Generally speaking, most of us do not even consider either the use, or its effect, of antenna polarization.  When we put up an antenna its polarization is strictly a matter of opportunity and circumstance.

      However, once we know better, the ability to control and switch an antenna’s polarization becomes almost a necessity. Analogous to the automobile, when we become aware of the use and advantage of it, it becomes almost indispensable almost immediately.

     Each signal coming into the radio receiver has a polarization to it.  It may be vertical, horizontal, circular, diagonal or something in between.  Of course, the actual polarization at the transmitting site may change many times before it arrives at the receiving site.  But the reasons for these changes, though quite interesting to most serious Hams, is outside the scope of this article.

     We have several reasons to possess the ability to switch an antenna’s polarization.  First and foremost is the ability and desirability to match the polarization of the receiving signal. When we do so, the signal strength increases most significantly and can make the difference between match and mis-match a difference between success and failure in two-way communications.

     However, that subject is also outside the scope of this article, although also extremely interesting to the serious Ham.  We must confine ourselves to the study of the relationship between antenna polarization and the noise in the radio receiver. We can actually use the antenna polarization to control and minimize receiving noise.
 

Some insights -

     A few years old, while monitoring the 80 meter band in the SSB mode, I discovered an horrendous hum, a low frequency murmuring sound with a strength of several S-units with a bandwidth of at least 10KHz.

     For some reason I decided to switch antennas with my remote-control antenna switch.  I have a selection of 6 different antennas.  Three of them are identical but mounted in a vertical, horizontal and diagonal configuration to match the polarization of the incoming signals. I switched from the horizontal to the vertical antenna.

     Immediately, there was a significant change.  The horrendous hum disappeared.  Now, there was a heterodyne nearly in the center of where the hum had been. Perhaps the hum came from the Cosmo?  Or some other unknown source.  It doesn’t matter.  What was important to me was the recognition the hum was replaced by the heterodyne when I had switched from one antenna polarization to another, meaning there were two AM radio signals “beating” against each other concealed originally by the hum, which disappeared immediately when I had switched antenna polarization. That was significant. When I switched to the diagonal antenna the heterodyne almost disappeared and the hum came back to a level less than a third of the original strength with the horizontal antenna.  That was significant, too!

     I went back to the vertical antenna and adjusted the width of my bandpass filter and rotated my VFO to search for the distance between these two AM signals.  There were about 3 KHz apart from each other, meaning appropriately the width of a typical SSB signal.

     I went to the AM signal on the left and found two AM stations in QSO with each other.  One was located in the state of Pennsylvania and the other in Maryland.

     Then I went to the AM signal on the right and found 4 AM stations in QSO.  One was in Maine; two in New Hampshire and the forth in Plymouth, Massachusetts.

     Both groups were unaware of each other and, of course, of me, too.  There was no reason to be aware of my existence. I was doing nothing to make them aware.

     Over the course of several years I have experimented with antenna polarizations to discover the best way to reduce noise in the radio receiver was to select the best antenna polarization at the time.  Sometimes it would be the vertical antenna and sometimes the horizontal antenna.  Almost every time the diagonal antenna would produce a compromise, meaning either a reduction or increase in noise with a corresponding reduction or increase in radio signal strength on the frequency.  It would almost never out-perform the other two antennas.

     There were exceptions!  These exceptions would occur when one polarization would produce a significant reduction in noise but, at the moment time, entirely eliminate the radio signal on the frequency.

     The diagonal antenna would produce a compromise.  Although the noise would immedi- ately rise, so would the radio signal to make it possible to participate in a QSO.  In that event, I would have to put up with a noisy but tolerable reception.

Conclusion -

     Frankly, I think the conclusion is rather obvious.  An ability to switch antenna polarization is an excellent way to reduce or to minimize receiving noise.  The most elaborate and expensive rig will not achieve the same results as easily.
 

      This demonstration is not meant to be a scientifically valid experiment to prove a relationship between the antenna's polarization and the receiving noise. The relationship is
clearly there ... we are just not performing a scientific test or experiment to conclusively measure the differences.  These differences vary enormously daily and seasonally, and with the
polarization of the Sun's radiation as the Poles periodically change their magnetic polariza-
tion. They also vary with the time of the day, with a pattern of the noise getting more severe
in late morning and petering off in late afternoon. In late afternoon, or early evening, these
differences between antenna polarization and receiving noise taper off and then, by early to
middle evening, disappear entirely.  We will find little or no difference in noise as we switch from one antenna polarization to another during these times.

     Instead, with the aide of a calibrated Tektronix Model 454 Oscilloscope, using the movie
mode on a Panasonic Digital Camera, we can easily see and hear the difference between the
noise from the HF receiver (3.940 MHz) and the antenna polarization at mid-day. By mid-
night, however, there will be very little or no difference at all.