The IFSK Tracker app can plot graphs of the signal-to-noise ratio of a narrow-band IFSK-modulated signal versus distance or time. It is designed for the analysis of geographically extended radio paths and estimation of the signal passing depending on the time of day. The operating principle is based on the reception of a reference IFSK-modulated beacon signal. A car (or any other vehicle) with a GPS receiver onboard, having fixed the starting point, sets off carrying a connected radio tuned to the frequency of the beacon. A low-frequency signal with a defined sound frequency and a sideband (in the case of SSB) is fed to the input of an audio card (or a smartphone). Recording can be activated or deactivated by pressing the corresponding button in the program menu. At the end of the recording, the data is saved in a predefined directory.
In the demo version, the settings and GPS coordinates are not saved and the duration of a session can not exceed 20 minutes.
After creating the HF pager app for text message radio communications on low-frequency HF bands and preparing for tests in the Komi Republic, I felt the need for a metering device that would show some numerical value of the quality of communication on a radio path. The original plan was to stop the movement and measure the signal-to-noise ratio using data from the WSPR beacon.
This technique is based on an excessive correlation of the received signal, which is recorded in the computer memory for 2 minutes. The bandwidth of the WSPR signal is only 6 Hz while the HF Pager has 50-200 Hz. This data would be a far from practical application and would not allow in any way to draw an analogy with the real signal of the correspondent.
Therefore, it was decided to create a separate measurement app based on the same principle of IFSK modulation, which will receive a reference signal, similar to that emitted by the HF Pager. Thus, such an app was developed and called the IFSK tracker. It works in the Android environment, which allows you to use it on a smartphone or tablet. The measurement principle is shown in fig. 1
The beacon consists of a generator of reference audio signals (a smartphone or a PC based) and a radio transceiver. It is set up at point A and connected to the transmitting antenna.
The reference signal is generated by the smartphone using the IFSKbeacon app
The signal from the sound card output is fed to the DATA connector of the transceiver in SSB mode or the microphone input in FM mode via an additional resistive divider.
Before starting the movement, the beacon must be started in a specific mode, so the app must be launched.
Go to the settings menu and set the following parameters:
- the baud rate should be set depending on the complexity of the communication regarding radio interference or the extended distance of the route.
- the audible frequency of transmission can be adjusted to personal preference, HF Pager should ideally be operated with the same setting.
- the volume level should be adjusted regarding to VOX sensitivity and the transmitter's ALC depth.
- the interval should be set to continuous in case of measuring the radio path in motion. If you intend to take long-term measurements (for example, passing over several days), tick the "Add CW-ID" box to amend the call sign encoded with Morse code at the end of each transmission.
For periodic transmission modes, the following options are available:
Each period will begin in minutes that are multiple of the above. For example: 00-15-30-45 or 00-20-40 minutes. The duration of each period is 5 minutes.
If it is required to measure the quality of radio communication along any route, then the Continuous radiation mode is set.
The receiver with a smartphone or a tablet connected to it is installed in a car or other vehicle (ATV, sled, boat).
On the receiving side, launch the IFSKtracker app on the measuring smartphone or tablet and apply the following settings:
The configuration options should be clear to any radio engineer, but I'll only note some important details:
- the baud rate must be the same as in the generator
- the reception frequency is selected according to the frequency of the beacon, for example, 1700 Hz, but if the measurements are made with two signals, it recommended to provide some distance of about 500 Hz or more between them, more on that later
- by default, it is customary to transmit with the upper sideband, so the box shouldn't be ticked, but if your transmitter only supports lower sideband of the LSB, then you need to tick it
- before starting measuring, do not forget to turn on your GPS receiver and set the coordinates of the beacon, otherwise, the distance will not be calculated.
- the brightness of the waterfall is selected based on the radio interference and the strength of the radio signal.
- some devices sometimes refuse to save GPS coordinates when the screen is off, so there is an option to keep the screen always on. It must be noted that the battery is drained much faster with the backlight on.
After setting the radio frequency and selecting the modulation, the speaker of the transceiver should emit a distinctive sweeping sound increasing with USB or falling in tones (LSB), and the app's waterfall should look like this:
On the blue background, "the waterfall", in the area of the left gap between two marks, our reference signal can be seen.
The green bar indicates the relative level of the input signal.
Yellow meander signifies synchronization.
A line of running "ones" signifies a correctly decoded package. If decoding fails, any other symbols may appear in their place. If you can see a bold line with the letter D, you made a mistake when choosing the sideband.
Line with the number "1" shows the parameters for the first sound frequency, namely:
- the rate of the measurements is 5.86 baud, the signal-to-noise ratio is +24.2 dB, the noise level is -47.6 dB, the error rate is 0%.
If everything looks good, select "Start recording to file" from the menu and you are ready to go!
It won't hurt to practice with some test files first and make sure that everything is working as it should.
Maybe the frequency may be reset to default (it is better to lock it), or the smartphone connector was broken. Or the satellite signal is being blocked by something. Maybe the GPS isn't working when the screen is not on?
After taking measurements, select "Stop Recording" from the menu and return to the main screen, where you will see the string
and the last saved file should appear, for example:
where the number represents UNIX time
To view a graph, long-press the file name to the graphs menu and select the corresponding graph, then select "Graphs by distance":
Based on the same principle, a measurement program, but with a wider band of testing and received signals, it allows you to evaluate the quality of the radio path and the transmission for voice radio communications, for example in SSB, FM, or AM modulations.
In this case, the beacon transmits an IFSK-modulated signal at a speed of 11.72 baud in the 450-2500 Hz band, which corresponds to the spectrum of a human voice, sufficient for voice transmission.
Two-ray propagation measurement
Before, we only considered the option with just one sound reference signal, which is labeled as "Receiving Frequency 1" in the settings.
However, our tool is capable of receiving two sound signals simultaneously!
Therefore, if you install a second beacon somewhere nearby of the first one and connect it to a separate antenna, for example, with a different polarization, you will be able to check the difference in the distribution of these two polarizations on the radio path at the same time.
The only thing you need to do is setting the second sound frequency on the new beacon in the line "Receive Frequency 2", for example 2200 Hz.
The radiofrequency and sidebands of both transmitters should remain the same.