Analyzing polar plots
Usually, there will be just a handful of harmonics that are over the limit during RE testing. By rotating the product at each of these frequencies and generating a polar plot of emissions, we can tell quite a lot about the potential sources. In addition, potential fixes may be tried and then the polar plot re-run to compare whether the source was identified and fixed.
Figure 1 - Products under test can include very complex I/O cabling arrangements.
Of course, it helps to understand the typical radiation patterns from antennas and any good antenna book will show typical polar plots, given the antenna type. Good resources might include Krauss's Antenna Handbook or the ARRL Radio Amateur's Handbook.
Let's look at a typical polar plot from a real product.
Figure 2 - A typical polar plot at frequencies below 300 MHz. Note the characteristic dipole plot.
Examining the figure, visualize this as the top view with the product facing up towards the top of the plot. Note that the shape of this plot, taken at 216 MHz closely resembles the traditional dipole "figure of eight", with major lobes side to side. If you were to compare this to the radiation plot for a tuned dipole antenna, we would deduce that the dipole elements are located linearly top to bottom (that is, front to rear on the product), thus creating side to side lobes.
If I were analyzing this plot, I would realize that because of the side to side lobes, I would suspect there were two dipole elements front to rear on the product. This could be a pair of I/O cables (front and rear) or even the front panel and rear enclosure, if they aren't well bonded to each other. By removing cables or applying copper tape for improved bonding, then re-running the plot, we may be able to confirm the problem.
As a second example, Figure 3 shows a polar plot at a much higher frequency of 848 MHz. Notice the number of lobes has increased dramatically. This type of plot is rather typical for frequencies above 300 MHz, but is much harder to interpret. this type of plot could be compared with the multiple lobes patterns from antenna structures longer than a half-wavelength or possibly multiple sources, such as a phased array antenna.The multiple lobes are generally the result of multiple cable emissions, multiple slot emissions, large apertures or a combination of all.
Figure 3 - A typical polar plot at frequencies above 300 MHz. Note the characteristic multi-lobed plot.
In troubleshooting multiple lobed responses like this, I often identify the angle of the largest lobe and try to determine what could be the source at that angle. In this case, most of the field level is coming from near the front of the product and I might be suspicious of an aperture or seam in the front panel. Sometimes it's better to first probe the enclosure seams with a near-field probe to identify potential leakages and copper-tape over suspect seams. if this doesn't help, then you can turn your attention to cables or other large apertures.
Hopefully, this technique provides you just one more tool to help identify potential emission sources.