DAB: Unique Software Qirx for SDR – smart, Feature-rich and free (2/2)
In this second part about DAB/QIRX, I will deal with anaylzing some results of QIRX’ log.
QIRX software provides several tools and data for DAB reception which it stores in a file called TII logger. TII stands for Transmitter Identification Information. Most important of these data are:
- Time of reception
- Ensemble ID – identification of the DAB-VHF channel received
- Signal-to-Noise ratio, or SNR, of the whole 1.536MHz wide VHF channel. Maximum values here are about 34dB from locals. Audio can be expected from about 9dB, reliable decoding of metadata from around 7dB
- Main ID and Sub ID of the physical transmitter’s location
- Strength – the average of the amplitudes (magnitudes) of the TII carriers of each transmitter at that moment. The strongest carrier within an ensemble gets value “1”, the other carriers a number from 0 to 1 in respect to their relative magnitude, compared to the strongest carrier. Scale is linear, not logarithmic.
For mobile use, also GPS data in 3D are stored, extracted from an NMEA stream, provided by e.g., an external GPS mouse.
There are two principal methods of collecting data:
- Scanning the whole DAB-band with all ensembles or scanning a couple of ensembles, as set in the Options’ tab, see Figure 2. This is done to get an overlook over all or many ensembles.
- Scanning of just one ensemble, mostly to scrutinize propagation from the physical transmitter’s locations – Figure 3.
For scanning, the position of the Threshold slider is important. This can be considered as “kind of a squelch”. It sets the threshold where an ensemble/service is logged. You can control this feature via the window “TII Carriers”. A high threshold results in reliably logging of the strongest station(s). A low threshold will save also weak(er) signals but may be prone to false positive logs which have to been checked/erased manually.
Scan the whole Band
A scan of the whole band with a high threshold (here 0.54) resulted in the ensembles of Figure 1. Reception has been done from a fixed location with a largely vertical-polarized discone antenna at a height of about 50m near Hannover, in the lowlands of Northern Germany. The radio horizon is about 30km, following the equation given by Armbrüster/Grünberger: Elektromagnetische Wellen im Hochfrequenzbereich, München/Heidelberg [Siemens], 1978, p.48. Their factor of 4.1 is a bit higher than other values also found, ranging around 3.6. Receiver is an SDRPlay RSP2.
Figure 4 shows the SNRs of three ensembles, transmitted by the local Telemax tower at 15.8km with antennas at a maximum height of about 340m above sea level, or ASL. This results in a radio horizon of 75km.
Both 10kW signals of ensemble 5C and 5D show a more or less similar SNRs, but at different medians of 27.0dB [ensemble 5C], 31.3dB [5D] and 27.1dB [7A], respectively. With (nearly) the same power and the same horizontal polarization – matching my vertical Discone antenna -, with 5D leading the pack by a whopping 4dB, or factor 2.5, presumably using another antenna pattern at the transmitters’ site. What puzzles me more is that the variance of ensemble 7A with 1.56 is more then double as high as with the other signal (0.62 and 0.70).
The next diagram (Figure 5) shows the SNR from ensemble 11C, transmitted from Brocken mountain. With a height of 1141m, it is also virtually line-of-sight. There we see a much lower SNR, due to the fivefold distance, plus the transmitter’s power of being only a fourth that of Hannover Telemax. With 10.2dB, the median SNR is barely above the reliable threshold of around 10dB to provide audio at all. Showing a variance of 0.9, it is prone to sink under this vital level – returning no audio then. The three bigger dips largely coincide with local sunrise, noon and sunset. Further studies are needed to get a clue on that.
The last diagram of this series, Figure 6, shows a splash of DX: From my location, the transmitter “Eggegebirge/Lichtenauer Kreuz” only provides marginal reception – with a median SNR of 8.6dB and a variance of 0.3 only rarely jumping over the threshold of 10dB. Sometimes, even metadata are lost, resulting in a somewhat thinned-out appearance of the diagram.
If you compare the diagrams from Brocken above and from Eggegebirge below, you may see some similarity in SNR over time with also pronounced dips around sunrise, noon and sunset.
Scanning one Ensemble
In a second step, I scanned just one ensemble for 24 hours, namely 9B “NDR NDS LG” on 204.640MHz with a choice of six stations – some easy, but e.g. Stade a bit challenging. Figure 7 shows the locations and some results, from a whopping number of 276’092 logs. For this, “Threshold” had been set to the lowest possible value, combining highest sensitivity with a maximum of false hits (here: nearly 30%) to be sorted out later – which of course had been already done in this example.
To get the performance of each transmitter’s locations within one ensemble, you cannot use the SNR values, as they refer to the strongest station within the ensemble: Visselhövede in this case. Hence, I had to use column “Strength” of the TII log, running from “1” for the biggest signal in the ensemble to “0” on a linear scale. Here, the smart guys of UKW/TV Arbeitskreis e.V. have invested much work in identifying the TIIs. If you match the Main/Sub Id of your TII log with their free publications, you can assign the IDs to their locations.
This has been done for Figure 8, sorted by distances of the transmitters. The Bispingen/Egestorf (74.2km) transmitter is running only 2kW, hence its strength is weaker and more patchy than e.g. 10kW transmitter Dannenberg/Zernien, despite its distance of 91.2km. Most prominent in the diagram of this transmitter, you see two peaks between 18:00 and 00:00 UTC. They occur – each time-shifted and weaker – also in the diagrams from Egestorf, Lüneburg and Rosengarten plus, much weaker, Visselhövede. Source of these peaks almost surely is a “moving reflector”, being more an airplane than an atmospheric phenomenon, enhancing reception currently. Websites like Flightradar24 with their playback function will help to find some suspects.
Finally, an alternative look at strengths. In Figure 9, I combined the strengths of just three transmitters, now having set a logarithmic vertical scale, rather than a linear scale to emphasize the weaker signals.
Some Notes on Propagation
Last but no least, I like to add some notes on propagation. In the DAB frequency range, of around 170 to 255 MHz, propagation largely follows “line of sight”, primarily controlled by the height of transmitters’s and receiver’s antenna – plus power of the transmitter and sensitivity of the receiver. Antenna polarization also plays a role – the polarization of the receiver’s antenna must match that of the transmitter’s antenna to avoid losses by a mismatch. Bear in mind that many transmitter’s antennas may have a non-omni-directional diagram.
This general propagation can be enhanced or degraded by atmospheric phenomenons, high or low pressure/temperature; by rain and fog, by aircraft scatter and other factors.
The SNR of an ensemble is mostly as better as the signal is stronger. There is an exception: if the same ensemble is received by two transmitters at a relative distance of more than about 75km, the “Guard Interval” is too short to sort them out. Result then is a reduced SNR at a high signal level. However, I never faced this situation.
Clem dropped my attention also to another most valuable tool, provided by fmscan.org. They maintain detailed databases also on DAB transmitters, their antennas, powers, ensembles etc., and a web service which will draw circles of coverage onto a map. This is a cool and free tool, you must not miss – see Figure 10.
The above mentioned tool does not take into account topographic data which may be important to calculate the coverage in mountainous regions. Here Nautel, a Canadian producer of transmitters, provides a free webtool after registration, see Figure 11.