Monthly Archives: April 2021

Raus aus dem DARC! Es ist eine Frage des Anstands!

Ham Spirit – aber manche scheißen drauf. Doch der Mehrheit der Funkamateure stinkt das zunehmend.
[Misericordia von Jörg Syrlin d.Ä., Ulmer Münster, um 1470. (c) 2014: Nils Schiffhauer]

Der Deutsche Amateur-Radio-Club e.V. (DARC) ist der größte Verein für Funkamateure in Deutschland. Vor und hinter den Kulissen sind einige seiner Funktionäre seit Jahrzehnten nicht zimperlich, mit allen Mitteln solche Funkamateure zu bekämpfen, die sich für einen fortschrittlichen Amateurfunk einsetzen. Zwar folgt der „Bundesverband“, wie er sich selbst nennt, dann in 90 von 100 Fällen irgendwann diesen „Abtrünnigen“. Aber der Hass der Funktionäre glüht weiter: Cancel Culture par excellence.

Wenn sie das alles doch wenigstens zum Wohle ihres Vereins unternähmen! Doch im Gegenteil: seit fast 30 Jahren verlieren sie kontinuierlich und massiv Mitglieder, und sie reihen einen teuren Skandal an den nächsten – worauf man sich ebenso verlassen kann wie auf das, was mancher für üble Nachreden zu halten geneigt sein könnte.

Unterstützt werden sie dabei von Anonymen und „Anonymen“, die die Drecksarbeit machen: Denunziationen bei Arbeitgebern und Behörden, Mordaufrufe und antisemitische Aufforderungen wie „Jagt ihn durch den Schornstein!“

Kein „Aufstand der Anständigen“

Dagegen hat sich trotz vielfacher Aufforderung bisher kein Vorstand, kein Amateurrat zu einem „Aufstand der Anständigen“ aufraffen können. Diese beredte Schweigen verstehen manche als Aufforderung (denn wie könnte man es auch anders verstehen?!), mit ihrem menschenverachtenden Tun munter fortzufahren. Dass sie daran nicht gehindert werden, fassen sie mit allem Recht als Ermutigung auf.

Denn: Wer schweigt, stimmt zu.
Und: Wer schweigt, macht sich zum Kumpanen.

Auch der im November 2019 mit hochfliegenden Reformplänen gestartete Vorstand tritt in die Fußstapfen solcher Ehrenmitglieder wie Dr. Horst Ellgering, DL9MH, und Steffen Schöppe, DL7ATE: Er weigert sich, diesem niederträchtigen Treiben Einhalt zu gebieten. Denn er weigert sich, ganz schlicht und einfach zu erklären:

  • „Für jene, die Mordaufrufe und Denunziationen als Mittel der Diskussion einsetzen, ist im DARC kein Platz!“

Und er weigert sich, diesem Satz umgehend Taten folgen zu lassen. Denn sonst wäre er ja ohne jede Wirksamkeit und würde eher als augenzwinkerndes Einverständnis betrachtet.

Bislang aber ist genau das: nicht passiert.

Das ist beschämend. Das ist unanständig. Das ist menschenverachtend.

Und genauso widerlich, wie die Beibehaltung von dem, was die Neonazis „Feindeslisten“ nennen.

Tretet aus – bis sich was ändert!

Gleichzeitig verlangt der DARC von jedem seiner Mitglieder Nibelungentreue noch zu seinen abwegigsten Vorstellungen. Sonst gibt ‘s Saures, sonst droht Rausschmiss. Mit Pauken und Trompeten. Und gelegentlich mit dem oben erwähnten Begleitkonzert.

Wer also als Mitglied des DARC nicht mit Mordaufrufen und Denunziationen einverstanden ist, wer einen offenen und fortschrittlichen Amateurfunk möchte und wer sich eine ebenso transparente wie effiziente Interessenvertretung wünscht, der hat bei diesem Stand der Dinge nur die Möglichkeit zu gehen. Und – ganz klar – dann wieder einzutreten, sobald sich etwas geändert haben sollte.

