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?