A series of three articles explored different wireless solutions for remote operation of a NanoVNA-H4.
The NanoVNA-H4 can use a USB connection, or a serial connection, the UART, selected from the configuration menu. The UART ‘channel’ is a conventional asynchronous protocol, 8N1, and supports a range of DTE speeds. Importantly, it does not incorporate:
- any form of flow control; or
- any form of error detection / correction.
Essentially, any form of wireless channel were the actual throughput is less than the DTE speed is likely to experience buffer overruns, errors.
For my use, direct capture of screens is very important. It is supported, but the RGB565 format used means a 307KB transfer for the -H4. This takes nearly 1min on an error free channel with throughput of 57600bps… so it is very slow.
The unprotected link problem will result in an unreliable communications link, though a reliable radio link of sufficient bandwidth might be reasonably successful.
Three possible solutions were tried:
- Bluetooth using HC-05 module;
- 433MHz data link using HC-12 modules; and
- SiK data radio.
Tests were conducted at DTE speed 57600bps unless stated otherwise.
Bluetooth using HC-05 module
The Bluetooth solution actually worked. Air speed is greater than the DTE speed used, and even at 115200bps DTE speed, data overruns were not apparent, throughput close to the theoretical 80% of 115200bps DTE speed (due to async encoding). Transfer time for a 307kB screen capture is just under 30s.
This appears to be a workable solution, though with a distance limitation typically of 10-30m depending on path obstruction.
Another advantage is that the radio frequencies used are at 2.4GHz, well away from the NanoVNA-H4’s measurement range.
433MHz data link using HC-12 modules
The HC-12 modules are a simple single channel data radio with a range of speed and bandwidths. The air speed is automatically chosen based on the DTE speed.
Distances of 1km or more are possible, though probably at lower data speed.
The problem is that at 57600DTE speed, throughput was around 20kbps for a very short distance at full power.
Since the throughput was way less than the DTE speed, in the absence of flow control this will suffer intolerable errors due to buffer overruns.
It is deemed unsuitable.
SiK data radio
The SiK data radio is designed for use with UAVs. It is a FHSS technology. It has the potential to work over distances up to 1km or more, depending on the configured transmit power.
The measured throughput was around 22kbps.
Since the throughput was way less than the DTE speed, in the absence of flow control this will suffer intolerable errors due to buffer overruns.
It is deemed unsuitable.
Bluetooth+improvements
The transfer time for screen capture would be improved if the user was offered an uncompressed TIFF format (which is already supported for SD card saves). The saving is 60-80% of download time, so a screen capture would take say 30% of 30s, so just under 10s for an average screen capture.
If the NanoVNA calculated a CRC and sent it at the end of the capture transfer, the receiving application could check the integrity of the transfer and the operator would know on the spot whether it need to be taken again.
An enhancement request was submitted on github: Enhancement: capturetiff command.
Time will tell whether that results in these improvements.