FlightAware Discussions

Two Antennas/SDRs Feeding FA

Hello Experts,

In my current setup, I have two FA antennas mounted outdoors to help deal with some ‘blind spots’ that my environment lends me. I used to run two separate instances on FA, but as time went by I decided to drop my secondary and just run my main antenna to feed the site. My ultimate goal is to utilize both antennas to feed FA under one instance with combined data, but even with searching the forum and the internet in general, I’m always left just short of being able to actually make that happen. I usually search and search until I eventually figure it out, but this one has me at a loss. I’m hoping you guys can lend me a helping hand.

My current setup:
1 Raspberry Pi 2 running FA-Image
1 Pro Stick > FA Filter > FA Antenna

My ‘spare’ hardware:
1 Raspberry Pi 2
1 NooElec RTL-SDR
1 FA Antenna
1 powered USB hub

What I’d like to achieve:
Existing FA Pi with two SDRs via powered USB hub, reporting combined data from both antennas to my one existing instance

From what I’ve read, I need to run two instances of Dump1090 on the Pi, each with different ports and then merge those with a ‘hub’ instance of Dump1090 which is on the port that FA monitors, but that’s where I get lost. I’m hoping for a guiding hand that will help me see the light (and maybe it could be a guide/sticky for those others who are scratching their heads as I have been for almost a year). Any help is greatly appreciated!

Pick one, use it as your primary site (probably easier to just use the one already running) and use the other one to just feed data to it. Pretty easily done with netcat and there are a bunch of posts on how to do it.

You can combine output of two antennas, but you will need a phasing harness.
The function of phasing harness is to make the two signals exactly in phase with each other, so that the signals add up. The more the two signals are out of phase, less is their combined output, and if phase difference is incresed, a point is reached where both signals are exactly opposite in phase (180 degrees or 1/2 wavelength). At this point the signals cancel each other and combined output is almost zero.

To achieve propper phasing:
(1) The Coax cable from each antenna to the point of combining should be from same reel/coil of coax so that their Velocity of Propagation (represented by Velocity Factor) is same.
(2) Length of Coax from each antenna to the point of combining should be exactly the same.

Even with all above, some trimming of length of coax is to be done to get a good match.

It is pretty tricky like tuning an antenna & reducing SWR.

As far as I know, MLAT will not work if output of one receiver is fed to another receiver.

Thanks for the reply! Unfortunately the posts on how to netcat is where I get lost. My goal is to use the existing feed as the ‘main’ and utilize the secondary to supplement. I feel like the steps before netcat is what I’m missing…

Honored to have you respond sir! To throw a wrench in the mix, I have my main feed served directly to the Pro stick via LMR-400 and the secondary feed is via Coax. Is there any way to achieve this with two SDRs?

As you are in the U.S., why not run dump978 for UAT on the second dongle. I run it on two of my RPIs but don’t see a lot of traffic at the moment.

It takes a few minutes to set up with Joe’s scripts ads-b-flight-tracking-f21/ads-b-receiver-project-setup-scripts-t36532.html

I believe that two dongles on 1090Mhz require two versions of dump0190 and two versions of piaware. I have not seen anyone do it but OBJ has referred to the possibility a number of times.

You can also use the second RPI to test code, cables, antenna and dongles. I have tested Nooelec and rtl-sdr dongles, an airspy(and soon airspy mini), FA Pro stick, FA Amp, FA Antennas, DPD Antenna, Nevis/Hab amp/filters etc. I am waiting on a miniADSB, cavity filter (due soon) and Mode S beast(due at the end of the month).

I did something similar using ModesDeco2 (RTL1) > Dump1090 (RTL2) > PiAware. You can also use ModeSMixer if you want to run two instances of Dump1090 or combine the feed of two Rpi’s and then feed PiAware with the combined feed.

Does MLAT work with combined feed of 2 receivers sent to Flightaware? It did not work with me.

No, you need to send each feed separately (you can combine them if you want for your own use but don’t point piaware at the combined feed)

Is it because of data duplication?

It is because, for mlat, the two dongles will have different clock characteristics and need to be processed separately. If you combine the feeds, the mlat server cannot tell which dongle each message came from.

Would the same be true if you fed the dongles with an external frequency reference instead of using the on board crystal, or does the usb transfer also interfere with the timing?

A common clock source would only eliminate the need for one half of the clock model (the frequency adjustment); you still need to somehow find the offset between the two receivers, the raw timestamps won’t be interchangeable.

You could probably combine two GPS synchronised feeds.

If we combine two antennas before feeding their rf output to the receiver, there will be some difference in time of arrival of same signal at two antennas.

The difference will be equal to spacing between antennas divided by speed of radiowaves.

If two antennas are 100 ft (30 meters) apart:
Spacing between antennas = 30 meters
Speed of radio waves = 300,000,000 m/sec
Difference in time of arrival of same signal at two antennas = 30 m / 300,000,000 m/s = 1/10,000,000 seconds = 0.1 micro second

I dont think such a small difference will affect MLAT.

Sure, if you only have a single receiver and the antennas are relatively close then mlat is fine with that. Whether you actually improve reception by combining antennas is a different question…

So, does two antennas approximately 30’ apart to cover blind areas (namely trees), not lend itself to some sort of merged feed? It’s sounding like maybe too much trouble than it’s worth?

How tall are the trees? Would it be simpler to put up a higher mast?

I’d love to place a higher mast, but I’m pushing it with my install height already (maybe in my next home)…trees are significantly higher unfortunately. When I ran two separate feeders with antennas around 30’ apart, I received unique hits on both since they were seeing around the shadows of the trees. My goal is to provide a better picture of flights without running two separate stations.

Where I get tripped up is the whole business of running two instances of Dump on one Pi. Like I said, I have two Pis available, so maybe one Dump per Pi and merge it somehow? Maybe it’s time to just hang up the dream…

Would it help if I take photos to describe my entire setup?