Elevation of Antenna Radiation page 2

2.1) Measurements

The measurements are taken from the 0° position at 45° intervals along the height of the antenna and above it. With an incremented spacing from the antenna to the flat sheet antenna in order to build a two dimensional image of radiation. The horizontal spacing used for figure 5 was from 360 mm to 1,800 mm distance from the antenna in 360 mm steps and the colours in the left chart used are for a temperature scale chart (going from blue for cold/low to red for hot/high). The data in the right chart represents lin mag readings from the VNA in mU. The antenna being measured is shown as a very dark red line at the bottom left hand hand side. As the measured antenna was placed up on a grounded bench, it was possible to centre the sheet antenna at the bottom of the sample antenna (AUT) for the lowest measurements.

 

Figure 5.

[Chart of radiation levels]

3.2 5.4 4.8 4.8 4.3
5.9 6.6 6 5.4 4.4
11.3 8.8 7 5.2 4.4
17.3 10.2 7.1 4.9 4.2
24.9 12.2 7.4 4.9 3.6
22.2 11.2 6.9 4.7 3.5
16 9 6.6 4.6 3.7

 


 


 

 

 

 

 

 

3.1) Analysis

 It can be seen from figures 5 that the radiation is greatest near the end of the antenna and tends to travel upward and outwards. This again demonstrates that the radiation direction is dominated by the greater upward (INCIDENT) current.  It may be that the results obtained for the points above the antenna have some inaccuracy due to the changing angle of radiation incidence as the flat sheet antenna was raised, particularly when it was very close to the AUT. Presumably there would have been higher reading levels if this had been corrected. There is also the problem that the cable from the flat sheet antenna passes through the area of high radiation to reach the upper measurement points of low radiation levels and there could be significant radiation picked up by the cable screen in relation to the radiation detected by the flat sheet antenna. High quality cables with good isolation are therefore essential for these measurements.

3.2) Conclusions

It is clear that the general upward direction of radiation is far greater than described in text books where doughnut shaped diagrams are shown that have presumably been generated with accepted formulas using sinusoidal current distribution and not by actual radiation measurements. The angle of the main radiation from the antenna does appear to be in the region of 17.5° as per GLASGOW'S  statement but an accurate measurement was not attempted because of the limited height of the lab ceiling. The real behaviour of radiation from a well matched, grounded monopole antenna is starting to appear from these measurements. It is now becoming obvious that the existing antenna theory does not match the true performance of this particular type of antenna very well at all.

 

 

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

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