T3A04: What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?
A. The modulation sidebands might become inverted
B. Signals could be significantly weaker
C. Signals have an echo effect on voices
D. Nothing significant will happen
The two component fields of an electromagnetic (EM) wave, the electric field and the magnetic field, each oscillate with specific spatial orientations. That is, the fields vary from positive to negative values in a particular direction as they move along through free space. Further, the two component fields’ directions of oscillation are perpendicular to one another. Check out the figure below depicting a simplified model of an EM waveform.
The orientation of the EM field oscillations is referred to as the polarization. The normal convention is to use the electric field’s orientation as the reference for describing polarization. So, if the electric field is waving itself up and down as it speeds along, we say the signal is vertically polarized. If the electric field is wavering left-and-right as it propagates, we say the signal is horizontally polarized. You can imagine that the electric field could oscillate in any orientation from perfectly vertical to perfectly horizontal, as compared to the earth’s surface. And the magnetic field will always be there with it maintaining an oscillation rotated 90 degrees from the electric field oscillations.
Of course, radio frequency (RF) radiation is a type of EM radiation, so RF has a characteristic polarization that describes the oscillating direction of the electric field component. You may now ask, “How do I know the polarization of my RF signals, or how can I control it, and why do I care?”
The polarization of your RF signals is determined by the orientation of your antenna – at least, it is determined by the orientation of the driven element of your antenna, where the feedline attaches. The electric field will emanate from your antenna with oscillations back and forth along the length of your radiating element. It is that simple for linear antennas like ¼ wave verticals, ½ wave dipoles, and Yagi directionals!
So, if you hold your HT antenna vertically, the signal polarization will be vertical. If you hold your HT antenna horizontal to the ground, the signal polarization will be horizontal. Hold the HT antenna at 45 degrees and the electric field oscillations will match that orientation. That ¼ wave vertical snapped onto your car’s rooftop with a magnetic base will produce vertically polarized signals. That wire dipole for 10-meter band strung up tight and level between a couple of trees will radiate horizontally polarized waveforms. The handheld 3-element Yagi that you hold skyward to make satellite contacts will be polarized with the orientation of the middle driven element.
Normally, VHF and UHF FM simplex communications and also FM repeater communications will be conducted using vertical polarization. Normally, single sideband (SSB) and CW communications on VHF and UHF will be conducted with horizontal polarization. And it matters that the stations at opposite ends of such line-of-sight communications are using the same polarization!
In order for a radio signal to induce voltages on a receiving antenna with the greatest possible strength, the radio signal and antenna must have identical polarization so that the signal’s electric field and length of the antenna element are aligned with one another. The greater the difference in polarization between signal and receiving antenna, the weaker the induced currents in the antenna, due to the relative lack of alignment.
A couple of related final notes:
- There is also circular polarization in which the polarization angle changes with wave propagation in a clockwise or counterclockwise rotation. See this excellent illustrationonline.
- With weak signal propagation using ionospheric skip, the polarization of the signals often becomes randomized, making consistent matching of polarization and receiving antenna impossible. Hence, weak signals due to both distance and polarization mismatch.
- The polarization of “loop” antennas, such as quads, delta loops, and others is determined by the feedpoint position of the driven element.
The answer to Technician question T3A04, “What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?” is B: Signals could be significantly weaker.
Related Question: T3A03, T3A09