Übrigens: Schon jetzt werden “Kritiker” mit Amateurfunkgenehmigung seitens des DARC zu nur “sogenannten Funkamateuren” abqualifiziert, also zu Nicht-Funkamateuren. Wer so handelt, ist eine Schande für den Amateurfunk. Handelt er überdies so als Funktionär des DARC, sollte er eine Schande für diesen Verein sein [was er natürlich nicht ist, denn der gesamte Vorstand nickt einträcht-/-fältig dazu]. Und sollte der Verein, wie er anstrebt, in Zukunft tatsächlich über den Zugang zum Amateurfunk entscheiden, so wäre hinsichtlich des Zugangs zum Amateurfunk überhaupt der Willkür Tür und Tor geöffnet.

Daher:

Tretet aus dem DARC aus!
Nutzt diese „Abstimmung mit den Füßen“!
Nutzt sie umgehend: Es ist eine Frage des Anstandes!

Ergänzung (5. Mai 2021): Danke für die vielen zustimmenden und auch für die wenigen ablehnenden Reaktionen! Erstere wollen anonym bleiben, um nicht “der Rache der Funktionäre ausgesetzt zu werden”, wie einer schrieb; letztere posten ebenso feige wie anonym.
Manche fragen nach weiteren Hintergründen, die ich u.a. mit meiner Rede in Cottbus und meinem Interview in der Fachzeitschrift “Funktelegramm” geliefert habe – bitte dort nachlesen.
Das Kalkül der Handelnden ist so schlicht wie effizient: schon vor den Kulissen bewegen sie sich weit jenseits des Ham Spirit (etwa mit den unverhohlenen Drohungen eines ehemaligen DARC-Vorsitzenden und heutigen Ehrenmitgliedes in einem seiner ekelerregend-demagogischen “Offenen Briefe”), hinter den Kulissen aber sorgt genau das für die augenscheinlich gewünschten verrohten Reaktionen, wie sie dann vom Strafrecht erfasst werden – “An den Eiern aufhängen”, ist da nur eine dieser weitverbreiteten Aufforderungen, gegen die sich bislang kein DARC-Funktionär stellen mochte.
“Entgegenarbeiten” (Ian Kershaw) nennt sich dieses Konzept, das auf offensichtlich kalkulierte Scheinheiligkeit der Führung baut, den somit selbstorganisierenden Hass der Speichellecker vollkommen unwidersprochen in ihren Dienst zu stellen und durch Nicht-Sanktionierung allem Anschein nach zu billigen. Feixend und auf fetten Spesenbergen hockend, waschen die zumindest mittelbaren Verursacher dieser kriminellen Auswüchse des Vereinsfunks ihre Hände in Unschuld, während sie nützliche Idioten mit keinem Wort daran hindern, sich strafbar zu machen. Bis heute.

P.S. Weil obiges “bis heute” bei diesem oder jenem auf Skepsis stieß, hatte ich Mitte Juli nochmals bei jedem Mitglied des DARC-Vorstandes und jedem Mitglied des DARC-Amateurrates angeregt,

“dass man … etwa Denunziationen bei Arbeitgebern und Behörden als Mittel der Auseinandersetzung über die beste Zukunft des Amateurfunks ebenso für unvereinbar mit einer Mitgliedschaft im DARC erklärt, wie die implizite Billigung von Mordaufrufen in diesem Zusammenhang. Und es nicht nur bei dieser Erklärung belässt, sondern dieser auch Taten folgen lässt.

Bis heute (2.9.) hat sich jedoch kein einziger dieser DARC-Funktionäre von dieser Vorgehensweise distanziert. Weder hinter vorgehaltener Hand (wie etwa: “Ganz im Vertrauen: ich bin ja privat auch gegen Denunziationen und so – aber das etwa öffentlich sagen?!”), noch und schon gar nicht öffentlich. Somit entsteht der Eindruck, als ob der DARC dieses Instrument der Disziplinierung weiterhin nicht aus der Hand geben möchte.

Will man, so lange sich das nicht geändert hat, wirklich Mitglied in einem solchen Verein sein?

Decoding ADS-B with free QIRX software

QIRX’ dashboard, decoding ADS-B: in the middle you see spectrum and spectrogram (“waterfall”) of the ADS-B signals. The window at the bottom lists alls received aircraft with additional data, whereas the top window places them onto a map.

In the last two blog entries, I took a look at the DAB capabilities of free software QIRX by Clem Schmidt, DF9GI, from Frankfurt. It directly works with RTL-SDR, Airspy and RSP2 SDRs. I tried this very smart software from my location near Hannover/North Germany now also with ADS-B, mostly with my RSP2.

ADS-B stands for “Automatic Dependant Surveillance – Broadcast” and is an automatic service where aircraft continuously transmits several vital data on around 1.090MHz. Most important part of these data is the 2D location of the aircraft which it gets by GPS plus height by a baromatric altimeter. From this position data, many other data are derived, e.g. climbing/sinking or speed. If matched to databases, you will also see type of aircraft, flight number and many other data.

“The internet” provides many services showing the results of ADS-B and other data, collected from receivers all over the world, among them Flightradar24, OpenSky, FlightAware and AirNavRadarbox. They each provide many additional data, somtimes available at different schemes. Most provide free access to much of their data, with some more specific data behind their paywall. OpenSky as a scientific and non-profit organization offers billions of datasets for free, see Scientific Datasets. QIRX uses an OpenSky data base with about 650’000 entries.

Backbone of all these services is a net of ADS-B receivers, connected via the internet and curated by each company.

QIRX shows some capabilities of such a receiving station, using a proper antenna and a simple SDR. It decodes the I/Q stream of it. ADS-B is transmitted via pulse-position modulation, or ppm. The system is explained in ICAO Annex 10 Volume IV [free download].

With QIRX, you must set the sampling rate of you SDR to 200000[Hz], as other sampling rates won’t work, see screenshot below.

To decode ADS-B, you must set the sample rate for your SDR to 2000000Hz.

After that, and having started QIRX in ADS-B mode, decoding is done automatically. Release your seatbelts, and simply relax by viewing the activities above your head. Coverage largely depends on the “view” of you antenna and a few other factors like te sensitivity of your SDR and the attenuation of your cable connecting your antenna with your SDR. Some web services, thanks to anticipatory obedience/security reasons/data protection etc., do mute some “special” flights . This is not the case, of course, with this setup. QIRX always provides stable decoding at even low SNRs – great!

Last, but not least, please find below a comparison of FlightRadar24 and QIRX setup with Flight Number TK1554/THY6KG, Hannover->Istanbul, starting from Hannover Airport. One difference between both screenshots is that at my location (Burgdorf), I got the Airbus only after it had climbed to an atlitude of 200m or so, whereas the FR24 receivers are placed at positions allowing for tracking the aircraft from even the runway.

Starting from Hannover to Istanbul: the airbus on track around Hannover. Top window shows the flight via FlightRadar24 web service, and even from the runway. Bottom window shows it received with QIRX from Burgdorf (red point in the northeast).

Also small aircraft is equipped with transponders, but not necessarily with ADS-B transponders, broadcasting the position, derived from their GPS. These small aircraft may haveonly Mode-S transponders on board, transmitting identification, height and squawk (transponder code) as assigned by their responsible ATC, or Air Traffic Control.

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.

Figure 1: Tag cloud of all DAB ensembles, revealed by a 24h scan of the whole DAB frequency range at DK8OK’s location near Hannover/Norther Germany. The bigger, the better.

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.

