- Technical explanations
- Transmitter hardware
- Frequency processing
- RF power amplifier
- Technical data in a nutshell
- Broadcasting program
- Further possibilities of the transmitter
- The story of Joe
40 meter (131,23 feet) long stainless steel wire drawn diagonally across to just below the top of the tower.
At 1476 kHz this is about λ/5.
However, only a few wire windings at the base are sufficient to bring the wire into resonance at 1476 kHz:
After approx. 30 meters long but thick coaxial cable feed, which itself only attenuates by 0.4 dB, you get 13 dB reflection attenuation at 1476 kHz:
After compensation of the remaining inductive part with a 4.7 nF series capacitor at the transmitter output, the reflection attenuation improves to over 35 dB:
The metal frame of the flat roof with the widely branched lightning conductor network serves as a counterweight.
To protect against lightning strikes in the antenna wire, two additional lightning interception rods over 10 m high are installed.
The transmitter consists of two “self-made” plug-in cards in Euro format
Power supply and transmitter unit with modulator (without modulation choke). The latter is housed separately in the aluminium housing, which accommodates the two plug-in cards.
For frequency processing, a commercially available 14.7456 MHz crystal is pulled to 14.76 MHz. A frequency division by 10 results in the desired 1476 kHz. A further frequency division by 369 results in 4 kHz. This frequency is the largest common divider of 1476 and 10000 kHz and is compared with a 10 MHz OCXO by PLL after division by 2500. The 1 kHz tone emitted during night is also derived from the 4 KHz by division, as is the sampling period (1024 ms on and off each).
RF power amplifier
The 1476 kHz TTL signal directly drives the RF output stage: A push-pull switching amplifier with two MOSFETs and a push-pull transformer leads into a 5th order elliptical filter (Cauer filter) on the secondary side. For sufficient suppression the zeros of the filter response are located at the first unwanted harmonics. The filter is designed in such a way that in the event of a short or an open loop circuit at the output, no short circuit transforms into the MOSFETs, which could endanger them. Without modulation the MOSFETs are supplied with approx. +15 V DC which is provided by the modulation choke.
The modulator is a standard audio IC with sufficient power headroom. It is operated with 36 V DC, so that 30 Vpp AF can be generated without distortion. This voltage is capacitively superimposed on the +15 V after the modulation choke at the MOSFETs, so that their supply voltage can vary between 0 and +30 V maximum, which results in 100% modulation degree. At the modulator input an 8-pole Cauer filter limits the modulation frequency to about 4.3 kHz. An upstream compressor allows the AF dynamic range to be reduced.
The on-board detector receiver with loudspeaker output placed directly on the PCB in the RF output area allows direct control of the transmitted modulated RF.
Technical data of the transmitter electronics in a nutshell
|antenna||40 meter long stainless steel wire|
|frequency||1476 kHz, OCXO stabilized with drift of a few mHz|
|modulator||Push-pull AB amplifier with drain choke, up to 100% AM, limited to 4.5 kHz AF range.|
|transmitter amplifier||Push-pull switching amplifier, 8 W carrier power, 32 W PEP|
|monitor||on-board detector receiver with loudspeaker output|
05 – 24 h: “Funklust – Deine Campusmedien”
00 – 05 h: periodically interrupted 1 kHz sine tone, 1024 ms off, 1024 ms on
All times are local time. Sporadic shutdowns of the transmitter or only modulation due to maintenance work are possible.
Possible options of the transmitter
The transmitting hardware allows a slow frequency shift of the carrier in the Hertz or sub-Hertz range (currently still deactivated) synchronized with the OCXO for better identification of the transmitter during DX experiments. In the long term, an improved processing of the modulation signal (compression and spectral filtering) is also conceivable.
The story of Joe: a medium wave transmitter is born
The trigger for my idea of an “own” small medium wave transmitter was primarily the successive shutdown of all German AM transmitters: First the short wave, then the long wave and finally the medium wave. But the motivation for such a station was not so much nostalgia but rather the realization that there was no easy way for people interested in radio technology, especially young people, to get to grips with this topic.
