Capturing the elusive APRS signal

I have been trying to capture APRS (Automatic Packet Reporting System) messages for over two

weeks now in vain. Finally today I managed to receive a message using the RTL-SDR dongle on my Windows laptop which was running SDR# software. I also had the captured message decoded using the AFSK1200 decoder as shown in the picture to the right. Here is how I got successful with that today. First of all I was using a monopole dual band antenna (2m/70cm) indoors. Of course I would not receive that signal which is known to be very weak. The monopole antenna needed a ground plane to make it act as a dipole. That alone improved the reception considerably and improved the VSWR in the two bands (2m and 70 cm). But that was not enough. I also needed to go outdoors. Apparantly the antenna does not receive well indoors due to walls, electronic equipment and wires etc.

The picture to the left shows the antenna placed outdoors standing on a metallic plate that extended about 10" all around the monopole. I placed the laptop inside with the cable from the antenna attached to the RTL-SDR dongle. The reception was tuned using the SDR-Sharp software to the ARPS frequency in North America which is 144.390 MHz. The demodulation was narrow FM (NFM) and I could see that every now and then a signal popped up at that frequency within the 7 KHz bandwidth or so. I used the AFSK1200 demodulator tool to decode the received signal but nothing got decoded for about two hours. I could see the decoder receiving something each time a signal popped up but nothing was decoded.

I forgot to add that I also used a 2m band filter shown on the right. This filter has low loss in the 144 MHz band of less than 0.7 dB and filters out anything received by the antenna except from about 138 MHz to 146 MHz. This ensures that any strong signals elsewhere in the spectrum do not overload the low noise amplifier (LNA) causing it to get desensitized. Apparantly this helped because after about two hours finally I got the first message decoded finally as shown above in the first picture, and the captured signal can be seen in the picture to the left popping up at 144.390 MHz.

Finally I got that APRS message captured. After that I kept receiving messages but nothing got decoded for another hour or so and then I got another decoded message which was identical to the first one. I did not receive anything else for the next hour and half or so at which point I stopped. So next I wanted to find out where the message came from and where it went to, so I looked up the call sign in the message (KN6UWK-7) on google APRS map and located that repeater at St. Clemente island near Los Angeles as shown in the map to the right.

This is pretty far from me (more than 50 miles) so it is nice that I was able to receive it twice. But I wonder why I was not able to receive anything else from the nearby repeaters that are all around me within less than 10 miles or so. Perhaps it has to do with the position of the antenna and the buildings around me. It would definitely help if the antenna were elevated up high to have a better line-pf-sight with the repeaters. But I was happy that at least I got this one message, which was routed via two repeaters in Ohio to the final destination (3S0PVP-0). I could not find that destination call sign on the map so I don't know what it is at. But the two repeaters WIDE1-1 and WIDE2-2 are both in Ohio. Pretty cool isn't it?

Clean 20 MHz Square Wave

Using the 5 times frequency rule, one can measure the wave accurately on an oscilloscope only if the scope's BW is at least 5 times the frequency of the wave. My old analog oscilloscope had a bandwidth of 20 MHz and therefore could not accurately display a square wave higher than 4 MHz in frequency. Recently I bought a new digital oscilloscope with BW Of 100 MHz, which meant it could accurately display a 20 MHz square wave. For that purpose I needed to generate a clean 20 MHz square wave to test my new 100 MHz digital oscilloscope.

I found this circuit on the right that utilized a 20 MHz crystal oscillator and a hex inverter IC. I built that circuit on a perfboard as shown below, and instead of the 40106 IC I used two other ICs that both worked fine: one was 74HC04N and the other one was 74HC14. difference between the two is that the 74HC04N inverters are also Schmitt triggers, whereas the 74HC14 they were regular inverters. Like I said both of them worked fine with no noticeable difference.

The picture above shows the front and back of the perfboard assembly. I used a 50 ohm SMA connector so that I can connect to the oscilloscope using a 50 ohm coaxial cable instead of a high impedance probe to minimize reflections.

But first I wanted to look at the square wave without using the SMA connector by probing directly on pin 3 of the IC which is the output of the circuit. The two pictures above show the probed waveform for the 74HC04N IC (on the left) and the 74HC14N (on the right). They are not bad and show a rise time of 2.8 and 3.8 ns respectively. However the square wave shows some reflections that distort its shape. I decided to go with the 74HC14N IC. I used the Math function in the scope to do FFT on the time domain signal to see its harmonic content. I also saved the time domain signal in CVS format and performed FFT on the signal myself on the computer using a Python program that I wrote which reads the Excel file and plots the FFT it performs on the data.