Figure 2: Scanning the whole DAB band or just some ensemble(s). The results are automatically written into the TII logfile – here opened, in the background.
Figure 3: Scanning just one ensemble, here 5D. Under the TII tab, your first must start TII Recording for logging. After that, you may access this file by double-clicking the page icon at the right of this row.

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).

Figure 4: Three ensembles from the local transmitter deliver different SNRs, see text.

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.

Figure 5: The SNR from ensemble 11C, as transmitted via the Brocken mountain, sometimes drops under the vital threshold of 10dB; see text.

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.

Figure 6: Only marginal reception is provided by ensemble 11D, Radio fuer NRW, from Eggegebirge. It rarely jumps over an SNR of 10dB which is needed for audio.

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.

Figure 7: Visselhövede transmitter provided continuous reception with nearly 80’000 hit counts when having scanned ensemble 9B for24 hours. From this diagram you see the relation of hits, distance and transmitting power of all received six locations.

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.

Figure 8: Strength of all six transmitters, observed at ensemble 9B over 24 hours. Please observe each different linear scale!

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.

Figure 9: Signal strength of three transmitters from ensemble 9B, compared over 24 hours at a logarithmic scale. Compare this view to the linear diagram above to see here a much better resolution of weak signal strengths.

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.

Figure 10: A web service of fmscan.org provides a tool for drawing the coverage (“footprint”) of specified stations, locations, frequencies etc. onto a map. This has been done here for ensemble 9B of six transmitters. Receiver “Burgdorf” can be found som 20km north-east of Hannover, at the bottom of the maß in the middle. Coverage from Dannenberg/Zernien had been highlighted.

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.

Fgiure 11: Radio Coverage of DAB transmitter “Brocken”, taking into account terrain data.

DAB: Unique Software Qirx for SDR – smart, Feature-rich and free (1/2)

The dashboard of QIRX shows the power of this software. My location [Burgdorf] is marked green (not red) because QIRX is getting the location via GPS; especially useful being mobile . Click for better resolution – as with all following screenshots!

Digital Audio Broadcast, or: DAB, now is common with most household and car radios – after a more than bumpy start. Pressed into market with voluptuous grants from the tax payer and unabashed blackmailing of ceasing all FM broadcast and, hence, making all analogue FM radios obsolete without any financial compensation.

After fierce protests, there is some coexistence between both ways to the listener – mostly thanks of the pressure of commercial broadcasters which often belong to media giants.

Software defined radios, or: SDRs, make an excellent start to discover both worlds. Here, I will focus on DAB with free software QIRX by Clem Schmidt, DF9GI, from Frankfurt. It directly works with RTL-SDR, Airspy and RSP2 SDRs. I tried this very smart software from my location near Hannover/North Germany, mostly with my RSP2.

This blog has two parts: in this first part (1/2), I want to get some ground under my feet – regarding DAB as well as QIRX. The second part (2/2) deals with some results of QIRX’ logs.

QIRX excels in a number of analytic tools, and an OSM-based map showing your location as well as the locations of the transmitters, all metadata transmitted plus other features like connection to GPS receiver’s NMEA output for mobile use. It can be also used very basically: just to listen.

DAB – an efficient concept

In the first step, you have to find out which transmitters you receive at your location. Each transmitter beams a so-called “ensemble” (also dubbed “bouquet”) into the country, or a bundle of programms. Many of these programms or services can be packed into just one physical DAB-VHF-channel (“block”, e.g. 5D) of 1,536MHz width, via a robust and spectrum-efficient mode, called OFDM. This is a special combination of phase-modulated carriers, commerically pioneered for DAB by Munich-based Institut für Rundfunktechnik from 1981. Each ensemble carries an Ensemble ID (EId), like 11F7 for “Antenne DE”. Thanks to the “Extended Country Code”, this EId is worldwide unique. In turn, each station/programme/service within an ensemble carries an unique Service ID (SId), like 121A for “Absolut OLDIE”.
Some identical ensembles may be aired from different locations/ transmitters within a service region. In this case, they work together as a presumably GPS-synchronized Single Frequency Network – to which we’ll come later.

