Antenna Polarization in Radio Communication
Although there are several types of antennas that produce circular (LHCP or RHCP) polarization, they are relatively rare in terrestrial communication application scenarios. Satellite-based radio communication often uses circular polarized antennas both at the satellite and the earth station end of the links. However, that’s a story for another day… Or else, check out the article Amateur Satellite Communications. Moreover, the circular polarization throws up its own set of challenges related to polarization rotation directions.
As amateur radio operators, generally engaged in terrestrial radio communication, we normally use linear polarized antennas. This is applicable to both HF as well as VHF/UHF communication. Typically, our antennas are oriented in ways to favor either Vertical or Horizontal polarization. This is notwithstanding the fact that some antennas may produce oblique angle linear polarization too. Let us now examine the typical scenarios and try to figure out what it means to us in practice.
Typical antenna polarization in VHF/UHF radio communication
The antennas for VHF/UHF bands are physically much smaller in size in comparison to the antennas used for HF radio. The typical VHF (2m band) and UHF (70cm band) amateur radio antennas quite often vertical omnidirectional antennas like a 1/4λ ground plane antenna, or even an HT mounted rubber Duckie or an extended vertical. VHF/UHF base-station operators often opt for directional antennas like the Yagi, Quagi, etc allowing better performance over longer distances.
The VHF/UHF terrestrial communication activity may be segregated into two types, the first one being the most popular FM radio either through direct one-to-one contact or via a repeater, while the second type is SSB/CW/Digi-mode VHF/UHF radio. Nowadays, most FM or the more recent DMR radio communication activity typically use vertical polarization as the preferred polarization. Although technically, there is nothing wrong with using horizontal polarization for FM radio, the vertical polarization is standard… One might ask, why was vertical polarization chosen as the standard for FM while it is horizontal for SSB on the same VHF/UHF bands? The answer is rather simple.
Most often, people constantly listen to the local FM repeater to monitor traffic and conduct QSO. The repeater is the central hub of traffic flow in most places. For the repeater to be able to listen and provide service all around in its vicinity, it needs to have an antenna with omnidirectional capability in the azimuth plane. In order to do so, the antennas at the repeater site would typically be a vertically oriented antenna. Usually, these are high gain, Vertical Collinear antennas. These antennas, therefore, produce and receive vertically polarized signals with a high degree of efficiency. Therefore, if someone uses a Yagi antenna to access a distant FM repeater, please use a vertically oriented (vertical polarized) Yagi and not a horizontally oriented one.
Due to the fact that that traffic routing hub (the repeater) is designed to function with vertical polarization, it becomes rather imperative for the sake of optimal performance to have a matching polarization orientation at our radio stations. Why is identical polarization orientation important? I have addressed it in the article Amateur Satellite communication under the section Antenna polarization for satellite communications. The principles are applicable in this case too. Please read it for more insight.
OK, so far so good… Why do the VHF/UHF SSB operators prefer horizontal polarization? Why doesn’t everybody stick to the same polarization standard regardless of FM or SSB?
There are two reasons for the above, the first one is historical, while the second one is related to performance. Long before repeaters became popular, radio amateurs have used the VHF/UHF bands to carry out terrestrial radio communication. They either used CW, SSB, or FM. However, since the QSO was point-to-point in nature and there was no intermediate repeater in the circuit, there was no need for a vertically polarized antenna that one finds at the repeater site. During the early days, VHF HT was relatively rare and most amateur VHF/UHF operation was done using base-station setups with external antennas. Operators usually wanted to optimize communication range using directional rotatable high gain antennas. The obvious choice was Yagi. A horizontally oriented Yagi was easier to deploy with minimal radiation pattern distortion on account of metal masts. It was easy to fabricate and deploy. Remember, nobody bought antennas in those days. All of them were homebrewed.
Secondly, and more importantly, a horizontally polarized antenna is less susceptible to the adverse effects of local QRM from electrical machinery, domestic appliances, and other sources. They pick up less local QRM and sound quieter thus providing leverage during the reception on account of enhanced SNR. The reason is that most of the aggregate local QRM contains vertically polarized noise since vertically polarized signals travel longer distances using surface-wave propagation phenomenon. For more information on the nature of noise, read the article Noise in Radio Communication.
These are the reasons why horizontal polarization still remains a general standard for VHF/UHF SSB radio since these are typically point-to-point operations without the use of repeaters, while vertical polarization for FM communication is standardized because substantial traffic on FM VHF/UHF is conducted through repeaters. Unlike the significant amount of FM traffic that is encumbered by the need to be routed via repeaters, the point-to-point SSB QSO does not carry that baggage and hence can easily leverage the additional advantages of high gain horizontally polarized directional antennas.
