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Task: Path loss modelling

Target frequency: 433 MHz;

Laboratory environment: in a basement where there were other equipment and metal shelves. A few acoustic absorbers were placed around the setup.

Scenario 1

  • TX antenna sealed in a small bottle (diameter 3 cm, length 10 cm)
  • TX antenna moving inside 50 L of a tissue-simulating liquid (held in a basin);
  • RX antenna fixed outside of the basin;
  • Both are connected to a Network Analyzer (very old, but calibrated before each measurement) that recorded the S parameters of different TX-RX distances;
  • Addition: The coaxial cables seemed to be very sensitive. A gentle twist could result in big changes in the S-parameters' curves in the frequency domain.

Scenario 2

  • TX antenna integrated to an electronic system (MCU+RF module+battery)

  • The whole electronic system was sealed in the small bottle (diameter 3 cm, length 10 cm), placed inside the aforementioned basin, and moved along the same trail as in Scenario 1;

  • RX antenna fixed outside of the basin and connected to a Spectrum Analyzer that recorded the received signal level at the same TX-RX separation distances as in Scenario 1;

  • S21 is regarded as Tranmitter power level - Received signal level - cable loss;

Illustration of the setup

enter image description here

Expected results

S21 increases (generally) as TX-RX distance increases

Actual results

Scenario 1: S21 against distance fluctuated within a small range (the measurement was repeated many times) enter image description here

S11 against frequency and S22 against frequency were normal reflection curves, and the values were reasonable.

Scenario 2: S21 increased with TX-RX distance enter image description here

Questions:

What could be the causes then? Is that because the network analyzer is too sensitive to the noises in the environment?

(The network analyzer was supposed to work well as other people were using it for measurement as well. )

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I would suspect that the primary reason for the seemingly odd results is that you are likely operating the experiment in the near field of the antennas. As your distance changes, the mutual coupling of the antennas changes.

As mutual coupling changes, antenna efficiencies, impedances, S11 and S22 parameters will change as well. You can execute experiments to confirm these effects. The result of this, however, is that your experiment has not isolated path loss but in fact has commingled other factors in the S21 measurement.

If you execute the test with at least 1 wavelength (i.e. > 0.7 meters) between the antennas, you should see more predictable results.

Given the lab environment, some of the observations could also be due to reflections of RF from nearby objects.

You will also need to take care that your transmission lines do not carry common mode currents as this will negatively impact the results. 1:1 choking baluns, or continuous ferrite sleeves consisting of the appropriate material, on double shielded coax is recommended.

As far as the effect of the lossy medium goes, the effect can be modeled as a loss within the transmitting antenna system as long as the medium is a homogeneous, non conducting medium (e.g. a dielectric). Thus the path loss, once outside of the medium and in the far fields of the antenna, will follow a classic free space path loss (FSPL) model.

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  • $\begingroup$ I have added some key facts: 1) TX antenna or TX was in a lossy medium 2) They were sealed in a small bottle. Given the existence of air inside the bottle, the effective wavelength would be shorter than 70 cm. $\endgroup$
    – luw
    Jan 17, 2019 at 17:31
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    $\begingroup$ @luw The wavelength in air is ~70 cm. The medium will likely shorten this wavelength a bit but only for the effective thickness of the medium. I added a bit more to my answer. $\endgroup$
    – Glenn W9IQ
    Jan 17, 2019 at 17:50
  • $\begingroup$ A further question: how to estimate the effective wavelength? In this case, the wavelength inside the lossy medium is around 10 cm. Besides, the air in the bottle should also be considered. Thereby, is it possible that the effective wavelength is much smaller than the thickness of the lossy medium (which is < 70 cm)? $\endgroup$
    – luw
    Jan 17, 2019 at 20:01
  • $\begingroup$ another fact was added to the original question: the coaxial cables used in scenario 1 (VNA) were very sensitive to its vicinity. $\endgroup$
    – luw
    Jan 17, 2019 at 20:51
  • $\begingroup$ @luw The wavelength is a function of relative permittivity. Permittivity also determines capacitance. Make two parallel plates and measure the capacitance with air between the plate. Then measure the capacitance with your medium as the dielectric. Compute the relative permittivity from that. $\endgroup$
    – Glenn W9IQ
    Jan 18, 2019 at 0:23

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