QIRX – the easy start

QIRX offers a scanner, catching all these ensembles from all the transmitters within the reach of your antenna:

First step: The DAB scanner of QIRX automatically catches all DAB transmitters which will provide a useful signal at your location.

At my location, QIRX scanner offers nine to ten such ensembles. For this example, I choosed the ensemble “Mitteldeutscher Rundfunk – Sachsen-Anhalt” (MDR-S-Anhalt), and clicked on the list of eleven services to “MDR Klassik” which shows up with some data on service quality plus multimedia:

Ensemble “6B/MDR S-Anhalt ” offers eleven services, from which “MDR Klassik” has been chosen. It shows also the Slide Show Data with additional information and also pictures.

It offers perfect reception, despite of delivering a signal-to-noise ration of just 10.9dB from a transmitter at a distance of 80+ kilometers.

Scanning is done in the background, and it may loop through (click: “Scan forever”) for hours or even days. It continually writes the results in the TII Logfile for future inspection – a great tool which will reveal even short openings over a specific time – scatter by tropo, aircraft or meteors among these.

How is the signal?

As an SDR aficionado, you will be pleased to see the spectrum and the spectrogram (“waterfall”) of the signal, the receiver is tuned to. The first screenshot below shows a near-perfect case from my local transmitter in 15.8km distance, delivering an SNR of up to more then 33dB, whereas the second screenshot of the ensemble “11D/Radio fuer NRW” at a distance of 115 kilometers shows a bumpy road ahead with SNRs well under 10dB. The “radio horizon” of this specific transmitter’s site already ends at about 85km, thus the margin is not too high.

Spectrum (top) and spectrogram (bottom) show the fine quality of a local ensemble, resulting in an SNR of nearly 30dB, whereas …
… this noisy/mixed reception of the ensemble “11D/Radio fuer NRW” from tranmitters at 100+ kilometers distance resulted in an SNR of more under than above 10dB.

Analytic Tools

One of the most exciting features of QIRX are its analytic tools. To make full use of them, a basic understandig of the concept of DAB is inevitable. ETSI, the European Telecommunications Standards Institute, is the umbrella organisation for maintaining also this concept. They provide a widespread number of different papers with standards and technical reports of which I found EN 300 401 (focusing on receivers) and TR 101 496-3 (focusing on the operation of a DAB network) especially helpful. Clemens, the software author of QIRX, has published some excellent information on these topics on his website, where I especially recommend the two parts dealing with TII, or Transmiter Identification Information. He had put a lot of work into it to present all information to get a clue what happens behind this rather complex and smart DAB system.

The window for the analytic tools comprises up to five sub-windows, with the Audio Spectrum skipped here:

Analytic tools: four sub-windows.

Let’s got through them step by step.

Constellation shows the four phases of the robust DQPSK modulation in a linear manner, representing each of the sub-carrier of the OFDM signal separately. By this, you may see which of the carriers actually is degraded by multipath propagation, caused e.g. by reflection from aircraft.
Above, you see the constellation of a near-perfect reception with an SNR of 33dB. Below, you see two examples at a much lower SNR. At the third example, the robust meta information from the Fast Information Channel (FIC) already is decoded, with however, the signal strength (more exactly, the SNR) just under the threshold to provide audio decoding.

Constellation at an SNR of 12dB …
… and at 8.3dB, just below the threshold of decoded audio.

