It’s more than three years that I monitor VHF for meteor scatter signals thanks to the French GRAVES radar near Dijon. This is a bistatic radar that transmits a CW signal at 143.050.
With a simple radio system is possible to receive the radar signal reflected by the meteors ionized plasma that these objects create when entering into the ionosphere.
Talking with my friend Andrea Miccoli, member of APA (Associazione Pontina di Astronomia) an astronomical association here in Latina, he told me that this activity could be an interesting allied for its works about astronomy for blind people.
Andrea and other members of APA teach, thanks to appropriate courses, astronomy and how the space is to people that can’t see by special high quality home made tools.
In the last year I took in account to better perform my reception tecnique to give a one more opportunity to blind that is to “watch” falling stars!
Here starts the project!
Before of the Perseids shower, this year in August, We (Andrea Miccoli, Andrea Alimenti, my second son Gabriel and me) installed a receiving station at the APA observatory so to have an indipendent 24h system that can streams the radio signal over the Internet so that is possible to listen the echoes from the impacting debris.
The hardware system is composed by an homemade four elements Yagi beam antenna, an also homemade bandpass two helical filter, a SDR RTL 0.5 ppm receiver, a PC and a smartphone for internet connection.
The antenna has a 1.5 lambda boom lenght, a gain of 8.8 dBd, it was build under the DK7ZB project (http://www.qsl.net/dk7zb/2m-port-SSB/4-Element.htm).
I decided to build this one because of its performance, high gain with a single antenna and both horizontal and vertical wide pattern.
The filter was necessary because of lack of filters in the SDR receiver, the sensitivity increases drammatically with a good bandpass filter. I built a very simple and effective helic filter which usually is used for transmission, too. The filter has a very low loss and an adeguate out-band rejection.
On the web there is a wide choice of cheap RTL 2832 receiver, for the project I focused to a 0.5 ppm model. It is provided with a TCXO and effectively is more stable then the cheapest models. The cost is not so high and for the moment We are satisfied about its job. Other differences compared to other models are the SMA connector, the lack of unnecessary IR-LED for remote control and power LED. It’s quite warm when powered on so We’ve fixed it to a heatsink plate so to keep its internal temperature stable. It’s plugged with an USB cable with a ferrite bead to limit any noise from the PC.
After a look to the this first part concerning the radio hardware system, a second and not less important part is composed by some software needed to manage the radio signal and transport it to the web for public sharing and for analizing and storing into images file the meteor events.
The SDR receiver is managed by SDR# (SDR sharp), a freeware easy to use software which can provide several functions to control this cheap receiver.
The frequency is centered to 143.049 MHz USB with a width of the audio band of 2500 Hz about, so the reflected signal of the GRAVES radar is centered at about 1 kHz.
Fast AGC was set.
Through a first VAC (Virtual Audio Cable) the output signal coming from SDR# is transmitted simultaneously to SpectrumLab and VSThost.
SpectrumLab is used to elaborate a coulpe of spectrograms, the first is a 6 minute long screen, saved on the HD every 6 minutes which shows the echoes received from the RTL2832, a second one is a 24h spectrogram that shows the entire day trend at glance.
A simple 6h plot is also created to see relative values in the time domain of peak values and duration of the events.
These three pictures are dowloaded every 6 minutes to my web server for a public view.
VSTHost is used, instead, to process the audio signal and reduce noise. This is done by a couple of plug-in: an expander and a multitrack equalizer.
The output audio is sent to a second VAC where is routed into another software, BUTT, so to be sent to an Icecast audio webserver. Butt takes care to prepare the signal with certain characteristics as audio sample rate, bit rate, informations of the trasmission, etc.
To have a minimum required band the digital transmission is only 8 kHz wide for 16 kbps streaming bitrate.
Icecast server is hosted at another location, running in a Raspberry PI, where is possible to have a public IP associated to the DNS: apalan.hopto.org:1081/stream. This is the address where is possible to listen the elaborated audio streaming and where blind people accede to listen for the meteors.
Internet connection is via smartphone because of a semi-remote position.
A lot is still to be done, improvements, upgrades, tests … just started and in the mind a lot of ideas as apply a preamplifier, build a second antenna to get more gain, etc…
I hope that these few notes can be useful for those who want to build a simple meteor scatter station with a little budget and if this project can be useful for people who can not see falling stars… well it’s welcome!
All this started when I decided to move my home made geophone from my lab room to my cellar at about 3 meters under the ground.
It was a better position for this device because of absence of electro-magnetic noise. Only a simple 220VAC socket and an incandescent lamp are located in the cellar, no other electric or electronic device, surrounded by reinforced concrete and innocent camping tent, skis, bikes, etc…
So, just for this electric “loneliness” it was difficult for me to have a connection with the PC located about 3 floors up. The only way was to use the wireless house net with a repeater on the first floor, but how to connect the geophone amplifier to SpectrumLab? Should I to transform analog signal into data signal? Than back to analog-digital signal. I’m too lazy for these.
An old Samsung smartphone was into my lab powered off from a while, so i tought: why don’t exploit the embedded smartphone technology for my goals?
I’ve dowloaded and tested some applications for the smartphone to stream audio by its wifi connection creating a simple audio server.
