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Vertical vs horizontal antenna polarisation benefits questioned

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Recent spectacular two-way communication between South Africa’s West Coast Radio amateurs and St Helena during good tropospheric propagation condition has raised the question whether horizontal polarisation (H-Pol) of the antenna would yield better results than a vertically polarised (V-Pol) antenna.

Fig. 1: 144 MHz contact between South Africa and St Helena

Operating on the simplex frequency of 145,500 MHz making a two-way contact over a distance of over 3000 km does not happen every day.  The amateurs on both sides were using vertically polarised antennas which started the debate. The convention says that H-pole antennas during tropospheric ducting conditions are preferred and should provide a gain advantage over V-pol antennas. It is, of course, important that both sides should use the same polarisation, which was the case with the St Helena contacts. This started a North-South debate that over the past years has become quite intense.

At a joint SARL/AMSATSA VHF workshop recently held in Gauteng, this subject was the main item on the agenda with a paper presented by Dick Coates (ZS6BUN) entitled “Are you serious about the last dB?” Coates did an intensive literature review and, coupled with experiential data from various radio amateurs, discussed the merits of both polarisations which proved that there was a marginal advantage using H-Pol over V-Pol. But does it really matter in practice? Well, yes and no!

Research into H-Pol and V-Pol started in the 1930s when the question was raised when one of the early TV stations in New York was planning to install their antenna on the Empire State building. The outcome was inconclusive so they decided on a compromise solution mounting the antenna at a 45-degree angle. It stopped the argument for some time but it did not prove anything.

By convention, the polarisation of an antenna is defined as the orientation of the electric field component of the wave it emits. For a yagi or a dipole this matches the orientation of the elements of the antenna. For many years, the “conventional orientation” for VHF and HF beams has been horizontal. Cross-polarisation losses, in other words a V-pole antenna at one end and a H-pole antenna at the other end, can be large, up to 30 dB is often quoted.

One of the misconceptions is that the polarisation of antennas is determined by the mode of transmission. For many years people believed that V-pol was best suited for FM modes. This idea came about because radio amateurs started using the FM mode when repeaters were established on the 145 and 435 MHz bands for mobile communication. It was simpler to fit a vertical antenna on a motor vehicle as it basically radiates omnidirectional while an H-pole antenna radiates directionally.

Fig. 2: Dick Coates (ZS6BUN) explaining the difference between vertical and horizontal polarisation of antennas and their effect on propagation.

“Just recording audio or video of contacts with different antenna polarisations does not provide any reliable proof. One would need to set up two identical stations on either side with the ability to physically switch the antennas from V-pol to H-pol several times and take accurate signal strength readings with the AVC witched off. Simple readings on the s-meter built into the receiver will not work as S-meters are notoriously inaccurate”, Coates said. “I have not come across any experiments as I have described.”

From science we learn that an H-pole antenna is more efficient, even if it is relatively marginal, as ascribed to the electromagnetic characteristics (magnetic susceptibility µR, conductivity σ and relative permittivity εR) of the soil. However, under absolute perfect conditions there should be no difference between the two polarisations.

The question to consider is whether the signal at the other end is received in or out of phase. Transmitted radio waves can follow slightly different paths before reaching a receiver. The waves can arrive at slightly different times and will be slightly out of phase due to the different path lengths. Depending on the magnitude of the phase shift, the waves can interfere constructively and destructively. The path between the transmitted and received signals is described as the Fresnel zone named after physicist Augustin-Jean Fresnel.

If a signal is vertically polarised and it deflects off the ground the resulting signal will be inverted relative to the original signal. This means the high points of the sine wave are now low points, and vice versa. Hence the bounced signal will arrive out-of-phase, which will weaken the received signal.

If a signal is horizontally polarised and it deflects off the ground the resulting signal will be received in-phase resulting in a stronger signal.

Wikipedia has an interesting an analogy. Place a mirror on the floor in the middle of a room, hold a flashlight on the other side of the room. The flashlight represents a signal and your eyes are the receiver. The mirror represents the ground. Move the flashlight move up and down representing vertical polarisation. Note that in the mirror, the flashlight moves in the opposite direction, that is, it moves down and up rather than up and down. This is out-of-phase. Now have the flashlight move to the left and right representing horizontal polarisation. If you look in the mirror, the reflected image of the flashlight moves exactly in tandem with the actual flashlight. Left is left, right is right. This is in-phase.

For poor ground the modulus of the reflection coefficient for V-pol is lower than for H-pol. It is also dependent on the angle. For grazing reflections, the modulus premium of H-pol over V-pol is less. Antenna height also plays a part. The additional signal strength of H-pole over V-pole antennas has been calculated as between 3 – 4 dB, hardly noticeable for FM or SSB signals. The mathematical treatment of the ground wave reflection was developed some years ago by two European radio amateurs, Gaeten Horlin (ON4KHG) and Palle Preben-Hansen (OZ1RH). The 3 – 4 dB gain comes into play in weak signals modes such as in the WJT digital modes. Between the West Coast and St Helena, one would not notice the difference when tropospheric conditions are good. On the other hand, one would not use V-pole in attempting tropospheric propagation communication between the West Coast of South Africa and Brazil, here the 3 – 4 dB of the H-pole polarised antenna would come in handy. In theory a tropospheric contact on 144 MHz between the countries is possible but definitely using H-pole antennas.


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