Hardware & Software

Hardware

Antenna


Position signals sent on the 868MHz band are weak: they are only 10mW and the transmitting antennas are simple 1/4 wave sticks installed "somewhere" in the glider, that means their placement is hardly ever optimal for achieving best range. Reception antenna, that picks up the signal is the most important and often neglected element in a radio system. Thus think well about this element: make it to have best gain possible with given space and budget and place it, where it can see the complete horizon with no obstacles in the way.

Receiving ADS-B is very popular these days. To compare: our signals are 1000 to 10000 (30-40dB) weaker in power, we gain a bit on the data rate which is 20 times slower (50kbps) than ADS-B (1Mbps) but in total we are still a factor of 100-1000 worse. Gliders fly lower than airliners thus antenna height and obstacles count much more.

Like for ADS-B a vertical collinear is a good choice for our antenna, here are some pointers:

Search on google for keywords like "collinear antenna coax homemade" to find more information.

Photo of a foil cone here. 10mm section of coax outer stripped back at base of cone location and 0.1mm craft cu foil cone soldered to braid. Remaining (short) coax is then terminated in a twist on F-plug, F to MCX adaptor.

Alternatively to building antennas on our own in May 2014 we organized the first bulk order of 9dB collinear antennas from China. These antennas proof to perform very well and come for a very reasonable price!
If you wish to order antennas from the same source please see website and e-mail directly to:

Mr.
Sallin Lu
Huahong Communication (HK) Co., Ltd.

ask for 868.3MHz base antenna, 9dB, fiberglass
M.O.Q. is 5 antennas.

Active Diapason Antenna

If you have a long run of coaxial cable with high losses, then , you may consider this antenna, designed by radio amateurs F1OPA and F5ANN, and manufactured with professional standards.
This antenna is an adaptation to FLARM frequency of their widely used ADS-B active antenna ( more than 500 antennas installed world wide).
This antenna support up to 18 dB of cable losses, without degradation of performances, and the low Noise Figure increase by more than 2dB the sensitivity of SDR dongle.
This antenna is sold exclusively by Jetvision in Germany

Preamps


To counteract cable losses it is advisable to use a mast head preamp as close to the antenna as possible. Following preamps are available:
- The Terra AB010 preamp works well for OGN stations. It features a weather-proof enclosure, which can be mounted right below the antenna. It required F-connectors. Available via EBay.
- Uputronics sells a filter + preamp combination. The case is not water proof.

Cabling


You may need some length of coax cable to bring the signal down from the antenna. Note that at 868MHz cable introduce significant attenuation: for example thin RG174 attenuates 1dB/1metre, thicker H-155, RF 5 or Aircell-5 0.2dB/metre. Plan your coax to be as short as possible, ideally put the receiver right at the antenna bottom.

  • Coaxial cables data may help you choose the correct cable (e.g. with acceptable attenuation) for your installation.

Filters


Depending on your RF environment your OGN-station may receive lots of unwanted signals coming from radio amateur stations, TV broadcast towers, TETRA stations, ….
To reduce the effect of these unwanted signals and to improve your OGN reception you can use a filter.
Following filters are available:
- Adam's SAW filter
- CBP-840
- Wevercomm WVC-881.5B-25M04. This is a high quality cavity filter with very low insertion losses.

SDR receiver


To make the radio signal readable to the computer and available for digital processing we use cheap SDR radios made from DVB-T USB dongles. They cost about $10 and are easily available on e-bay or from other sources. From our experiences the dongles based on the R820T tuner work best on 868MHz but for example EDMC receiver is running on a E4000 dongle and has a great range !
To find quickest your dongle you may type "R802T" into e-bay or a search engine. If you have already got such dongles give them a try !

CPU Board


To Run the OGN receiver you'll need some kind of computer running Linux. This can be a regular desktop PC or server, but by far the most popular option is to use a small Single Board Computer (SBC) like the popular Raspberry PI.
These small devices are very energy efficient (using < 5 W), and can be placed close to the antenna, minimising cable losses. They are cheap and easy to install, even without any experience.

The current receiver acquires 1MHz of spectrum around 868.3MHz and processes this searching for radio packets on 868.2MHz and 868.4MHz. The amount of data to be processed is about 2MBytes/sec (RF data is digitized to 8-bit I/Q values). To process this data, a multi core processor board like the Raspberry Pi 2 is recommended.
Other brands of computer boards are also available, a comparison can be found in the links below.
For desktop PCs, performance is not an issue. A Pentium 4 or comparable system should easily be powerful enough to run the OGN receiver among other tasks.

Photos


  • Photo gallery presenting pictures of selected "technical solutions". It may be useful and give you inspiration/ideas for your own installations. Because at the moment this gallery has has no option to be edited by the group members(to be fixed soon!), do not hesitate to send the photos you wish to be included to the group e-mail. Please, always remember to give some comment to each. NEW!!: You can now link your new installation's photos together with your OGN receiver's description at List of Receivers page.

Prototyping


Software installation instructions


Additional configuration

OGN infrastructure

APRS server status:

OGN Client Apps

  • Live 2D Web display of Flarm equipped gliders and aircrafts. (Google Maps)

FLARM

OGN Tracker

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