Introduction: (Inexpensive) NOAA APT Ground Station Utilizing the RTL-SDR and 3D Printing.
This is a guide aiming to help you build an inexpensive APT ground station. The QFH antenna described in this tutorial has been measured by a vector analyzer to correspond with NOAA APT frequencies at 137-138 MHz. The cost of this project varies from 10 € to 50 € (~55 $) and makes use of 3D printing and multifunctional hardware such as amplifiers, filters and the RTL-SDR.
You will need:
1. RTL-SDR, AliExpress
2. FM Band Stop Filter 88 - 108 MHz, AliExpress
3. SPF5189Z LNA, AliExpress
4. 1 m of PVC pipe with a diameter of 25 mm.
5. ~200g 3D Printer Filament, I used PETG for its durability.
6. 5 m copper wire with a diameter of ~2 mm.
7. A 50 Ohm transmitting line (RG174, RG58, RG178).
8. 50 Ohm RF connectors corresponding to your SDR and transmitting line (in my case SMA for RG174).
9. A 5.4 pf capacitor (I used 2 capacitors with 2.7 pf capacitance in parallel).
10. 0.5 mm wire to create a 248 nF coil inductor.
11. RF connectors for PCB.
12. A 18650 battery to power the amplifier.
13. A laptop or a desktop running SDR reception software (GQRX, CubicSDR, SDR#) and APT decoding software (WXtoImg).
14. A soldering iron.
15. A drill.
16. A crimping tool.
These are the things you are going to need in case you want to copy my system, alternatively you can just use the antenna and a transmitting line to connect it to the SDR. The reception will still be good!
Step 1: Printing and Building the QFH Antenna.
A QFH (Quadrifilar Helix) is a circularly polarized antenna and consists of two loops which form four helixes, to receive the APT signal we need a QFH with right-hand circular polarization (RHCP). Since the antenna is used as a receiver, the source should be on the top. Download and print QFH_BOTTOM 2 times, download and print QFH_BODY as many times you need to guarantee that the loops form helixes ( I printed 3) and cut 2 pieces of 2.5 m wire. Ideally you need ~2.20 m (2203 mm) of wire for the longer loop and ~2.16 m (2158 mm) of wire for the shorter loop but it may be easier to cut longer wires and then match them to the body of the antenna.
- Fit the first printed QFH_BOTTOM part to the PVC pipe and place it close to the bottom.
- Fit the second printed QFH_BOTTOM part to the PVC pipe and place it 45 mm far apart from the first.
- Fit the printed QFH_BODY parts to the PVC pipe, the distance between the top part and the bottom QFH_BOTTOM part should be 742 mm. You should fit the rest of the QFH_BODY parts between them, the distance between them doesn't matter but you have to make sure that the wires form four uniform helixes.
- Drill holes below the first and the second QFH_BOTTOM part so the wire can pass through the pipe.
- If you want, you can use the drill to straighten the wires after you have placed them to the antenna body. Be careful and don't apply too much pressure as you may break the wire and hurt yourself!
- Form the helixes in right hand motion as it is shown in the picture below.
- Stip your transmitting line.
- On the top of the antenna, solder one end of the longer loop and one end of the shorter loop with the center core of the coaxial line.
- Solder the remaining ends with the shield of the line.
10. Make sure there is no short circuit (the core is completely isolated from the shield).
11. Strip the other end of the line and use the crimping tool to fit it to a RF connector that matches your needs (male RP-SMA for RG174 in my case).
The standing wave ratio (SWR) of this antenna was measured to be 1.27 for 137.5 MHz using an Agilent E5062 vector analyzer.
The input impedance of the antenna was measured to be ~50 Ohm.
Step 2: Filters and Amplifiers.
For this system I used a FM bandstop filter, a SPF5189Z LNA and I created a bandpass filter.The signal is transmitted from the antenna via the transmission line to the FM bandstop filter where noise from radio stations is eliminated. The signal is then transmitted to the bandpass filter for further isolation and finally the filtered signal is amplified for ~25 dB by the LNA, which is connected to the SDR.
I purchased the bandstop filter from AliExpress, here's the schematic in case you want to built it yourself.
To create the bandpass filter, I used two 2.7 pF Capacitors in parallel (total capacitance of 5.4 pF) connected in series with a 248 nF inductor (I used normal coil of 0.5 mm diammeter to create the inductor with the help of this calculator). I soldered everything to a PCB and used corresponding RF connectors (SMA for PCB).
I used a 18650 battery case and I soldered it to the PCB of the SPF5189Z LNA.
Finally, I connected the system to the SDR.
Step 3: Further Antenna and Filter Study
The antenna should tune at 137.5 MHz, for this to happen the larger loop should tune to a lower frequency and the smaller loop should tune to a higher frequency. For this design, the longer loop consists of 2.2 m wire and tunes at 136.26 MHz (wavelength of 2.2 m) when the shorter loop consists of 2.16 m wire and tunes at 138.79 MHz (wavelength of 2.16 m). 136.26 + 138.79 = 275.05 and if we divide by 2 we get 137.525 MHz as the antenna's center frequency.
