Medium size vertical magnetic loop
In free space
When the size of the loop is not very small compared to the wavelength, then the loop doesn’t behave any more like a small loop. As with the folded dipole one can distinguish two modes. Talking about two modes makes it easier to understand what is going on.
The two modes in a folded dipole.
When we “open” the folded dipole we get the magnetic loop. In the loop the transmission line mode is called the loop mode. The antenna mode we call the dipole mode of the loop.
For a medium size loop the dipole mode can not be neglected. It contributes significantly to the radiation pattern. The big difference is the horizontal polarised field that comes up on the Y-axis.
In the next figure the radiation pattern in the YZ-plane (horizontal polarization) is shown for a 1.3 ´ 1.3 meter square loop.
A 1.3 * 1.3 meter square loop shows on 10.1MHz only a -10dB nulling dip!
So in the XY-plane (at the higher frequencies) there is not only vertical polarization, but now also a horizontal polarised component.
Important to remember is that the vertical polarised component in the XY-plane is made by the loop mode and the horizontal polarised component by the dipole mode.
The result is that both component are 90 degrees out of phase !
In other words: elliptical (circular) polarization.
In EZNEC it is easy simulated. Place the loop (free space) in the origin and oriented 45 degrees to the X and Y-axis. Then calculate the ´near field´ (including phase) on the X or Y-axis at a far distance (100mtr) from the origin. On the Y-axis the Ey component is almost zero as expected. At the higher frequencies the Ez and Ex are about 90 degrees out of phase and are about equal in magnitude.
What are the consequences for the reduction of QRM at the higher frequencies?
Almost all noise sources are linear polarised !
When the noise source is pure horizontal or pure vertical polarised the nulls are very deep on the X-axis or Y-axis respectively. This is the same for the medium size and small size loop.
But when both components (vertical and horizontal) are present we can not get a deep null because of the elliptical polarization. When both components are equal in magnitude the maximum reduction (null) is only -3dB.
Last update: September 24, 2006