Radiation Polarisation page 2

4.1) Rotation Measurement

  As it is now established that there is definitely an upwards and a downwards polarisation an investigation into gain and phase around a complete revolution is performed in order to understand better the effects of different polarisations. The transmit antenna is placed at the same height and in the far field from the Rx and simply revolved at right angles to the Rx antenna as shown in figure 2. This arrangement ensures that the distance between the antennas does not vary and any changes will be mainly due to polarisation. As the rotated (Tx) antenna does not have a ground plane, there will be variations in it's return loss and this will cause small variations in gain measurement accuracy but they are not sufficient to obscure the general characteristics measured. Gating with the VNA is required and the gate times can be kept constant because the distance between the antennas does not vary.

Figure 2 Alignment of Antennas

[Drawing of two aligned antennas]

4.2) Rotation Results

  The results obtained from one series of measurements are shown in figures 3 & 4. The degree markings around the perimeter of the polar display represent the physical orientation of the antenna, with the axial scale representing level in dB and signal phase in figures 3 & 4 respectively.

Figure 3 Gain

[Polar S21 plot of rotating antenna]


Figure 4 Phase

[Polar phase plot of rotating antenna]


4.3) Rotation Analysis

  From the above results, it is possible to come to three simple conclusions.

  1) There are gain minimums with the antenna physical position at 60° and 300°. This approximately agrees with the conventional understanding of there being lower gain with one antenna in the horizontal polarisation and one in the vertical polarisation. However, it would be expected that these nulls would appear at the exact angles of 90° & 270° and the reason that they do not appear at these precise angles will become apparent in the later article on wave tilt.

  2) There is a jump in signal phase when the antenna physical positions are moved between 60° and 90° due to a distinct preference for the two antennas to be in opposite orientations. Note that the shift of phase from -180° to +180° with the antenna returned to the upward position is due to the phase completing one whole rotation and is a familiar effect to experienced users of the VNA but does not represent a real shift of phase.

  3) There is noticeably greater gain with Tx and Rx antenna in opposite orientations rather than similar orientation (this varied between 3.6 dB and 4.6 dB in various measurements) and it is demonstrated graphically in figure 3 that there is a preference for the polarisation of the two antennas to be in the opposite directions. The difference in gain is of course related to the different INCIDENT and REFLECTED powers in the Tx antenna but it is not directly proportional to the difference of the original powers. The reasons for this power difference will become apparent when wave tilt and radiation mechanism is studied in a later articles.

5.1) Swept Frequency Phase Measurement

  Now that it can be seen that physically rotating the Tx antenna 180° causes an opposite radiation polarisation, there rises the question as to how this happens. One possibility is that the current induced in the Rx antenna travels in the opposite direction and therefore goes towards the end of the antenna and is reflected back resulting in a longer distance traveled and consequentially greater re-radiation. At the matched frequency, the antenna has a phase shift of 90° in one direction, giving a total possible phase shift of 180°. If this change of current direction were the case, then the difference between an upward and a downward antenna would only be in the order of 180° at the matched frequency of the Rx antenna. Figure 4 shows the normalised result of a swept frequency (gated) phase measurement with the Tx antenna in the upwards and downwards positions and there are no observable deviations of change of phase around the matched frequency, thus demonstrating that current direction is not the cause of the 180° phase shift.

Figure 5

[Graph of swept frequency phase measurement]


5.2) Swept Frequency Analysis

  It can be seen that there is an almost constant phase difference between the upwards position and the downwards position of the antenna across the frequency rage of 0.3 MHz to 500 MHz. Thus proving that the phase difference is frequency and antenna length independent. It can be concluded therefore, that the polarisation of the greater radiation received from the Tx antenna dictates the polarity of the current induced in the Rx antenna.

6.1) Conclusions

  It has been firmly established in the diametrically opposed antenna measurement, the antenna rotation measurement and the swept frequency phase measurement that there is 180° phase difference between the received signals from an upward antenna and a downward antenna regardless of the frequency of operation. It has also been shown that the gain between antennas can be greatly reduced if there is a polarisation mismatch or a polarisation cancellation.

  It can now be deduced how the dipole antenna shown in the Forward manages to transmit two opposite phases without there being a total cancellation. As there is already a 180° phase difference between the two radiations from there being an upward antenna and a downward antenna (causing opposite polarisations), as well as a 180° phase difference between the two transmit voltages resulting in a final sum of the two radiations in the receive antenna. Therefore the omission of radiation polarisation in figure 1 in the Forward is one of the sources of confusion in the understanding of dipole antennas.

  This all suggests that the polarisation of the radiated signal cannot always be ignored. It may not be important with two intercommunicating simple monopole antennas where electrical polarisation matching theory is adequate but when either of the antennas becomes more complex or the behaviour of the antenna needs to be understood in detail, then the comprehension of the radiated polarisation can make a major difference.


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First Published by William J Highton on 23/7/2015

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