To achieve the best results, the polarization of both the transmit and receive antennas must have identical polarization orientation. Any amount of orientation mismatch will produce proportionately additional communication circuit loss. This is applicable to the direct-wave Line-of-sight (LOS) type circuits that usually prevail during VHF/UHF terrestrial QSO. On the other hand, HF radio communication through Ionospheric Skip is immune to antenna polarization mismatch. It does not matter whether the antennas at two far ends of an HF radio link are horizontal or vertical, or altogether different… Read on…
Significance of polarization in HF radio communication
As I mentioned above, the HF radio communication is rather immune to the adverse effects of polarization mismatch between the transmitter and receiver antenna. It does not matter. HF Radio stations around the world may freely use either vertical or horizontal polarization and yet continue to conduct QSOs without any problem… Why is it so?
There are several reasons for this. To begin with, HF radio terrestrial communication does never really occurs via the direct LOS mode. This is technically not feasible in practice because of the proximity of the earth’s surface to the HF antenna. When we speak of proximity (low height) above ground, we do not measure it in feet, meter, etc. The height is measured in terms of the wavelength (λ). Typically an HF antenna could be a fraction of λ or at best nothing more than 1-2 λ. On the other hand, the VHF/UHF antenna might easily be 10-20 λ high or even more.
For direct LOS propagation to be possible, it is important for the propagation path between the two points to clear a certain height above the ground. This clearance height is determined by the wavelength. If this clearance condition is not met, then path loss increases and typically makes communication very difficult. This clearance zone that is required is called the First Fresnel Zone. To learn more about it read the article Ground Wave Propagation. I have explained the fundamentals of the Fresnel zone and its limiting factors in sufficient detail in that article.
The bottom line is that practical HF radio station antenna installations are most likely never to be able to clear the First Fresnel Zone. Hence, LOS (direct-wave) communication that would have required a good polarization match between the transmitting and receiving won’t occur. Hence, it is pointless to aim for identical polarization at the TX and RX ends.
On the other hand, the typical HF radio communication would occur through ionospheric skip propagation. Let us see what might happen in this scenario…
Interestingly, the ionospheric skip mode propagation does not preserve the polarization orientation of the RF from the transmitting station. Actually, it totally messes it up and makes the polarization of the propagating wave random and unpredictable. It not only changes the wave polarization randomly, at times, it even might produce multi-path propagation conditions with each signal wavefront having a different polarization.
Therefore, not only will the signal reaching the RX station at the far end receive the signal with an unknown polarization orientation but it might also receive multiple simultaneous components of the signal with different polarization and phase. Moreover, the new altered polarization of the received signal does not remain constant but continues to change freely as we conduct the QSO. These phenomena occur due to various reasons. Without going into their detail which is beyond the scope of this post, I will cite a couple of prominent factors.
Primarily, as the RF wavefront enters an ionospheric layer that typically has a charge density gradient, in the presence of the earth’s magnetic field, the signal polarization begins to change its orientation either clockwise or counterclockwise depending on the direction of magnetic field lines. The duration spent by the signal before being bend back to earth and the prevailing maximum ionosphere charge density determines the amount of polarization rotation that occurs. This phenomena is called Faraday Rotation. Since the ionospheric charge clouds continually float around, its density also varies resulting in a constantly varying amount of polarization rotation.
Another factor that causes additional random polarization change occurs when the HF propagation path has multiple skips. The reflection that occurs from the ground that happens at the start of the next skip also randomly changes the polarization of the incoming signal. The quantum of change is determined by the type of terrain and the surface unevenness.
All said and done, the signal that finally reaches the receiver has unpredictable and random polarization. There are at least two important takeaways from these phenomena.
Firstly, one may use an antenna at either end with any polarization. It simply doesn’t matter. There is no way anyone could match the antenna polarization and keep them in sync on an HF radio communication circuit… So, why even bother?
Secondly, due to the continuous rotation of the incoming signal polarization and also due to multipath propagation, the signal voltage generated at the receiver antenna will constantly vary in amplitude. This will result in the effect of signal fading. The fading effect may occur at a slow rate or it may be fast. The depth of fading can be either shallow or quite deep. It will entirely depend on the prevailing behavior of the ionosphere at that specific time and also the multiple propagation paths that might exist.