Datasheets/Dx64FN.ashx).

bob k6rtm

Bob / k6rtm,
Do you think that the FXR unit will provide some filtration below 800mhz? Sort of the same idea as the satellite TV diplexers?

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.


bob k6rtm

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?

And, can you point me to the software?

Much thanks in advance, Tom

rtl-power is part of the rtlsdr suite of tools you probably installed on your Pi as part of bringing up PiAware.

rtl-power is better described at http://kmkeen.com/rtl-power/

I’d look at the spectrum from 400 MHz to 1300 MHz – that should cover broadcast television and some other commercial stuff. Compare sweeps with and without the splitter in line.

bob k6rtm

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?


Newby question, but why would one want two receivers connected to one antenna?

Wouldn’t they just report the same thing?

It’s handy for comparing different configurations / different receivers / different software.

Or if you wanted to do something like handle 978MHz and 1090MHz at the same time.

.CSV = Comma Separated Values

Excel can import that file type and parse it into cells using the Text Import Wizard.

Yes, that’s what I did and I was able to graph the scan results.
Thank you, Tom

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.


If you make your own DIY colinear antenna out of RG-6 coax you can do this…

or similar


GECEN A04-20-20dB In-Line Amplifier

I would recommend a 12-15vdc 1.0a switching wall wart instead of the mag wart shown with injector.

You can get injectors on ebay as well.

You would need a short rg6 cable 6" to connect wall wart to the injector.

Also you need F connector to MCX for the ADS-B receiver.

Also there are 4 / 8 way splitters.

One more thing, if your willing to wait, 10-14 days or so, these work ok as well…

DVB-T SDR+DAB+FM HDTV TV Tuner Receiver Stick RTL2832U+R820T2

Anything from Noo takes 7-10 days anyway fur twice the price and going up…!!

edit to change link to different in-line amp.

This setup will double your stats at least…less then $20 bucks…


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…

Antenna—DC/Blocker—Amp—Power/Inserter—Splitter—Dongle #1 & 2

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.

splitter spec sheet - http://fxr.com/products/power-splitters/~/media/Microlab/Datasheets/Dx64FN.ashx


OK, after a week of testing the two system through the splitter on the same antenna, here are my observations.

Dump1090-Mutability produces consistently 3% more position reports over the FA PiAware SD image. In my specific case this translates into about 1500 more position reports per day.

I’ve now swapped dongles and splitter ports between the two system to ensure that any difference between those hasn’t slued the results.