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Posts Tagged ‘Polarisation’

Making Waves – Antenna Polarisation Issues

June 8, 2017 2 comments

 

Someone asked me to explain why he was unable to hear some horizontally polarised stations on his vertical even though the vertical operates at 360degrees.  This was the explanation I gave him:

Dipole1fig.1

Fig.1 shows the radiation pattern of a 1/2 wave dipole, showing strong signals off either side with nulls towards the ends.

Dipole2fig.2

This is better shown in fig.2.  Assuming that the antenna runs from north to south in a straight line, any station to the east or west will be able to hear / work it but if they are located north or south of it they will struggle to hear or be heard.

Vertical1 fig.3

fig.3 shows the radiation pattern of a vertical antenna.  A vertical antenna, when placed over a good earth, will radiate evenly in all directions and so can be heard/worked by any station in any direction that is either vertically or horizontally polarised.  However, those who are horizontally polarised must have their antenna running in the correct direction for the reasons outlined above.  There is also a 3 – 6dB loss in signal due to the change in polarisation although this effect is only true in a vacuum as scatter caused by other objects and reflection changes the polarisation of the radio waves anyway.

 

 

 

 

 

 

 

 

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