RF Radiation Beam tilt question

[r_2016]

Hi,


I am reading about how a radio station may "null fill" to direct RF radiation downward from the broadcast antenna; without this the area where the antenna is located would not receive a good signal, if any. Does anyone know what limits apply to this? If someone stood at the base of the tower with a radio tuned to the frequency, would they receive no signal? Thanks

 

[sophiecentaur]

Hi and welcome to PF

For the most complicated transmitting antennae, which are used in high power stations, with large service areas, the vertical radiation pattern is tilted downwards so that the peak of the power aims at around the edge of the service area and minimal power is fired upwards (reduces wasted power and also interference into other areas). This is achieved by applying a phase tilt across the radiating elements. The vertical aperture is often several wavelengths in extent and often consists of a set of dipoles, approximately 12 wavelength spaced. The pattern from a simple, uniformly illuminated aperture will have significant 'sidelobes' and serious nulls. Nulls can be filled by splitting the array into two, interleaved arrays and feeding the two arrays with different phases (in quadrature, iirc) Good null filling is a clever business. The radiation pattern is always frequency sensitive and transmitting antennae need to handle four or five UHF channels, spanning a frequency range of more than 10%. The pattern in the main beam can be made to be well behaved but, in the area near the foot of the mast, the change of the pattern with frequency will produce severe distortion of the frequency response. Some people who live close to a main transmitter need to get their signal from a relay station, rather than from the mains station but the siting of these big stations is often chosen to be up on a hill and not in the centre of a large city.

 

[sophiecentaur]

This ebook (link) tells you a lot of stuff about broadcast antennae in general. Beam tilt is dealt with pretty comprehensively.