28 January 2013

SDR on a netbook

Since sdr# runs fine on the 2004 laptop (Centrino CPU), I wanted to test the RTLSDR hardware on the eeePC 901 with a dual-core Atom N270 processor. That is not an hyperfast processor, but the netbook is small and it has a solid-state drive (SSD, as opposed to "spinning" hard disk or higher capacity).

First test involved using ADSBsharp to upload data to the common hub. This piece of software runs the RTLSDR dongle at 2048 Msamples/sec, and the machine was coping with the load, delivering as high as 120 decoded packets per second.

Then I tried latest sdr# development release and, despite the additional load for the display, the eeePC 901 behaved exactly as the older laptop. At 250 ksamples/s (total bandwidth of 250 kHz) there is CPU to spare.

Sure, the screen is tiny, but for a portable operation the netbook is small and solid. Note: I forced the "Super performance" mode, which apparently consists in a 5% overclock. With a reduced sample rate the CPU can be slowed down (to be tested).

21 January 2013

New RTLSDR dongle has arrived

Simply amazing. The sensitivity difference between my first RTLSDR dongle based on E4000 and today's with R820T is about 3 times, in favour of the latter. Even with the stock DVB-T antenna I could pick up more signals than the E4000 and the external GP. Then I "sacrificed" the stock antenna to use the cable with MCX connector for building an adapter to the TV plug: with the GP antenna outside I could receive airplanes as far as 300km/160nm. The ADS-B software was showing an average of >50 signals per second, against less than 10 for E4000.

So, as many others have reported, the R820T stick is more sensitive above GHz than the E4000 counterpart. I also noticed a much stronger frequency drift in the first minutes of operation, which also result in a warmer RTLSDR dongle.

Two experiments will follow:
- retry 23 cm SWL'ing during the monthly activity event
- test the ADS-B reception in a location with a 270 degrees wide horizon

14 January 2013

ADS-B ground plane antenna

This is not rocket science: dipole and ground plane are the most basic antennas you can build. And they can be good performers too! Here is a picture of the 1090 MHz GP I assembled for my ADS-B reception tests.

It is built around a panel-mount BNC socket. Each of the three solid copper wires is about 6.6 cm long and there is a 10 kohm 1/4W carbon resistor across "hot" and "cold" points to discharge static electricity right at the source.

This GP performs equally well with and without resistor, so leave it in place. This antenna is omnidirectional, and depending on its horizon it can bring up signals as far as 100 km away (E4000 tuner, airplane above 10'000 m). In this screenshot my receiver is below the ISSxyz airplane:

07 January 2013

ADSB antenna

I am not a fan of aircraft monitoring, but their ADSB 1090 MHz transmissions represent a widely available and geographically distributed beacon for testing new antennas.

My antenna test-field is the usual balcony open to N-NE.

First I tried a biquad without reflector. This is somehow bidirectional in an "8" shape. The advantage of this antenna is the intrinsic short-circuit, which avoids the problem of static electricity build-up (and frying the RTLSDR dongle).

Then I read many people suggest a collinear antenna. Projects documented online do not agree on sizes of straight elements (13 vs 19 cm) and coils (2 vs 4 cm diameter), so I opted for a simpler ground-plane: one vertical element, and 2(4) radials. All 1/4th wavelength long = 300 / 1090 / 4 = 6.8 cm (a bit less, in practice). It is short, simple to tune and easy to handle. The 1090 MHz GP antenna can be made of 1mm dia. solid copper wire, even keeping the insulation. In order to discharge static I inserted a common 10kohm 1/4W carbon resistor across antenna terminals.

Performance? They receive signals from the same distance, the GP being slightly better (but I would need a side-by-side realtime comparison) probably due to the biquad needing some form of tuning, making it a bad match outside resonance window.

Conclusion. Unless you are absolutely sure in your interest in ADSB monitoring or you have access to proper instrumentation to measure antenna impedance above 1 GHz, go for the GP antenna. According to my observations, ADSB signals propagate in line-of-sight, so look for a good, open, position: I can draw my antenna electrical horizon by looking at what/where it can receive.
Use a low-loss coax (known quality SAT-TV coax is a good choice) and keep it short: this trick will easily save those couple of dBs so hard to achieve with a different antenna without tuning instrumentation.