ADSB receiver differences

Do the same thing on an airspy/ADSBspy setup and a rspduo/dump1090 combination, and yes there are differences in performance:

difference airspy and rspduo

The airspy result is always within the rspduo result, the latter is a newer receiver, I got the airspy in 2015, so it is probably it is an early release, the rspduo is two months old. The airspy software is somewhat easier, it simply always works, and with the rspduo/dump1090 I oftentimes need to restart it until the ADSB results become visible in the virtual radar server.

The difference is range is approximately 50 km in range at 200 km distance, translated into dBs using the path loss equation I guess this is 2.3 dB in the receiver sensitivity favoring the rspduo. Receiver sensitivity is actually not really tested because we are using different decoders.

Last update: 7-Apr-2019 8:32


ADSB antennas

With the new mast I can easily test different ADSB antennas. I already reported a first test with a low gain antenna in this article which looks like this:

lowgain antenna
Figure 1: the low gain antenna with 0 dB gain

With the mast the situation is slightly different, first because there is more coax cable between the receiver and the antenna, it is Aircell 7 cable which has a loss of 0.2 dB per meter at 1090 MHz. So with 50 meter that is 10 dBs going up in thin smoke, which is really a pity but unavoidable if you haven’t got the LNA (yet) to compensate the cable loss. The easier way out is to build a high gain (9 dB) collinear antenna like explained here. I added a 1/4 lambda sleeve balun and a 1/4 lambda whip to the tip, in the end it looks like this:

highgain antenna
Figure 2: High gain antenna with 9 dB gain (to calculate via 10 log10(n) where n is the number of elements in the collinear antenna (here n=8)

The high gain antenna is called high gain because of its expected radiation pattern which is more pancake like than torus or sphere like with the low gain antenna, so it favors more the signals at low elevation.

rspduo lowgain_03apr2019_a
Figure 3: Low gain performance, so you see aircraft data up to 70km
highgain 6-apr-2019 ADSB
Figure 4: High gain performance, with reduced coax cable length (I managed to bypass 20m) You get to see aircraft data up to 250 km

Figure 4 shows the best result that I got so far, namely, with the high gain antenna and 20 meter reduction in length of coax cable for which I had to drill a new hole in the ceiling. We improved the range by almost a factor 3. To summarize:

  • Low gain antenna, 50m cable, max range is about 70 km
  • High gain antenna, 50m cable, max range is roughly 100 km
  • High gain antenna, 30m cable, max range is about 250 km.

Since we know that the loss is caused by Aircell 7 cable we also know what the difference is between the last 2 results, it is 4dB which is the same as a factor 2.5 in the free space loss equation where the loss = (4 pi d / lambda)^2 with d denoting the path length and lambda the wavelength.

For the 30 remaining meters we should get 6 dB of damping, and this would be a factor 4 if it were to improve the path loss. Curvature of the earth and the height of the involved antennas constrain the maximum range to something like 430 km, so the factor 4 will never be obtained.

Maybe another suggestion is to not use coax cable at all and place the SDR receiver very close to the antenna, and then feed it with USB cable or turn it into a wireless device with a raspberry PI and a SDR. This is a personal choice that people have to make, I want to be able to use my SDRs for various experiments, so this is not an option.

There are various things to learn from this experiment:

  • 1 GHz is a frequency where cable losses become a serious factor in the design
  • The antenna gain is relatively easy to improve
  • The antenna pattern shows that we are easier affected by obstruction, for instance, the south west sector has more obstruction not because the antenna is below the rooftop heights (it is above it), but according to me due to the city and the port area in that domain.

Last update: 6-Apr-2019

Increase stiffness

A mast of 9 meter of aluminum is like spaghetti, it bends typically by a meter at the end just under its own load. So normally you add guy wires to stabilize the mast, but this is a bit of a pain when it is behind the house. So I added two stiffening bars in the middle and spanned wires from the top to the hoist point. This takes away the bending moment of the aluminum and translates it in to forces  over the added wires. I will first test this with PVC tubes and rope, which is all available and cheap. If it needs to be stiffer then I will replace it by steel cable and aluminum crossbars.

Note the crossbars
Bottom view
Top view, the matchbox is for sealing the polyester rope
In the end celebrate, drink some wine and join your wife in the local pub

Last update 31-Mar-2019 at 9:20 AM

Tiltable mast

In June 2017 I had two brackets and a stack of four fiberglass elements from the army dump. I wanted to have something that could tilt and came up with this design:


In total there are four brackets, 1 is the hinge and three brackets to attach the support mast to the wall. The support mast is the part that needs to be strong, it is 5 mm thick aluminum with a length of 2 meter. The mast consists of 2 tubes of 6 meter that fit together. The dimensions are in the drawing. Without guywires and without any wind load other than the mast itself the structure should survive a storm (they can go up to 120 km/h). So this is what it became:

Tilted configuration, so I can add easily something to the mast whereas I could not when I would have installed it on the top of the roof, the intention is to experiment with different antennas
Two U shaped aluminium brackets form the hinge, MFJ makes them but you could also make them yourself, there is a metal shop in our city, a big help with these projects
Above the kitchen block there are 8 chemical anchors holding three brackets, also there is a winch. You can lift the mast yourself but you soon start to appreciate the presence of a winch and a reduction in force (note the V shape of the tow on the top wall bracket, it reduces the load by a factor 2). The polyester rope should survive the 100 kg force easily (it could go up to 300 kg), but I think I will replace it by steel cable.
View from the other direction, this would also be the parking position if we ever get into a gale
And a separate PVC tube to keep the balun and the ladder line away from the mast, another detail (not shown) is the grounding, I installed thick copper cable to the bottom of the mast and a lightning arrester.
In its upright position it looks like this.
The thickest copper cable I could get was promoted to grounding line. Directly get the clamps at the hamradio market because they are hard to find
And on we go via the rain pipe to eventually a two meter copper tube that you hammer down under a tile in the garden, hopefully it is enough to dissipate the energy if we are ever hit. (Note: if you want to claim via an insurance then this should be certified, this one isn’t).

Last update: 27-Mar-2019 at 8:24