I designed the aforomentioned antenna using the software 4nec2.
We can see that the SWR of the antenna tuns out ~1.24 at 137.5 MHz.
And the impedance turns out ~45 Ohm for the real part and ~8.5 Ohm for the imaginary part, ideally the imaginary part of the impedance should be 0 but it's still low and shows us the "inductive attitude" of the designed antenna.
Regarding the gain of the antenna, we can see that it is ~4 dBi for right-hand polarization and "follows" the satellite in its pass from the horizon.
I also ran simulations for the filters using the software RFSim99.
For the FM BSF I got.
For the BPF I got.
We see that the simulations agree with the design of the antenna and the filters.
Step 4: Software
I work on Elementary OS which is a Linux distribution and I use Gpredit to monitor the NOAA satellites which you can also run on Windows.
The satellites broadcast at 137-138 MHz or more specifically:
- ΝΟΑΑ-15 - 137,62 MHz
- NOAA-18 - 137,9125 MHz
- NOAA-19 - 137,1 MHz
The bandwidth of the emitted signal is ~ 34 KHz and the modulation is FM. Open CubicSDR or else (GQRX, SDR#) and set the modulation to FM. Tune to the frequency of the satellite you want to receive and set your filter's width to be at least 34KHz.
As long as the satellite approaches your location, you should see something like this.
You should hear something like "tick-tock" between a 2400 Hz and a ~1000 Hz tone. Record the "tick-tock" sound as the satellite passes over your geographical position and save the .wav file.
Open WXtoImg and load the .wav file, you may see something like this.
Bare in mind that this is not the original image of the satellite but one modified by the program, for the original image you have to go to Enhancements -> Normal.
If you choose normal, you would see two images. In the top you can notice something like "NOAA (ch 1-4)", this means that one image comes from channel 1 of the satellite's radiometer and the other image comes from channel 4. Channel 1 of the satellite's radiometer is the earth image in visible light when channel 4 is an infrared image. If you move the cursor of your mouse over the infared image you can notice the temperature of a particular position on the map (bottom right). The 2400 Hz tone tells us about the colors (shades of grey) of the earth image when the ~1000 Hz tone (1040 Hz for the visible image and 832 Hz for the infared image) is a sequence of synchronization pulses.
Step 5: Conclusion
You can now see the satellite image on your computer!
In case you face some problem decoding the .wav file with WXtoImg, download Audacity and resample the audio file to 11025 Hz (Tracks -> Resample).
You may also need to normalize the audio file before and after resampling it, go to Effect -> Normalize.
Please let me know if you need further information.
Thank you, Than.
3 years ago
Excellent detail, thanks for sharing! The later advice for re-sampling the signal is greatly appreciated, as I've not read that before and makes sense for signal pre-processing. Amazing how some signals are present, though not even audible until the sampling settings are correct... not only for the carrier signal, for the modulated sound/signal too. Thanks again!
3 years ago
Ypu have done an excellent job of explaining the particulars of building the antenna and electronics for a project of this type. Good work! I look forward taking a shot at this project sometime in my near future! As a question, would parabolic amplification be useful for a project like this or is this the only type of antenna that works best?
Reply 3 years ago
Thanks a lot.
Typical parabolic antennas used for satellite TV are designed to tune at 10-20 GHz, the gain of these antennas for 137.5 MHz will be way too small and they also need to sychronize with the satellite's polar orbit. For example, the gain from a parabolic dish with a diameter of 1 m and 60% efficiency would be 0.95 dB and you would still need to combine it with a feed antenna which tunes at 137.5 MHz and uses circular polarization. You will need a way larger parabolic dish for NOAA APT/VHF to get a relevant gain (40 dBi), think of the size of the Stanford dish. Not only you need a dish of a similar size but you also need to install a mechanism in order for this antenna to follow the satellite's orbit.
The benefits of the QFH are:
1. It can receive circularly polarized waves.
2. Its gain of ~4 dBi is decent.
3. Its gain is stable for 120° to 180° into the horizon, you don't need to install a mechanism to follow the satellite's orbit.
4. It can be built within 2 hours.
A better alternative (NOAA also suggests it) is to use a circularly polarized Yagi (cross Yagi, helical Yagi). The gain of this antenna could be as high as 14 dBi (more than 3 times stronger than the QFH) but it needs a mechanism so it can always point towards the satellite and think that each dipole of the Yagi should be ~1.09 m (around three times longer than a common TV Yagi). It's an interesting/intriguing concept to build one in the near future but not as comforting as the QFH.
3 years ago on Step 5
Many thanks for a detailed information I am very impressed with your instructable.You have done a lot of work I have found so informative.
Reply 3 years ago