Antenna Near Field Radiation page 2

2.1) Radiation Upwards

  The measurements are taken at 10° intervals along the height of the antenna and also with an incremented spacing from the antenna to the probe measurement plate in order to build a two dimensional image of radiation. The horizontal spacing used for figure 4 was from 2 mm to 62 mm distance from the antenna in 15 mm steps and the colours used in the left chart are for a temperature scale (going from blue for cold/low to red for hot/high). The data in the right chart represents linear magnitude readings from the VNA in mU. The antenna is shown as a very dark red line at the bottom left hand side in both charts.


Figure 4. Chart of radiation upwards.

[Chart of upward radiation levels]


  It would be expected from the large upward (incident) current that there would be more upward radiation at the base of the antenna than at the top (end) but this is not the case. The upward radiation in fact builds up as it travels up the antenna and reaches a peak at the very top. The radiation appears to stay very close to the antenna and pass through the gap between the probe and the antenna surface. Virtually skipping along the antenna surface and cumulating in a 'hot spot' at the very end of the antenna. It is interesting to note that the increase in level going up the antenna fits the curve for the small plate used in the previous current distribution measurement and suggests that the error in the toroidal ferrite probe measurement was mainly due to radiation. The logic for this being that the measurement plate being used in this radiation measurement (see figure 1) is at right angles to the antenna and would therefore not be very susceptible to electric field effects. It follows that the error seen in in the antenna voltage distribution curve in relation to the top two samples in antenna-voltage-power-and-impedance-article2.htm Figure 8 is also probably due to radiation.

There also exits some small radiation at the base of the antenna which may be regarded by some experienced antenna engineers as a side lobe and its presence will be easier to understand once wave tilt and polarisation have been studied in later articles.


2.2) Radiation Downwards

  The measurements are again taken at 10° intervals and 15 mm incremental spacing from the antenna to build a two dimensional image of radiation. The spacing used for figure 5 was also from 2 mm to 62 mm and the colours are again for a temperature scale chart.


Figure 5. Charts of radiation downward

[Chart of downward radiation levels]


  It can be seen that there is a lower magnitude of radiation downwards and that it originates from just above the centre of the antenna. There is no sign of a side lobe this time and it is not obvious from the data collected so far to establish exactly what is happening.

3.1) Analysis

  It was proven previously in the antenna current distribution article that the greater current within the antenna is the INCIDENT current which is going UP the antenna. It can be seen from figures 4 and 5 that the greater radiation is in the upward direction and therefore the upward radiation is obviously related to the upward (INCIDENT) current and that the downward radiation is related to the downward (REFLECTED) current. Both the upwards and downwards currents produce radiation which appears to travel very near the surface of the antenna until they meet radiation coming from the opposite direction; where they then radiate outwards with a level related to the lower level of the two radiations. It is becoming clear that the upwards and downwards radiations have to first be treated separately in order to understand and predict the overall radiation.

3.2) Conclusion

  The near field radiation from a grounded monopole antenna is complex to understand and with the data collected in this article there is insufficient information to make an accurate assessment of what is happening. As already stated, more will be revealed in the articles on wave tilt and polarisation, particularly with a more detailed analysis in the latter article on wave tilt.

  This basic measurement method could be extended to other antenna types and could possibly enable an approximate picture of current distribution to be built up for structures that do no lend themselves to the current transformer method of measurement i.e. a discone made from sheet metal.

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First Published by William J Highton on 16/7/2015

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