I am afraid today’s post will be disturbing to some readers. We are going to destroy several myths about loop antenna polarization.
So, as you know, I am a big fan of wideband active receiving loops. Love them. I have been comparing magnetic loop performance to my traditional ham antennas (dipole, beam). They stack up really well.
We tend to think of near-ground vertically mounted loops as having sharp nulls and vertical polarization. As you will see, this is only partly true. I decided to explore these myths by modeling patterns for a one meter loop, vertically oriented, six feet above ground. Examples are shown above. Two dramatic results follow.
First, as shown in the top row above, loop antenna polarization is mainly vertical (red line) only for low angle radiation. As incoming signals have higher angles of arrival, your ground mounted vertical loop picks up more horizontal polarization (blue line). When you add these two patterns together, the null disappears and you have a nearly omnidirectional antenna.
Keep in mind this is for far-field propagation. You still get magnetic loop benefits for nulling nearby interference.
Second, in addition to angle-of-arrival variation, your small loop antenna polarization depends on how small it is, relative to your frequency. The ratio of loop circumference (C) to wavelength (λ) is a huge factor.
Loop Antenna Polarization – Size Matters
Check out the lower half of the above diagram. When your loop has a C/λ ratio of 1%, pattern is almost pure vertical polarization (lower left above). For a one meter loop, this is in the MW broadcast band. At 10% or 10 MHz, less so, but still advantage vertical by around 2:1. By the time you reach upper shortwave (30% or 30 MHz) your loop is about equal in both polarizations.
So, to recap, your small loop antenna polarization is indeed mostly vertical for very low angle-of-arrival and very small size relative to wavelength. But as frequency or angle-of-arrival increases, you get mixed polarization. So, generally, think of your ground-mounted vertical small loop as mostly omnidirectional in the far field.
I wonder if the modeling takes into account the ground reflection being some unknown distance beneath the earth. Wouldn’t the effective height be higher?
Good question, Guy. I did the modeling for a one meter loop with the base (feedpoint) at six feet above a real (average) ground.