Thursday, July 25, 2019

Windows Override Command Execution (Image File Execution Options)

When you run a command at the windows cmd prompt, such as schtasks.exe, klist.exe, driverquery.exe, etc, Windows searches a registry setting for default options to execute the file. 

The location is:

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\<nameOfEXE>

Oddly, Windows checks the same registry key regardless of where it actually found the exe or who is executing it. 

If you create a registry entry at the right spot with the "Debugger" string you can make it execute another program instead:

I feel like this could be handy in a situation where you can edit the registry but file integrity prevents you from modifying files on disk. You can kind of stitch execution together to somewhere you control.

Wednesday, July 10, 2019

Calculate proper length of antenna for better signal

To increase signal quality, make sure the antenna length matches the frequency you are receiving at. I've had a 20-40 db increase in signal just by extending or retracting my antenna a couple of inches. A simple process is this:

  1. Have on hand a ruler with centimeter markings (I use a retractable tailor tape measure)
  2. Open your calculator and do 300 / your_frequency (in mhz) = wavelength
  3. wavelength / 2 = how many meters long a half wave dipole would be. If this is too long, divide by 2 again to get the length of a quaterwave dipole in meters.
  4. Extend the length of your antenna (I use ANT500s from my SDRs) to one of those lengths using your ruler. You should prefer half wave, but quarter wave is fine too.
For example, I'm listening on the national calling frequency for APRS (144.390) i'd do the following:
  1. 300 / 144 = 2.08
  2. 2.08 / 2 = 1.04 (half wave dipole length, way too long for my ANT500)
  3. 1.04 / 2 = 0.52 (quarter wave dipole, 52 centimeters can be done easily)
  4. Measure out 52 centimeters on the tape measure, use it as a guide to extend the antenna to the proper height.

How to loopback audio from one app to another for SDR

Piping the decoded audio from one application to another can be super annoying in linux. Fortunately there is a utility in apt that makes it dead easy (at least on Ubuntu). Simple steps:

  • apt install pavucontrol
  • start your applications (gqrx/fldigi/direwolf/whatever)
  • execute pavucontrol
  • In the "Playback" tab, make sure your "output" app is showing up and bouncing the sound meter
  • In the "Recording" tab, make sure your "receiving" app is showing up. Click the sound card button and select the "monitor" entry
  • Should start working immediately.
For FLDIGI to show up it needs to start recording from the sound card, I had to go to Configure > Sound Card > Audio > Devices > select "PortAudio" > leave capture and playback at "default"

Decoding APRS via SDR

APRS uses packet radio and FLDIGI doesnt support it for some ungodly reason. You can chain a few tools together to get the decoded output.

  1. Plug in and attach your RTLSDR, or whatever, to your VM
  2. start up GQRX, start receiving
  3. Tune to 144.390 to get an APRS signal (North American calling freq for APRS)
  4. Set the following: 
    • Filter Width: Wide
    • Filter Shape: Sharp
    • Mode: Narrow FM
    • AGC: Fast
    • set the squelch to silence the noise
  5. In the audio section, hit the "UDP" button which should start streaming the audio over UDP port 7355.
  6. In another tab: apt install direwolf
  7. direwolf -r 48000 udp:7355
You should get output like below: