Electric field polarization in different stations

[Gizew]

evelam00@gmail.com how is the electric field from AM broadcast stations polarized? from TV stations? from FM broad cast stations? is there any cionnection between this polarization and orientation of receiving antenna?

 

[Pumblechook]

AM stations are vertical. The aerials are usually mast radiators or T aerials. This gives low angle radiation. A hozirontal dipole would fire too much radiation at a steep angle due to ground reflections.

At VHF and UHF there is no deep technical reason why either should be used. The choice is more of convenience. It is said Horizontal can be a bit better over hilly areas and where there are a lot of trees but it may be marginal or just a theory.

TV is either. As the wavelength is so much shorter you can get transmitting aerials many wavelengths above ground and ground reflections have little effect. A mixture of hor and vert is used to reduced interference.

In Britain FM was horizontal because it was intended for home use and roof yagi aerials are easier to mount horizontally and look neater. These days slant or circular (both sometimes called 'mixed') is used to accommodate both home listeners and people in cars. It is also better for portable sets with telescopic aerials. Circular allows reception with the receiving aerial at any angle.


A vert transmitting aerial requires a vertical receiving one. Circular polarisation requires both ends to be phased in order for the electric vector to rotate in same direction but broadcasting receiving aerials tend to simple hor or vert or at at any angle.