Channel Impulse Repsonse (CIR) shows the time of flight from all transmitters to the receiver – referenced to the strongest signal, showing up as “0”. You may switch the scale from samples to time in microseconds to (relative) kilometers. These data also show up in the TII window at left-hand, and are used to populate the map.
It is the easy-to-read surface of heavy work under the hood to which also some other radio enthusiasts had contributed.
Below you see first the CIR display, where you see signals from three transmitters. The X-scale is in microseconds, time-of-flight, referenced to the strongest signal. On the left you see a list of all three received transmitters, ensemble 5D, with their real distance (km abs.) from the receiver, their distance relative to the strongest signal (km rel.), and their direction as seen from the receiver (AZM) – to turn your antenna into the right direction …

Right window: Signals from three channels with each different times of flight to the receiver. They are referenced to the strongest signal (“0”). The left window lists the received DAB-VHF-blocks plus its main and the sub identification.
Here you see the three received locations of one DAB-VHF-Block (“5D” – frequency) and one ensemble (11F7) plus their main ID (“51”). The following sub-IDs 1, 2 and 4 mark the actual transmitter’s site. The location with the green mark (5D/11F7/51 1) is the one which actually is decoded at this moment – the strongest one which in this case is also the nearest one on Telemax in Hannover at a distance of just 15.8km.

TII Carriers is a unique and exciting tool of QIRX software to look a bit deeper into the structure of the Single Frequency Network to which DAB is organized. Let’s take the map above with three transmitter on the same DAB-Block, or: VHF channel. TII or Transmitter Identification Information tells us just what transmitter(s) we do receive. Clem, the author of QIRX, put a lot of work not only to get this tool running, but also in describing the background and how to use this feature – you must not miss this (there are two parts …)! I can give only a weak echo of his very well placed explanations there.

Basically, it decodes the “Null symbol” of the TII which is transmitted with low power within what seems a “pause” of only 1.3ms of duration between each frame of DAB stream, being itself 96ms long.

The most easy situation is to receive and decode only one transmitter. The following screenshot shows this situation with DAB-VHF-Block 9B, transmitter Visselhövede. In the TII window you see 4 x 4 carriers, separated within four compartments by a dashed vertical yellow line. Each of the four groups of carriers contain the same information, but each taken from a different part of the spectrum to enhance overall sensitivity for weak(er) stations. The position of the TII subcarrier defines the sub-ID, and, hence, the individual transmitter. In this case, the sub-ID is “1”, denoting Visselhövede as transmitter location.
The mapping of DAB-VHF-Block, Main/Sub-ID and transmitter site has been mainly done by UKW/TV-Arbeitskreis e.V., a smart group of enthusiasts dealing with reception above 30 MHz.

4 x 4 single TII carriers show reception of just one transmitter site.

If you play around with the “Threshold” detecting TII carriers this may reveal also other transmitter locations, transported via the same DAB-VHF-Block. So, I lowered the threshold to 0.010 (x10). As a result, much more TII carriers become visible in the four compartments. They belong to other sites, hence, bearing other sub-IDs. To decode them, they must show up almost similar within all four compartments of the carrier spectrum window (right). Only then they are duly listed under the TII tab on the left side, and show also up in the map at the top.

With a much lowered threshold, four additional transmitter sites show up, carrying DAB-VHF-Block “9B”. Audio-decoded is the best signal, namely Visselhövede, marked green in the map. (Remember: click picture to enlarge for showing more details!)

You see 4 x five carriers, jumping over the gray threshold. On the left, they all are listed with their metadata including their sub-IDs of:

  • 1 Visselhövede – 65,8km/10kW,
  • 2 Dannenberg/Zernien – 91.2km/10kW,
  • 5 Bispingen – 74.2km/2kW,
  • 6 Lüneburg/Neu-Wendhausen – 96.1km/4kW and
  • 4 Rosengarten/Langenrehm – 106,8km/10kW

The additional four sites duly show up in the map, in red with the fifth, Visselhövede, marked green as carrying the best signal.