The easiest and lighter application that I supposed it to work well is free and it’s name is Wireless Mic.
It is very easy to use and configure, moreover latency is very low, it didn’t load the old smartphone CPU, in general it don’t require many resources, this absolutely avoid audio flow interruption.
I connected the output from the geophone amplifier to the headphones-microphone input of the mobile phone, a capacitor separates eventual DC continuity. After some regulations for audio strenght and IP configurations for intranet comunication I tried to receive the stream in my LAN.
To get and listen the stream I’ve used the freeware VLC media player software, it can simply manage a network flow just writing server IP address and port.
Just started the player I was sure that connection was ok, of course audio was non audible because of the low audio frequency, this was the big question, would this system transport a so low frequency audio signal? To see if all was right I’ve send the signal from VLC media player to a VAC (Virtual Audio Cable) to pass the signal to SpectrumLab, unfortunately SL at the moment can’t receive direclty a compressed audio stream.
With my big surprise all was working perfectly! Great! Giving some weak touch close to the probe I saw the signals on the low frequencies of the spectrum. Another test was made injecting a sub audio signal to the smartphone.
I’ve tested with success this system also for VLF reception for about a couple of months before to move it all into the cellar.
I’ve tried this system with other smartphones and I’ve find that not all models can manage well the audio, some phones use to cancel what they consider an audio noise, like for embedded microphone, also from headphone-microphone plug so to get out a too digitized or weak sound. Some models can manage this some not. The old Samsung made its dirty job well.
As power supply you can choose for a big 12V gel battery and a 5 V stabilizer to keep in charge the phone battery or a wall power supply, this second solution doesn’t give the safety from AC noise but you can build your own low noise one.
An example of the needed material with another cellphone brand
Here some pictures of the remote cellar seismograph:
After a while and some technical problem the VLF station SAQ from Sweden was again in the air today.
Signal was very strong despite the noise created by statics and thunderstorms.
I received the station more than 20 db over the noise level. Unfortunately a local noise appeared time to time exaclty on the same frequency (17200 Hz) so I had to narrow down the band to get a better signal.
Here you can hear the CW message:
Signal was received by my VLF loop and SpectrumLab via sound card.
Tra qualche giorno la stazione spaziale cinese Tiangong-1 entrerà nell’atmosfera terrestre disintegrandosi, speriamo per gran parte della sua mole.
Per l’occasione ho predisposto il mio sistema per la ricezione meteor-scatter VHF tramite il radar GRAVES, in modo da provare a ricevere un’eco radio prima della totale disintegrazione della stazione. In questo caso, visto che attualmente l’altezza della Tiangong è di soli 250 km circa, ci si aspetta una prima riflessione di tipo Airplane scatter, ossia una riflessione sulla superficie metallica della stazione spaziale, simili a quelle viste spesso con la ISS. In seguito, durante il momento di attrito con l’atmosfera terrestre, a seconda della velocità di discesa, forse sarà possibile rivelare l’eco dovuto alla ionizzazione dell’atmosfera stessa.
Alla solita pagina Meteors Radar è possibile vedere la schermata aggiornata ogni 6 minuti dello spettro radio preso in esame a 143.050 MHz. E’ inoltre possibile ascoltare gli echi meteorici cliccando al link suggerito in alto nella stessa pagina.
Seguirà un eventuale aggiornamento sperando che la Tiangong si schianti in mare aperto.
Qui sotto una immagine delle zone dove potrebbe rientrare la stazione spaziale in Europa.
After months of test I finally detected the VLF carrier signal from Riccardo IW4DXW.
Today, probably thanks to a particular propagation condition on VLF, was possible for me to “read” during the entire daylight the signal from Riccardo’s transmitter. Unfortunately with the first rain drops the signal was cancelled from my spectrogram. But I don’t know if this is the real cause of the signal disappearance.
Here the detail of captured spectrogram where is possible to see the weak signal. Blue arrow for the trace, the red one the signal peaking.
The distance from me and Riccardo is about 385 km, 10 km more than fromRenato Romero that received and decoded some EbNaut signal from IW4DXW.
To receive the 14uW radiated from Riccardo’s antenna I’ve used a 40 turns loop on NW-SE orientation.
On top is possible to see a whip antenna always for VLF, I’ve tried to use both antennas together, but their proximity caused some problem mixing the signal of the whip in the loop and viceversa. So the next step will be to separate them.
Despite the nice free space that you can see on the photo I’ve a large amount of manmade noise from houses on the environs and this is a real big problem for my radio activity, especially for very weak signal reception.
In these days Stefan DK7FC is transmitting with a special license on 17470,1 Hz a CW signal. I and other Hams are trying to receive his signal.
I already received Stefan’s signal on 8 kHz with a 47 uHz waterfall grabber, now I successfully tryied also a 424 uHz grabber.
Here is an image of the signal:
In this page http://www.ik0vve.net/?page_id=377 is possible to see my online grabber updated every 30 minutes.
To receive the transmission I’m using the MF-LF TX antenna because the whip and loop rx antennas are not installed at the moment. The antenna is connected to the sound card of the PC by a 1:1 trafo without any preamplifier.