A long odyssey to the destination
After switching off the long wave on Dec 31st 2014, I contacted the KEF (Commission for the Determination of Financial Needs), which was mainly entrusted with the switching off of the AM transmitters. I was told that nowadays AM only had a “museum character”. This gave me hope that we would be able to keep at least one of our large transmitters alive and perhaps put it into operation on special events, similar to the historic SAQ machine transmitter in Grimeton, southern Sweden.
But unlike in Finland, for example, where the long wave transmitter Lahti was converted into a museum and the antenna towers were kept alive as a local landmark, or in Switzerland, where the antenna tower of the Beromuenster station was preserved and even a radio hiking trail was set up, we destroyed almost all of these landmarks in Germany. Finally, on March 2nd 2018, the two 360 m high antenna towers of the DLF in Mudau/Donebach (one of the highest buildings in Europe!) were blown up. Now this remainder of hope for a remaining AM transmitter with a museum character was gone.
An own transmitter?
By chance I heard about Bernd Schmid (DL2MFP), who had the frequency of 801 kHz reserved for a small museum transmitter at the radio museum in Wertingen near Augsburg. He was already on air on special events. Later I also heard about Michael Heller, head of the radio museum in Cham, who together with his team saved the BR transmitter formerly located in Munich-Ismaning for the museum and also transmitted sporadically on 801 kHz. I found out from both sites how to start such a small medium wave transmitter. However, I also realized very quickly that the way was very hard – without any guarantee of ever reaching the goal.
First steps, first setbacks
Since I didn’t have a museum in my background, I first tried to arouse interest in my project through the large DARC association, to which I belong since 1972, primarily for the purpose of promoting young talents. But there other things were obviously much more important, my E-Mail inquiries were answered only after months (after renewed inquiry), but without recognizable enthusiasm. Although they regretted the shutdown, this was the only “activity” on the part of the DARC.
Another idea was to win over the major corporation Siemens (my long-term employer in Erlangen) for this project, especially as Siemens is currently building a new campus here for many millions of Euros. But here, too, there was no support. The expressed “understanding”, but when you’re ultimately referred from one department to the next, you know what’s between the lines. Another timid attempt at BR at the Dillberg broadcasting location was unfortunately just as unsuccessful, although some of the other employee would have liked to support it there. But there were clear guidelines from there bosses.
Positive turnaround on New Year’s Day
I had been in contact with the Fraunhofer Institute for Integrated Circuits (IIS) in Erlangen Tennenlohe many years before. I know the director, Prof. Albrecht Heuberger, personally for a long time. This personal contact should ultimately lead to success. He invited me to his apartment on New Year’s Day 2018 and had an open ear for the idea of a MW transmitter – not for museum reasons, but for educational reasons. Since the IIS has been broadcasting test broadcasts on MW and SW in DRM and later also in DAB+ for quite some time, I was in good hands.
Cooperation with Funklust Campus-Radio
Now one thing led to another: For these broadcasts there is already a radio program, namely the one of Funklust association and the required media license. The association agreed that I could also broadcast this program on MW later. For this purpose the media license was adapted accordingly.
Search for a suitable frequency
The application for a desired frequency at the BNetzA is then only a formality (i.e. with an existing media license). For this purpose I had been searching for free gaps in the MW band by waterfall diagram for months. The goal was a gap at the upper band limit because of a shorter antenna length and a better chance of sky wave propagation. In addition this gap that should be free in the evening hours. The frequency 1476 kHz seemed perfect and was applied for.
Unfortunately, I could not know that shortly after my broadcast permit was granted, a British MW radio station with far higher power also chose this frequency.
Happy end: the perfect broadcasting location
The biggest problems “media license, radio license and program design” were apparently solved, but the IIS was also able to offer a dream solution for the next two larger problem areas “antenna” and “accommodation for the transmitter”: A concrete tower almost 60 meters high with various other antennas stands directly next to the institute buildings and can be used for the MW-station as well.
In addition, there is the amateur radio club station DK0FHG in the attic of one of the buildings. The responsible radio amateurs welcomed my medium wave transmitter as a permanent guest.
Ralph Oppelt (DL2NDO) is founder and developer of the medium wave transmitter Joe on 1476 kHz.
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