Far field radiation pattern in free space @28MHz for circular polarization
Using EZNEC4+ it is easy to measure the circular polarization which pops up when a magnetic loop is no longer small compared to the wavelength. And EZNEC4+ also shows the lack of circular polarization when the Alford loop is used instead.
On the left side are the results for the vertical medium size magnetic loop, on the right side the results for the vertical magnetic Alford loop. Upper plots are at 45 deg. and the lower plots at 0 degrees azimuth angles. Both loops are 1.3*1.3mtr.
The medium size loop shows clearly full circular polarization at 45 deg. and the Alford loop shows no circular polarization at all. Remember that ´zero´ degrees is perpendicular to the loop.
For QRM reduction the near field is of more importance.
To show the effect in the near field I wrote a ´matlab´ routine. This routine makes the near field visible in 3-D (EZNEC only makes 3-D plots of the far field).
The colour in the figures below indicates the amount of circular polarization: blue is no circular polarisation, red is full circular polarisation.
The distance to origin is a relative measure of the field strength (linear scale !).
The axis give the distance at which the field is measured, so no info about field strength.
Remember we see the near field, so ground wave effects are included.
Left column is the medium size magnetic loop, the right column the Alford loop.
Near field measured at 10, 20 and 100 meters distances.
The Alford loop shows a very small spot with circular polarization caused by ground reflections.
At close (QRM) distances the medium size loop shows very much circular polarization.
The amount of circular polarization decreases with distance. The ground wave horizontal component of the field decreases with distance and only the vertical component remains.
Last update: September 24, 2006