As expected the frequency content of the square wave consists of the fundamental at 20 MHz, and the odd harmonics. The third harmonic is supposed to be at about -9.8 dBc relative to the fundamental which is close as seen in the picture. The fifth harmonic is supposed to be at -14.5 dBc from the fundamental which is also close, but as you see the higher order harmonics are not consistent.

The calculated FFT from the CVS data file is shown to the right, and as expected it shows a fundamental at 20 MHz, third harmonc at about -9.5 dBc from the fundamental and a fifth harmonic at about -14.5 dBc from the fundamental as the oscilloscope measured. I limited the FFT components to 140 MHz in the plot but they go higher of course.

I tried to clean the waveform by filtering the signal using a 100 MHz filter (the purple curve above left) and 50 MHz filter (the purple curve above right). Of course the filter gets rid of some details but that did not fix the shape of the waveform itself, just the reflection details. So next I took the signal out of the SMA connector and into the oscilloscope using a 50 ohm coaxial cable instead of the probe.

I had to purchase a 50 ohm adapter because the oscilloscope did not have a 50 ohm input setting. You can see the adapter connected to the second BNC connector of the oscilloscope (CH 2). You can also see that now the square wave is much better and cleaner than before.

The clean square wave is shown above on the left, and the FFT content is shown above on the right. As expected the third harmonic is again is at -9.5 dBc from the fundamental and the fifth harmonic is at -14.5 dBc as before, but you can also notice the higher order harmonics keep going down as they should instead of the inconsistent behavior we saw before from the p robed waveform. So I was happy with this result and I verfied that my generated square waveform is indeed clean with a rise and fall times of 3 and 2.2 ns respectively. I also verfied the operation of my new 100 MHz digital oscilloscope which measured the 20 MHz square wave properly as it should.

Catching Views in Catalina Island

The trip to Catalina island from Dana Point takes about 1 hour and 30 minutes. When we arrived we saw the famous Catalina casino which no longer shows movies as it used to in the past. People can still get inside if they buy a $20 ticket but we did not think it was worth it.

The first thing we did when we got off the boat was to rent a golf cart for two hours. We needed to spend a couple of hours as our hotel check-in time was 2 pm and we ararived 11 am. So we rode the golf cart around the capital Avalon and explored the scenery and all the places that we visited later on on foot. After we returned the golf cart we had lunch in a steak house in Avalon and then we walked to the hotel which was about 15 minute walk. The view from the hotel ballcony is hown in the picture to the right.

We spent the rest of the day walking around Avalon till it became dark. The weather was a little cold in May, and it was Eid Ul Fitr so we enjoyed three days during this short vacation in this lovely island. A scene in the evening of the capital Avalon is shown in the picture to the left.

We were quite tired from a whole month of fasting and a very busy day, so we went to bed early that evening to get ready for our adventure awaiting us in the morrow as we planned to go parasailing in the morning. So next morning we had breakfast in the hotel, which was a regular continental breakfast, not that great if you ask me but was enough to get us going for our parasailing adventure. We were the only two on the boat and we went up 600 ft in the sky for about 15 minutes as you see in the picture to the right. It was scary after a while as we kept going up, but when we started to go down we started to cheer and all fear went away. We enjoyed this adventure as it was the very first time we ever do parasaling, and probably the last time too.

After that adventure we had a very good seafood lunch at a restaurant in downtown Avalon by the sea. In the afternoon we walked all the way to Wrigley's Memorial and Botanic Garden. It was hot away from the coast but it was worth seeing all the special seculant plants inside. I also managed to fly my drone and take some panoramic video from the sky of Wrigley's Memorial building.

Luckily we discovered there was a bus that we could ride to take us back to the hotel for $2 each. We had to pay exact change to take this ride. The ride was quick and smooth and saved us from walking back in the heat, although walking back was downhill and much easier than our walk in the morning. We walked some more in the evening and explored the area around the Casino, and again we retired early in the evening to get ready for the final day adventure which was a safari ride inside the forests of the island for two hours before finally going back to the return trip boat around 3 pm. So the next day we checked out from the hotel but left our bags at the desk, had our lunch and then took our Safari ride from downtown Avalon. There were no bisons or anything as theey advertised, so the only things we saw were trees and vista landscapes from high above the coast. The ride took us to the Catalina airport and back. At 3 pm we took our boat back to Dana Point and drove back home to arrive around sunset. That concluded this short vacation in May 2022.