I/Q Data: The diagram always shows the time sequence of IQ data, in units of samples. Here, one sample corresponds to the system clock time of 1/2048000 sec, i.e. about 1/2 microsecond. The Y-axis can be switched between “Magnitude” (roughly the absolute amplitudes of I/Q), or just the amplitudes of the I-data (“I-Data” ticked). The first is the tool of choice to reveal the above mentioned “Null symbol” of 1.3 milliseconds, see screenshot below. For a detailed explanation, which is out of scope of this blog entry, please refer to QIRX’ website.

Two transmitters with each different time of flight: the strong and nearer signal’s end frame end first, followed by the much weaker end frame of station 2. After these frames of 96ms follows a period of 1.3ms, in which the null symbol is hidden. A slight notch in this period marks that both signals do comply with ETSI specs.

One additonal feature of DAB, much worthwhile to be mentioned, is the so-called “Guard Interval“. It guarantees that all transmitters involved in an Singe-Frequency Network, with their individual stations distinguishable only by their TII codes, can all transmit on exactly the same frequency – whereby the relative distances can be up to approx. 75km apart without interfering with each other. This has the consequence that e.g. the Bundesmux (5C – DR Deutschland) needs only one frequency nation-wide, which is e.g. selected once in the car and then works in the whole republic, not requiring any re-tuning by the driver. By the way, all locations of a block stored in the database can be displayed in QIRX with one mouse click.

Caveat: “Threshold” is as sensitive, as it is sensible. Too low a threshold may result in errors, too high a threshold may miss some transmitters. It is a good idea to start at a threshold allowing only one or two transmitters coming through, and then reduce this threshold by carefully checking the results for probability (etc. by their distance).

At some locations, there may occure a collision of the same sub-ID from different transmitters. This can be de-fuddled by QIRX’s function “Show Collisions for Sub ID”, but this is beyond this mere introduction. I have also to skip many more interesting applications of this software, e.g. using it for multipath detection by carefully observing the spectrum and measuring its deviations from a near-perfect brick-like shape – so that you can even calculate the delay caused by this effect.

We all have to be indebted to Clemens not only for his smart achievement in writing QIRX software, but also for his explanations and examples on his website!
He also helped in explaining some details for this text.

P.S.: Don’t miss the second part of this blog, showing some examples of analyzing QIRX’ logged data!

P.P.S: The best: the QIRX story isn’t over with just DAB. It features also a ADS-B decoder for those flight messages on 1.090MHz, drawing them on a map, and filing them. I will come back to this also stunning feature soon – stay tuned!

HF: Doppler, Signal Level and Time

Two views of the carrier of Sofia-Kostinbrod on 9400kHz from 15:30 to 18:30 UTC: On top the frequency within a window of 2Hz height only, at the bottom the synchronized HF level of this carrier; see text. [Click onto the picture for a better view.]

What you see in the picture at the top, is a mostly hidden gem of HF propagation. I took the carrier of Sofia-Kostinbrod transmitter form Bulgaria (250kW) on 9400kHz and observed it for three hours. In the upper window you see the frequency wihtin a window of 2Hz height only. You see two strong carriers: one nearly in parallel to the x-axis, the other snaking some fraction of one Hertz below it.

With one transmitter only on this frequency: How does this happen?

It’s multipath propagation. The signal takes one way via a groundwave-like way, the upper trace. It reveals a very slight drift downwards. As I use a GNSS-controlled receiver, the FDM-S3 from Elad, this miniscule drift should be happen within the transmitter, not the receiver.
The snaking trace stems from a second way, most likely via the F2 layer of the ionosphere. As the ionosphere is prone to winds and an ever dynamic change of its ionization, it is moving. And with all moving objects, also this causes a Doppler effect to waves. This is exactly what we see – the angular speed of the ionosphere, relative to the “groundwave-like” signal.
You may also see at least two weaker traces, caused by two further ways, hence showing other Doppler shift.

In the diagram at the bottom, you see the combined level of all traces. Because they reach the reeiver at different time and, hence, different phases, their addition leads to an ever changing signal level, called: fading.

I hope to continue this work with some other examples in the future, also taking fade-in and fade-out into account.