View Single Post
Old January 27th, 2010
Nine_c1's Avatar
Nine_c1 Nine_c1 is offline
Armchair Expert
Join Date: Jul 2009
Posts: 2,577
Default Polarization of Radar Waves

While Radar Polarity has been discussed before I think it is something that still is not very well understood by some enthusiasts. In fact, some of the very knowledgeable and technically astute still get this backwards or more correctly.......90 degrees off!

Most of us know that there are basically three types of polarization used in police radar.......Vertical, Horizontal and Circular. The confusion begins when we incorrectly apply the orientation of the radiated radar waves to the polarity of the signal.

Radar waves are more correctly known as Electromagnetic waves or Electromagnetic radiation. All Electromagnetic waves have two components, the Electric Field and the Magnetic Field which are two sinusoidal waves that oscillate in phase but perpendicular to each other, or at 90 degrees.

Polarization of any radar antenna or signal is always given in reference to it's Electric Field and NOT the magnetic field.

To explain the concept it's easiest to think of a pool of water. Drop a stone into the center of the pool and you get concentric waves that ripple outward to the edge of the pool. This would be a perfect example of how a vertically polarized radar works. The Electric Field is the peak and trough of each wave measured in the vertical plane. The Magnetic field works in conjuntion with the Electric Field to propogate the waves in a concentric pattern in the horizontal plane.

So a vertically polarized radar initiates a signal by oscillating a voltage (dropping a stone) in the vertical plane which is radiated by the antenna in the horizontal plane (in the form of concentric waves).

For our RDs to be most efficient, they need to have antennas oriented with the same polarity as the transmitter. For a vertically polarized signal that means that the waves enter our horn antenna in the horizontal plane and get converted to an electrical signal in the vertical plane.

Why do the Police choose vertical and circular polarization for their mobile radar units.? Range! First we need to know that circular polarization rotates through both the vertical and horizontal planes. That means that the vertically polarized horns in our RD's are perfectly compatible with circularly polarized radars. Next, the electromagnetic waves radiated by a vertically polarized antenna travel above (parallel to) the ground unimpeded or skip off the ground since they travel in the horizontal plane. Horizontally polarized waves, however, travel in the vertical plane perpendicular to the ground (think of a flat tire here) and where each wave contacts the ground that portion of the wave gets cut off, thereby sapping the strength of the signal near the ground. Also, vertically polarized signals are more directional in the vertical plane which means the beam is more focused near the ground in the horizontal no energy is wasted radiating the signal out into all the bouncing around occurs in the horizontal plane (near the ground) where our RD's can pick it up.

This brings up again the point why we have such a hard time with Horizontally polarized units like the RedFlex radar vans. Horizontally polarized radar waves travel in the vertical plane and the signals lose power quickly near the ground due to cutoff. Also, a good part of the reflections are into outer space, and beam width is tightly focused in the horizontal plane making it very directional and difficult to pick up off axis. A vertically polarized antenna can pick up a horizontally polarized signal but the loss in gain by the antenna is tremendous. The phenomena called multipathing can actually work to our advantage depending on the particular situation. Multipathing is where the radar signal bounces around randomly off the terrain, and depending on unique circumstances, a portion of the radiated energy can actually change polarity.

So how does knowing any of this help us? It just gives us some ideas on the do's and don'ts for mounting our RD's. Some of this is old hat for most but it just goes to explain why.
  1. Mounting your RD level with the horizon or slightly downward is best for picking up direct waves or waves reflected off the ground.
  2. Stay away from curved glass! The slope of your windshield is not as important as the curve. Curved glass will distort the wavefront by creating a phase delay from side to side. To perform optimally the wavefront (which is in the horizontal plane) needs to be planar or as flat as possible. Thats what the little bubble lense on the 9500ix and M3 flattens out the wave front as it enters the antenna.
  3. If you want better range against the RedFlex Radar vans go with one detector mounted vertically (which orients it for horizontally polarized signals). However, now your windshield slope is working against you. A remote vertical mount should work best.
  4. Stay away from vertical members when mounting your remote RD. Again, any object (albeit a low loss medium like plastic) can cause a phase delay on a portion of the wavefront entering your antenna which causes phase errors and lowers the antenna's gain. The same object placed horizontally in front of your antenna would create a uniform phase delay which should not have a negative effect. Only trial and error can tell for sure though if something blocking your antenna is creating a loss in sensitivity.

Horizontal polarization (Electric Field)

Vertical polarization (Electric Field)

Circular polarization (Electric Field)

Electromagnetic Wave Diagram

In practical terms, how much difference can be realized by having your Radar Detector inproperly oriented with respect to a polarized signal? At minimum you can expect detection range to be cut in half! Lacking the correct environmentals that induce multipathing, and thus changes in polarity, your range could drop to virtually nothing.

Here is a video of (2) Escort 8500 X50s going up against a Multinova Ka Radar unit that is horizontally polarized (again, the radar wave radiation pattern would be in the vertical plane). One is mounted normally with vertical polarization, and the other is mounted vertically (giving it horizontal polarization with respect to the Electric Field). Notice the X50 that is mounted vertically has double the range over the X50 using the standard mount. The detection on the standard mount was probably only as good as it was due to the surrounding reflective surfaces.

Last edited by Nine_c1; August 15th, 2014 at 10:19 PM.
Reply With Quote