Vector measurements with the rtl-sdr, 7

16.10.2020 18:44

Looking back at my last few posts I feel that I was going into increasingly theoretic guesswork about the impedance mismatches in my rtl-sdr measurement setup. Even though I still think it could perform better, I would like to share some actual antenna measurements I made with it. I've also now got my hands on a NanoVNA-H, one of the many variants of the popular low-cost vector network analyzer. It's interesting to compare the measurement results between the two instruments.

The collection of antennas I've measured.

Yesterday I've measured the VSWR of a few compact LTE antennas. These antennas have an adhesive backing and are meant to be mounted into mobile devices that use the cellular network for connectivity. All of these antennas come with a coax cable tail that terminates with a female U.FL connector. This was a bit problematic since both my instruments terminate in a SMA connector. So to make a connection I've used a U.FL-to-U.FL "thru" on a RF demo kit PCB I got with my NanoVNA and another short U.FL-to-SMA patch cable.

I've calibrated both my rtl-sdr setup (vect-meas-mux in the following) and the NanoVNA with the short-open-load one port method. I've used the U.FL calibration standards on the same RF demo kit. This resulted in the calibrated measurement plane on the U.FL end of the U.FL-to-SMA patch cable. Hence the measurements include effects of the "thru" and the coax cable tail that was attached to each antenna.

Calibrating the reflection measurement setup.

The vect-meas-mux setup consisted of the ERASynth Micro, rtl-sdr, my custom multiplex board and the Transverters RF bridge as the directional element. It allows measurements up to 2000 MHz, with a gap around 1200 MHz, defined by the rtl-sdr's tuning range. It's the same setup as I described in the past blog posts in this series. The NanoVNA-H was bought from Nooelec and the particular version I have is no longer listed on their website. It shows firmware version 0.4.5-1 and allows measurements up to 1500 MHz. I transferred the S11 measurements from the NanoVNA to the computer using code based on this example notebook and then calculated VSWR from S11.

In the graphs below, the VSWR measured with vect-meas-mux (orange) and NanoVNA (blue) is compared to the VSWR given in the datasheet (black) of the respective antenna. I was mostly interested in how both instruments agree with each other and how well the results were repeatable. The datasheet curve is there as a reference. I wasn't expecting to get a good match. I took care to have a reasonably clean measurement environment: as much open space in all directions around the antenna as possible, antennas mounted on a non-conductive plastic holder, etc. However obviously that can't compare with a measurement in an anechoic chamber and random objects in the vicinity were affecting the results. I also didn't use a choke on the coax tail to prevent it from radiating.

VSWR measurement results for 2JF0624P antenna.

VSWR measurement results for 2JF0924P antenna.

VSWR measurement results for 2JP0424P antenna.

VSWR measurement results for SRFC015 antenna.

I'm pretty happy with how these measurements turned out. The results from my own instrument match those from the NanoVNA quite well. The agreement with the datasheet is often reasonably good as well although, as I said before, that's probably due to my poorly improvised environment around the antenna.

After doing many measurements the main problem with the rtl-sdr method turned out to be its slowness. NanoVNA displays the result nearly instantaneously. On the other hand a 200-point sweep from 55 to 2000 MHz with vect-meas-mux takes around 7 minutes and 30 seconds. Most of this is due to hugely inefficient way I collect signal samples from the rtl-sdr: I run the rtl_sdr binary once for each of the 200 frequency points. This results in the setup and tear down time of the rtl_sdr binary dominating the total run time. I only need to dwell on each point for about 100 ms, which would mean a sweep time of about 20 seconds. I'll probably rewrite my code to use the rtl_tcp server to fetch the I/Q samples, which should reduce the overhead significantly.

In the future I would also like to be able to estimate the error intervals of the measurement, like I did with my scalar experiments. Because this was a vector measurement and I used calibration, the relatively poor directivity of my RF bridge affected the results much less than with scalar measurements. A bigger factor was probably the limited dynamic range of the rtl-sdr. scikit-rf has some support for estimating error intervals, but I still need to figure out how to integrate that with my code.

I'm also looking for extending the frequency range of my measurements, but that will require switching to a more expensive SDR. Perhaps something like the HackRF or an USRP.

Posted by Tomaž | Categories: Analog

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