Simple microwave distance radar from nanovna TDR function

[neazoi]

Hi, a project calls for a simple microwave radar to measure relative distance of moving objects that are up to 15-20m apart (maximum).

A thought that I had, was to use the nanovna TDR function, as shown in this video

However, instead of connecting a coaxial cable, I would connect an antenna directly at the nanovna port and set the velocity factor to 1 (air).

Will this setup work as a simple distance measurement radar in my case?

 

[FvM]

It's basically a FMCW radar. Not sure if NanoVNA TDR function is suitable for moving targets, should work at least for slow moving objects with sufficient radar cross section.

 

[neazoi]

It's basically a FMCW radar. Not sure if NanoVNA TDR function is suitable for moving targets, should work at least for slow moving objects with sufficient radar cross section.

Humans, it is basically for humans at a distance of 10-20m at max.
Thanks so much for the valuable information.
I thought it was working in the pulse method, because in my mind reflectometers output a pulse and wait for a return pulse, then measure the time the pulse has taken to arrive back to the source, taking the velocity factor into account.
I can see from the video that he is using a start and end frequency, but I thought that this has something to do with something else. So you say this sweeps actually the frequency range?
If that is the case wideband antennas like Vivaldi must be used.

 

[FvM]

Yes, wideband antenna is needed. TDR display is provided by some VNA to support e.g. cable diagnosis. It's a pure software feature (kind of inverse FFT).

 

[vfone]

I do have a NanoVNA and I set it for a 10m TDR reflection measurement and connected a 5m coax cable. The VNA TDR measurement indicated a length of 5.05m, which is pretty close to what it is.
I replaced the coax cable with a high-gain antenna and pointed it to a wall placed at 5m. I changed the scale for maximum dynamic range and the port output power to maximum but nothing was shown on the display, which make sense because the sensitivity of the VNA receiver is way below to what should be for a over the air "radar like" test.
A normal FMCW radar receiver have a sensitivity better than -100dBm (even down to -130dBm in some cases).

 

[D.A.(Tony)Stewart]

It can work but may be overkill. For 20~30 m max all you need is <=200 MHz for nanoVNA
Using a well-tuned antenna for return loss >> 20 dB you can easily detect reflections with a splitter to measure reflections with a fixed Tx and suitable RF diode and DMM or signal conditioner to detect mV of reflections from DC to motion rate. Tune antenna for null reflections.

I recall circa 1977, when i was tuning length of dipole for a sounding rocket using VHF, I could detect people moving at the far end of the lab. Even if higher than have wave length. As I thought to myself, this would make a great motion security detector. Years later I found companies doing that. So you may find cheap solutions in the alarm or IR motion detector business which can work to 20 m easily.

Then choose desired beam-pattern and non-interfering* Tx channel to optimize gain, loss diversity. Even add FM music to it if you want. lol.

Then count the ships getting close or going by with a suitable counter with signal conditioning.

* as in won't get you in trouble with coast guard or regulatory agencies.

 

[FvM]

I wasn't aware of rather limited NanoVNA V1 performance when I wrote in post #2 it should work. You need to consider at least V2 plus 4 (4 GHz, 90 dB dynamic).

Another point is that you can't legally operate a FMCW radar in the low GHz range, because there's no free band with sufficient sweep bandwidth. Available FMCW solutions are using 24 and 77 GHz band.

Motion detectors can operate at 2.4 and 10.6 GHz, but they don't provide distance discrimination that you probably want.

 

[vfone]

Just to measure the distance to an object an FMCW radar don't need wide bandwidth.
Wide bandwidth is necessary for range resolution (to be able to distinguish between two close objects). The bandwidth also limits the maximum distance where an object can be detected.
So with a NanoVNA can use the ISM 2.4GHz band, or the 5.6GHz band if have a 6GHz LiteVNA.

 

[volker@muehlhaus]

Just to measure the distance to an object an FMCW radar don't need wide bandwidth.

The FMCW radar designers that I know go for high bandwidth only for one purpose: resolution

See here
"The range resolution of FMCW radar is proportional to the reciprocal of the bandwidth, i.e. delta x = c / (2 * delta f)."

In the 2.4 GHz ISM band we have 100 MHz bandwidth, that would give 1.5m resolution which is pretty bad.

 

[vfone]

Yes, but the main reason the radar was invented (and still exist) is to measure the distance to a target. Resolution is an extra requirement.
Now depends for what distance the 1.5m resolution is about. If have 1.5m resolution at few kilometers distance, is not so bad.

 

[D.A.(Tony)Stewart]

The specs for {radial, angular} Range/Resolution * diversity, size, reflectivity, sweep/step(time, BW} , SNR, are all tradeoffs, but defining all the object and user interface requirements ought to be done 1st and not just how many objects can we see with a range/resolution ratio of 20/1.5 with a requirement of at least 1 per tbd seconds.