THE TRANSMISSION GALLERY
MIXED POLARISATION FOR VHF RADIO TRANSMISSION
In 1979 the decision was made to begin re-equipping the BBC's VHF-FM radio networks so that they would broadcast with mixed, rather than horizontal polarisation.
Mindful that many more people were now tuning in to VHF radio in their cars and in portable receivers the IBA were already transmitting most of the ILR stations to date with mixed or circular polarisation. Meanwhile the BBC had conducted their own tests on transmissions made from Nottingham and Crystal Palace. The results of these tests encouraged the BBC to begin work, in 1979, on the re-engineering of Wrotham.
Tests from Wrotham
The new mast, antenna and transmitters were brought into service at Wrotham in December 1981, replacing those used since the start of VHF-FM radio in 1955. With the introduction of the new antenna, the polarisation of the transmitted signals was changed from horizontal to mixed. The change also involved a doubling of the transmitter power to maintain approximately the same level of HP component together with an additional VP component of similar amplitude.
The opportunity was taken to compare reception of Wrotham before and after the change. The tests included some overnight periods when both new and old systems were available simultaneously allowing direct comparisons.
The new MP antenna at Wrotham is 29 metres higher than the old HP antenna and the mast position was changed by about 100 metres. The radiation pattern of the new antenna is not as omnidirectional as the original slot antenna, and because of this, bearing in mind that the maximum e.r.p. is restricted by internationally agreed limits, the mean e.r.p. of the horizontal component of the new antenna is 1.3 dB lower.
Polarisation of received signal
At each point at which field strengths were recorded, the field strengths of the HP and VP components were measured by rotating the receiving antenna between HP and VP. The antenna was also rotated to the angles at which maximum and minimum values of the signal were obtained and these values and angles were noted.
Even in those directions towards which Wrotham radiated truly circular polarisation, the received polarisation was elliptical rather than circular. This was due to ground reflections and obstacles along the propagation paths altering the the phase and amplitude of the received signal components.
Taking into account the propagation paths and variation between the radiated HP and VP components with direction, it would be expected that on average the received polarisation would be elliptical with maxima in random directions. Examination of the results showed that although this tended to be the case there was a bias towards the maximum field being near the horizontal. The maxima were within 10 degrees of horizontal at 28 % of points measured: at only 12% of points were the minima near horizontal.
The average improvement a listener could gain by rotating his receiving antenna from horizontal to the maximum was only 1.6 dB. At only 16% of the points measured was the ratio of maximum relative to the horizontal signal more than 2.5 dB. The received HP components were, on average, 0.6dB stronger than the VP components.
The received signal from the nominally circularly polarised transmission was rarely truly circular and thus any advantage to be gained by the use of circularly polarised receiving antennas is minimal. Only in exceptional circumstances is it worth mounting a fixed linearly-polarised rooftop receiving antenna other than horizontally (perhaps when screened by buildings where there may be standing wave effects, or to null out horizontally polarised interference). The great majority of fixed installations with external HP antennas will have experienced negligible change to their received signal.
Reception in cars
Following the re-engineering of Wrotham from HP to MP, despite generally much-improved reception, a few reports were received of increased multipath interference to car radio reception. Initial checks revealed that the problems were limited to fairly restricted areas. Tests were therefore carried out to assess the causes of this increased multipath and to locate the sources of the reflections.
Measurements were made outside programme hours with the Radio 3 transmitter feeding Wrotham's old HP antenna, whilst the Radio-4 transmitter fed the new mixed polarised antenna. Both directional and omni-directional receiving antennas were used.
The tall buildings of central London generally give the effect of reflecting VP more strongly than HP. although Other Measurements on isolated tall buildings showed no significant difference Ill the more rural Horsham area both polarisations were reflected with equal intensity and mainly from the hills.
0verall, the benefits given by improved car reception and greater coverage considerably outweigh the slightly increased multipath found in some areas.
A slanted dipole viewed from the side appears to be vertical and therefore radiates vertical polarisation in that direction.
|Transmitting antennas for circular and slant polarisation
It is useful to understand the problems of designing transmitting antennas for circular and slant polarisation.
Taking the simple case of a slanted dipole it can be seen that although in a broadside direction the antenna is obviously slanted, when viewed from the side the antenna appears to be vertical and indeed radiates vertical polarisation in that direction.
The illustration [right] shows a slanted dipole viewed from the side appears to be vertical and radiates vertical polarisation in that direction.
The same type of problem occurs with nominally circularly polarised antennas: it is not practical to maintain the exact phase and amplitude characteristics over a wide arc. Even with carefully designed high power antennas the ratio between the HP and VP components varies by up to 4dB. Similarly the phase relationships vary by up to 45 degrees.
Thus the angle of slant from a slant polarised antenna varies with direction and the actual polarisation of a nominally circularly polarised antenna can best be described as elliptical with the parameters of the ellipse varying with direction. In practice, this is of no consequence as we are seeking only to excite HP and VP receiving antennas and do not expect listeners to attempt to match the radiated polarisation with their receiving antennas. Hence the use of the more general term, 'Mixed polarisation'.
|Types of polarisation
Linear polarisation is the basic form and is usually termed 'Horizontal Polarisation' (HP) or 'Vertical Polarisation' (VP) to describe the direction of the electric field in the plane at right angles to the direction of propagation. This orientation also corresponds to the optimum orientation of the receiving dipole for the corresponding transmitted polarisation.
If the transmitted power is divided (usually equally) between the horizontally and vertically polarised components, no importance being attached to the phase difference, then the transmitted polarisation is called 'Mixed Polarisation' (MP).
Special cases are 'Slant Polarisation' where the two components are equal and in phase and 'Circular Polarisation' where the components are equal but differ in phase by 90 degrees. Slant polarisation is another form of linear polarisation where the electric field lies in a plane at 45 degrees to the horizontal. With circular polarisation the electric field rotates with time.
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