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hanhealthy
Antenna-Theory.com Newbie
Joined: 25 Mar 2012
Posts: 1
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 Posted: Sun Mar 25, 2012 8:37 am Post subject: which antenna only radiates toward half hemisphere |
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I want to fix my antenna on the ground. So the radiation pattern of exactly upper hemisphere is preferred.
Are there any antennas' radiation pattern focus only on hemisphere?
Thanks in advance. |
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bigSteve
Antenna Wizard
Joined: 14 Mar 2009
Posts: 265
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R. Fry
Antenna Theory Regular
Joined: 06 Jun 2011
Posts: 49
Location: Illinois USA
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| Posted: Thu Mar 29, 2012 12:17 pm Post subject: Re: Antennas |
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The pattern graphic in the above link shows that a monopole over real earth has a radiation pattern that is "skewed" upward, away from the horizontal plane. However that is a conclusion based on the radiation that exists an infinite distance from the monopole, over a ground plane of infinite extent.
In reality all monopoles of 5/8 wavelength and less radiate maximum field in the horizontal plane, regardless of earth conductivity. If this was not true, then AM broadcast stations would have no groundwave coverage -- which clearly they do
It was shown by careful experiments by Brown, Lewis and Epstein of RCA (1937) that the groundwave field 3/10 of a mile over a real earth path of about 4 mS/m conductivity from monopoles of heights of about 45 to more than 90 degrees using 113 x 0.412-wave buried radials is within a few percent of the theoretical maximum for the applied power radiated by a perfect monopole over a perfect ground plane. |
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Schubert
Antenna Wizard
Joined: 08 Apr 2009
Posts: 161
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| Posted: Thu Mar 29, 2012 5:03 pm Post subject: |
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Hi R. Fry,
You wrote:
In reality all monopoles of 5/8 wavelength and less radiate maximum field in the horizontal plane, regardless of earth conductivity. If this was not true, then AM broadcast stations would have no groundwave coverage -- which clearly they do
This isn't true, because you do not have to be at the peak direction of radiation from an antenna in order to communicate.
I do concur that the monopole fields are more towards the horizon in general though.
Thanks |
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R. Fry
Antenna Theory Regular
Joined: 06 Jun 2011
Posts: 49
Location: Illinois USA
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| Posted: Thu Mar 29, 2012 8:03 pm Post subject: |
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| Schubert wrote: | | This isn't true, because you do not have to be at the peak direction of radiation from an antenna in order to communicate. |
However the last part of your quote above (after the comma) does not mean that the first part of it is valid.
Groundwave fields radiated by the monopoles used by AM broadcast stations have been accurately measured for many decades, and show that close to the monopole (within several wavelengths), they are within several percent of the theoretical maximum value for the relative field envelope existing in the elevation plane. |
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R. Fry
Antenna Theory Regular
Joined: 06 Jun 2011
Posts: 49
Location: Illinois USA
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| Posted: Thu Mar 29, 2012 8:37 pm Post subject: |
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Picking up on the last paragraph of my last post in this thread, the link below shows the fields measured in the horizontal plane from a 90 degree monopole. The measurements were taken by a well respected broadcast consulting firm.
The inverse distance field plot line there is the theoretical maximum field for a perfect monopole over a perfect ground plane, for that applied power.
http://i62.photobucket.com/albums/h85/rfry-100/MWFieldIntensityMeasurement.jpg |
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